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Abstract

Sharks are diverse and ecologically important predators and are also highly varied in their biology and behaviour. Prior studies have posited basic relationships between body form and lifestyle; previous investigations of body shape in sharks, however, have been restricted to a few species, or measured dead sharks, which may show artefacts of preservation or distortion and/or require lethal sampling. Therefore, using non-lethal field methods, we examined body and fin shape in a group of eight different shark species that co-occur in coastal waters of the Western Atlantic but vary to different degrees in biology and ecology. We measured a series of 12 morphometric variables and body size (pre-caudal length) from wild individuals (N = 90 sharks total) belonging to the families Carcharhinidae [order: Carcharhiniformes (tiger, bull, blacktip, lemon, blacknose, Atlantic sharpnose and sandbar)] and Ginglymostomatidae [order: Orectolobiformes (nurse)]. By taking phylogeny into account using the SLOUCH method, our analysis revealed isometry of all 12 morphological variables measured relative to body length among all species, indicating that despite substantial lifestyle differences, the general body form of these carcharhiniform and orectolobiform species is overall highly conserved. Univariate analyses were consistent with this result in showing no substantial differences among species once the effects of body size were accounted for, although there was a modest difference among the species in leading edge of the caudal fin, which was also revealed by an elliptic Fourier analysis. A multivariate principal component analysis showed some differentiation among species in the height of the dorsal fin, the length of the lower lobe of the caudal fin and in overall body girth, but the lack of significant variation in the univariate analyses suggests that such differences may not be biologically substantial. Our conclusion was that these sharks are similar in gross morphology, which underscores the generality of the shark body form for different niches. Indeed, the most important variable distinguishing the species was variation in body length, which in sharks is generally linked to variation in diet type or breadth.

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... Ecomorphology, a field which seeks to relate morphological variation to ecological conditions through evolutionary theory (Andrew Barr, 2018;Bock, 1994), is increasingly being used to rationalise observable morphological variation between elasmobranch taxa (Irschick & Hammerschlag, 2015;Irschick et al., 2017;Sternes & Higham, 2022), thus at least to some extent aiming to reduce the uncertainties mentioned above. This approach makes several biological assumptions and the validity of which will vary between systems, however, provides a valuable process by which inferences can be made about the selective forces underlying the evolution of morphological structures, and how these may differ between populations and taxa Sternes & Higham, 2022). ...
... Such differences result from allometric growth, which refers to the growth of a morphological structure such that body proportions do not remain constant as a function of body size (Gayon, 2000). Not all elasmobranch taxa for which previous studies have been conducted show pervasive allometric growth (Reiss & Bonnan, 2010), and even in those that do, not all morphological structures have been shown to follow a clear trajectory (Ahnelt et al., 2020;Gayford et al., 2023;Irschick et al., 2017). Initial explanations for such a trend related to differences in maximum body size indicate that larger-bodied sharks would exhibit more pervasive allometry (Ahnelt et al., 2020;Irschick et al., 2017), however, more recently the role of ontogenetic niche shifts as a selective driver of allometry has been highlighted and formalised as the Allometric Niche Shift (ANS) hypothesis . ...
... Not all elasmobranch taxa for which previous studies have been conducted show pervasive allometric growth (Reiss & Bonnan, 2010), and even in those that do, not all morphological structures have been shown to follow a clear trajectory (Ahnelt et al., 2020;Gayford et al., 2023;Irschick et al., 2017). Initial explanations for such a trend related to differences in maximum body size indicate that larger-bodied sharks would exhibit more pervasive allometry (Ahnelt et al., 2020;Irschick et al., 2017), however, more recently the role of ontogenetic niche shifts as a selective driver of allometry has been highlighted and formalised as the Allometric Niche Shift (ANS) hypothesis . This hypothesis suggests that taxa which undergo ontogenetic niche shifts should exhibit relatively allometric growth, compared to those which do not exhibit ontogenetic niche shifts. ...
Article
The central tenet of ecomorphology links ecological and morphological variation through the process of selection. Traditionally used to rationalise morphological differences between taxa, an ecomorphological approach is increasingly being utilised to study morphological differences expressed through ontogeny. Elasmo-branchii (sharks, rays and skates) is one clade in which such ontogenetic shifts in body form have been reported. Such studies are limited to a relatively small proportion of total elasmobranch ecological and morphological diversity, and questions remain regarding the extent to which ecological selection are driving observed morphometric trends. In this study, we report ontogenetic growth trajectories obtained via traditional linear morphometrics from a large data set of the brown smoothhound shark (Mustelus henlei). We consider various morphological structures including the caudal, dorsal and pectoral fins, as well as several girth measurements. We use an ecomorphological approach to infer the broad ecological characteristics of this population and refine understanding of the selective forces underlying the evolution of specific morphological structures. We suggest that observed scaling trends in M. henlei are inconsistent with migratory behaviour, but do not contradict a putative trophic niche shift. We also highlight the role of predation pressure and sex-based ecological differences in driving observed trends in morphometry, a factor which has previously been neglected when considering the evolution of body form in sharks.
... Such a growth trend differs from that of basking sharks, which show negative allometric growth in the caudal fin and head [15]. It also differs from the growth patterns of white sharks and tiger sharks, in which the caudal fin scales negatively [14,15,19,20,22]. Isometric growth trends in caudal fins, as we observed for megamouth sharks, were reported for a variety of small-bodied sharks, such as nurse sharks Ginglymostoma cirratum Bonnaterre, 1788, and spiny dogfish [14,18]. ...
... It is possible, therefore, that no change in feeding ecology is reflected by little changes in the shape or proportion of caudal fins. Indeed, isometric growth in the caudal fin was reported for nurse sharks [14], which also prey on slow-moving organisms, including benthic fishes and invertebrates, without showing dramatic changes in diets during growth [20,39]. Of note, basking sharks also prey on small zooplankton and do not exhibit ontogenetic changes in diets, yet this species shows negative allometry in the caudal fin [15]. ...
... The anterior margin and the height of pectoral fins show isometric scaling. This result is consistent with the studies on other shark species, including other lamniforms [20,21]. In contrast, pectoral fin bases showed positive allometry. ...
Article
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Megamouth sharks Megachasma pelagios Taylor, Compagno, and Struhsaker, 1983, are a large-bodied, planktivorous, deep-sea species with peculiar morphology. Since their initial description in the late 20th century, many individuals of different sizes have been reported, but few studies examined ontogenetic changes in body shapes. Here, we assess the growth changes in their heads and fins based on length measurements from nine different-sized individuals (177–544 cm in total length). Bivariate analyses showed that the head becomes larger relative to body length with increasing body size (i.e., positive allometry), whereas the relative size of the caudal fin remains constant (i.e., isometric growth). This trend differs from basking sharks and apparently resembles whale sharks and some baleen whales, although they are all large-bodied filter feeders. Given that relative mouth size is linked to feeding modes, our results suggest that megamouth sharks have different feeding modes from ram-feeding basking sharks and may have some similarity with suction-feeding whale sharks and engulfment-feeding baleen whales.
... The evolutionary 'success' of sharks has previously been partially attributed to conservatism in their body form (suggesting that all sharks possess one of a small number of generalized body forms) and locomotor function (Thomson & Simanek, 1977). Despite these notions, it is clear that sharks exhibit a wide range of body sizes and shapes, representing adaptations to a range of distinct ecological niches (Irschick et al., 2017;Sternes & Shimada, 2020). Different shark species employ alternative locomotor strategies, facilitated by interspecific variation in the shape, size and physiology of body appendages (Wilga & Lauder, 2002;Gleiss et al., 2011;Iosilevskii & Papastamatiou, 2016;Maia et al., 2017). ...
... Ontogenetic morphometry has also been studied in some sharks, providing evidence of both allometric and isometric growth, depending on the species under investigation and the morphological traits in question (Lingham-Soliar, 2005a;Reiss & Bonnan, 2010;Irschick & Hammerschlag, 2015;Fu et al., 2016;Ahnelt et al., 2020;Sternes & Higham, 2022). Some studies have suggested a relationship between ontogenetic scaling patterns and body size, with smaller-bodied taxa likely to grow isometrically, and larger species more likely to exhibit some degree of allometric growth (Irschick et al., 2017;Ahnelt et al., 2020). ...
... Moreover, there is a marked size difference between these two species, with S. lewini typically reaching substantially greater length and mass than R. longurio (Compagno, 1984). In light of the aforementioned hypothesized relationship between body size and ontogenetic scaling (Irschick et al., 2017;Ahnelt et al., 2020), the inclusion of taxa varying in maximum body size is important. Additionally, S. lewini and R. longurio are categorized by the IUCN as critically endangered and vulnerable, respectively (Rigby et al., 2019;Pollom et al., 2019); as such, further studies into their growth patterns are vital to help inform local management plans and conservation efforts. ...
Article
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In addition to the selective importance of interspecific morphological variation, ontogenetic morphological variation may reflect different selective regimes to which successive developmental stages are subjected. The typical body form of carcharhiniform sharks is considered relatively conserved, yet sharks exhibit a wide range of body sizes and shapes, representing adaptations to distinct ecological niches. Previous investigations of ontogenetic shifts in shark body form have provided evidence for both isometric and allometric changes, depending on the morphological characters and species investigated. These findings have led to suggestions of a relationship between body size and allometric growth in sharks. In this study we present evidence of ontogenetic allometric shifts in two species of carcharhiniform sharks (Sphyrna lewini and Rhizoprionodon longurio) from novel measurements. Our results are generally consistent with previous suggestions of body form conservatism across shark phylogeny, yet also suggest potential selective factors underlying observed instances of ontogenetic allometric shifts, and highlight where additional studies are required. We propose the 'allometric niche shift' hypothesis for interspecific differences in scaling trends, suggesting that long-distance movements and ontogenetic trophic niche shifts represent key drivers of allometry in sharks.
... To accomplish our objectives, we collected a set of pictographs for two lineages of sharks, the sister families of Carcharinidae and Sphyrnidae, and the order Lamniformes, from multiple illustrators. Sharks make particularly strong candidates for the use of pictographs as their cartilage can be distorted through preservation (Irschick et al., 2017;Jones & Geen, 1977), the generally large size of taxa limits the application of tomography, and there is a lack of readily available whole specimens (Irschick et al., 2017). Additionally, sharks, while sharing a general body plan, have a wide range of morphological diversity (Sternes & Shimada, 2020;Thomson & Simanek, 1977) that can reduce the descriptive power of common morphological measurements across species. ...
... To accomplish our objectives, we collected a set of pictographs for two lineages of sharks, the sister families of Carcharinidae and Sphyrnidae, and the order Lamniformes, from multiple illustrators. Sharks make particularly strong candidates for the use of pictographs as their cartilage can be distorted through preservation (Irschick et al., 2017;Jones & Geen, 1977), the generally large size of taxa limits the application of tomography, and there is a lack of readily available whole specimens (Irschick et al., 2017). Additionally, sharks, while sharing a general body plan, have a wide range of morphological diversity (Sternes & Shimada, 2020;Thomson & Simanek, 1977) that can reduce the descriptive power of common morphological measurements across species. ...
... The percent variance explained of the two PC SL axes is labeled on the x and y axes. coordinates to hundreds of global shape measurements or tens of thousands EFA coefficients and thousands of retained PC scores enabled a quantification of typically qualitative, discrete traits (Baken et al., 2021;Irschick et al., 2017). This reduced the subjectivity of qualitative categories, facilitated the discernment of the principal features that change across shapes, and enhanced the trait information for downstream analyses. ...
Article
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Body shape is a foundational trait on the differences between species. However, morphological measurements can be simplifying and, for many taxa, can be distorted upon preservation or are difficult to collect due to a species' habit or size. Scientific illustrations, or pictographs, provide information on a species' morphology but are rarely used as traits. Here, we demonstrate the use of pictographs using two shark clades: Lamniformes and Carcharhinidae + Sphyrnidae. After collecting 473 pictographs from 67 species across 12 sources, we used landmarking to show that measurements derived from pictographs do not substantially differ from those garnered from specimens. We then used Elliptical Fourier Analysis and principal components analysis to construct a multivariate morphospace. Using global shape measurements, we evaluated whether substantial variability in body shape was introduced by habitat association, endemism, or illustrator. We found that a species' habitat preference strongly influenced the discovery rate of pictographs and the within‐species similarity. While illustrations varied within a species, only a limited set of illustrators exhibited significant systematic variability. We also demonstrated the utility of pictographs in two common applications. For ancestral trait reconstruction, we developed a simple extension to estimate body shapes from principal components and, in doing so, observed that the Lamnid body plan diverged from the rest of Lamniformes ~100 MYA. For phylogenetic generalized linear mixed models (PGLMM), we found that the pictographs had greater explanatory power than traditional morphological measurements. We used the PGLMM to show that higher endemism across Carcharhinidae + Sphyrnidae taxa correlates with body shapes that have caudal fins with small heterocercal angles and more pronounced second dorsal/anal fins. We concluded that pictographs are likely an undervalued and easy‐to‐digitize data source on a species' body shape with numerous established methods for comparing pictographs and assessing variability. Scientific illustrations are useful traits that should be included in trait‐based studies. We show there similarity to specimen‐based measurements and find little systematic variability across illustrators. We highlight their utility in enhancing two common trait‐based analyses: ancestral trait reconstruction and phylogenetic generalized linear mixed models.
... Subsequently, Iosilevskii and Papastamatiou (2016) compared the body morphology of sharks with their buoyancy and energetics, and showed that larger sharks increase buoyancy to compensate for longer pectoral fins. More recently, Irschick et al. (2017) examined eight shark species to determine if ecology influenced body form. Despite all these studies, the four body groups proposed by Thomson and Simanek (1977) have been assumed to capture the body form diversity in sharks (e.g., Maia et al., 2012;Shadwick and Goldbogen, 2012;Irschick and Hammerschlag, 2014;Lauder and Di Santo, 2016;Irschick et al., 2017) even though their study was based on only about one-tenth (56 different species) of all known shark species using simple morphometrics. ...
... More recently, Irschick et al. (2017) examined eight shark species to determine if ecology influenced body form. Despite all these studies, the four body groups proposed by Thomson and Simanek (1977) have been assumed to capture the body form diversity in sharks (e.g., Maia et al., 2012;Shadwick and Goldbogen, 2012;Irschick and Hammerschlag, 2014;Lauder and Di Santo, 2016;Irschick et al., 2017) even though their study was based on only about one-tenth (56 different species) of all known shark species using simple morphometrics. ...
... We used an Analysis of Covariance (ANCOVA) to determine if taxonomic order ('factor') and shark size ('covariate') had an effect on overall shape in sharks ('response variable'). In this study, we chose to use the results of the precaudal body analysis, because many comparative papers on shark morphology (e.g., Irschick and Hammerschlag, 2014;Fu et al., 2016;Irschick et al., 2017) relied on the use of the precaudal length (PCL). Furthermore, certain taxa (e.g., Alopias and Stegostoma) have caudal fins nearly the length of the body, possibly causing statistical outliers. ...
Article
Sharks are among the oldest vertebrate lineages in which their success has been attributed to their diversity in body shape and locomotor design. In this study, we investigated the diversity of body forms in extant sharks using landmark-based geometric morphometric analyses on nearly all the known (ca. 470) extant sharks. We ran three different analyses: the ‘full body,' ‘precaudal body,’ and ‘caudal fin' analyses. Our study suggests that there are two basic body forms in sharks, a ‘shallow-bodied’ form (Group A) and ‘deep-bodied’ form (Group B), where all sharks essentially have one basic caudal fin design of a heterocercal tail despite some specializations. We found that swimming modes in sharks are highly correlated with body forms where Group A sharks are predominantly anguilliform swimmers and Group B sharks are represented by carangiform and thunniform swimmers. The majority of Group A sharks are found to be benthic whereas pelagic forms are relatively common among Group B sharks. Each of the two superorders of sharks, Squalomorphii and Galeomorphii, must have gone through complex evolutionary history where each superorder contains both Group A sharks and Group B sharks, possibly involving parallel evolution from one group to the other or at least one episode of evolutionary reversal.
... Univariate ANOVAs were run on each morphological variable using three terms in each model [body size (TL and SVL, respectively), species and the interaction between body size and species] to tease apart the specific morphological differences among Hyperolius species (Irschick et al., 2017). The key term of interest is the interaction between species and size, which informs for a given body size that there is heterogeneity among the species in the variable (Irschick et al. 2017). ...
... Univariate ANOVAs were run on each morphological variable using three terms in each model [body size (TL and SVL, respectively), species and the interaction between body size and species] to tease apart the specific morphological differences among Hyperolius species (Irschick et al., 2017). The key term of interest is the interaction between species and size, which informs for a given body size that there is heterogeneity among the species in the variable (Irschick et al. 2017). We performed the ANOVAs on all log10-transformed and non-size-adjusted morphological variables other than TL and SVL, respectively. ...
... The first PC consists of approximately equal loadings from all variables and primarily reflects differences in body size among species (e.g. Bookstein et al. 1985;Berner 2011;Irschick et al. 2017). PC2 and PC3 represent those aspects of shape that contribute most to variance. ...
Article
Reed frogs of the genus Hyperolius inhabit a wide range of habitats from open savannah to montane forests in Sub-Saharan Africa with sympatric species frequently exploiting the same localities at tadpole and adult stage. Niche partitioning with respect to morphological traits related to diet and locomotion is expected to shape local Hyperolius communities. In a case study on Albertine Rift Hyperolius in Rwanda and Burundi, we analysed the partitioning of larval and adult morphospace (three-dimensional volume describing size and shape variation) as affected by phylogenetic relationships and environmental constraints. Morphological features and molecular barcoding demonstrated that the Albertine Rift Hyperolius resolved into 12 terminal groups (¼species), which represent five phylogenetic lineages. The major morphological source of variation among species was isometric size variation at tadpole and adult stage. Among-species variation of tadpole size (maximum size reached before metamorphosis) predicted the range of snout-vent length variation in adult males indicating a strong coupling between the two stages. The adaptive decoupling hypothesis is not supported at the taxonomic scale of the Hyperolius genus. Size-independent shape traits were assessed using a principal component analysis. The two traits which explained most of the morphological variance at the two ontogenetic stages were related to feeding (mouth architecture in tadpoles, head shape in adults) and to locomotion (body/tail height in tadpoles, leg morphology in adults). Partitioning of the shape axes indicated that savannah-dwelling and forest-dwelling species exploit different volumes of the available morphospace. Transition from open savannah habitats to closed-canopy forests seems to induce modifications of both tadpole and adult morphology.
... vertebrate clade occupying a diverse range of ecomorphological niches (Navia et al., 2017;Sternes & Shimada, 2020). Allometric growth has been recorded in varying proportions in several elasmobranch taxa, with functional hypotheses for observed trends proposed (Ahnelt et al., 2020;Bellodi et al., 2023;Gayford, Godfrey, et al., 2023;Irschick & Hammerschlag, 2015;Irschick et al., 2017;Reiss & Bonnan, 2010;Sternes & Higham, 2022). As of yet, the only formalised hypothesis concerning body form in elasmobranchs rather than specific structures, the allometric niche shift hypothesis suggests that species that exhibit ontogenetic shifts in habitat usage and/or trophic ecology are more likely to display allometric growth than species exhibiting relatively static ecologies through ontogeny . ...
... In the total data set, and data sets sorted by sex and maturity, there are multiple aspects of morphology that scale allometrically with body size, but others that scale isometrically (Tables 2-6). This is consistent with scaling coefficients reported for other shark species (Bellodi et al., 2023;Irschick et al., 2017). We also note the presence of unambiguous differences in morphometric scaling between the sexes and between size classes (Tables 3-6). ...
Article
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Recent studies have uncovered mosaic patterns of allometric and isometric growth underlying ontogenetic shifts in the body form of elasmobranch species (shark and rays). It is thought that shifts in trophic and spatial ecology through ontogeny drive these morphological changes; however, additional hypotheses relating to developmental constraints have also been posed. The bull shark (Carcharhinus leucas) is a large-bodied coastal shark that exhibits strong ontogenetic shifts in trophic and spatial ecology. In this study, we utilise a large data set covering a large number of morphological structures to reveal ontogenetic shifts in the body form of C. leucas, stratifying analyses by sex and size classes to provide fine-scale, more ecomorpho-logically relevant results. Our results indicate shifts in functional demands across the body through ontogeny, driven by selective pressures relating to trophic and spatial ecology driving the evolution of allometry. We also find significant differences in scaling trends between life stages, and between the sexes, highlighting the importance of utilising large, diverse datasets that can be stratified in this way to improve our understanding of elasmobranch morphological evolution. Ultimately, we discuss the implications of these results for existing ecomorphological hypotheses regarding the evolution of specific morphological structures, and pose novel hypotheses where relevant.
... In our study, we used preserved museum specimens, unlike others, who have used live sharks (Irschick & Hammerschlag, 2014;Fu et al., 2016;Irschick et al., 2017). That said, several studies (Reiss & Bonnan, 2010;Anhelt et al., 2020) have examined ontogenetic change using museum specimens. ...
... To document morphological differences in S. lewini, we followed the approach of Irschick & Hammerschlag (2014) and Irschick et al. (2017) and quantified the following morphological measurements (Fig. 1) using a standard metric tape measure (accurate to 1 mm): (1) cephalofoil or head size (EE), the distance between the inner part of the eyes; (2) lateral span (LS), the distance (i.e. around the curved dorsal side of the shark) from the insertion point of the anterior edge of one pectoral fin to the insertion point of the other pectoral fin; (3) frontal span (FS), the distance (i.e. ...
Article
Major shifts in habitat often occur during life history and can have significant impacts on the morphology and function of an animal; however, little is known about how such ecological changes influence the locomotor system of large aquatic vertebrates. Scalloped hammerheads (Sphyrna lewini) are large sharks found in warm temperate and tropical waters. Smaller scalloped hammerheads are generally found in near-shore habitats, but as they grow larger, individuals spend time in deep-water, pelagic habitats. We measured a number of morphological traits of scalloped hammerheads, ranging from 32 to 130 cm, to determine whether there are allometric changes in morphology in association with this shift in habitat. We found that head morphology, caudal fin area and lateral span scaled with negative allometry, whereas the lengths of their pectoral, dorsal and caudal fins, and their pectoral and caudal fin aspect ratios, scaled with positive allometry. Furthermore, the largest shark in our dataset exhibited an optimal body fineness ratio for locomotor efficiency. This suggests that the changes in ecology have profound influences on the functional morphology of scalloped hammerheads. We discuss how these drastic morphological changes relate to potential changes in scalloped hammerhead swimming function and performance.
... Supporting the findings of this study related to a possible lemon shark position within the genus Carcharhinus, Irschick et al. (2017) carried out another study. For comparison; 12 morphometric values and body measurements of 8 different individuals from Carcharhinidae and Ginglymostomatidae families were obtained and used in the calculation of this study. ...
... For comparison; 12 morphometric values and body measurements of 8 different individuals from Carcharhinidae and Ginglymostomatidae families were obtained and used in the calculation of this study. In terms of the general shape of the pectoral fin or the dorsal fin, the first four PC values obtained as a result of the harmonic analysis for each fin were found to be close to each other for these 4 species (lemon, blacktip, nurse and sandbar) (Irschick et al., 2017). (Dosay, 2000). ...
Article
Full-text available
Many doubts have not been answered about the phylogenetic relationship of the sharks. The morphological models and molecular studies, frequently used these days, can put some species into different order or suborder. The sharks contain about 1% of all fishes, separated into 8 orders. Within these, the largest group is the genus Carcharhinus, which includes economically important sharks. A lot of different analyses were done to determine the relationship among these genera. Most of them indicate that phylogenetic relationships at most taxonomic levels remain mysterious for this genera. This study was applied to determine the interrelationship between Carcharhinus and Negaprion genera based on the lemon shark position and to find out the possible paraphyletic situation of genus Carcharhinus, via using ribosomal ITS2 region and mtDNA D-loop for comparison and to get more reliable findings. As a result, based on the ribosomal ITS2 analyses, the lemon shark is placed within the genus Carcharhinus, on the other hand, the lemon shark finds a place outside of the genus Carcarhinus according to the mtDNA D-loop analyses results. Different findings regarding the position of the lemon shark indicate that it is necessary for more accurate results of the study by using more samples and more gene data.
... In our study, we used preserved museum specimens, unlike others, who have used live sharks (Irschick & Hammerschlag, 2014;Fu et al., 2016;Irschick et al., 2017). That said, several studies (Reiss & Bonnan, 2010;Anhelt et al., 2020) have examined ontogenetic change using museum specimens. ...
... To document morphological differences in S. lewini, we followed the approach of Irschick & Hammerschlag (2014) and Irschick et al. (2017) and quantified the following morphological measurements (Fig. 1) using a standard metric tape measure (accurate to 1 mm): (1) cephalofoil or head size (EE), the distance between the inner part of the eyes; (2) lateral span (LS), the distance (i.e. around the curved dorsal side of the shark) from the insertion point of the anterior edge of one pectoral fin to the insertion point of the other pectoral fin; (3) frontal span (FS), the distance (i.e. ...
Article
Major shifts in habitat often occur during life history and can have significant impacts on the morphology and function of an animal; however, little is known about how such ecological changes influence the locomotor system of large aquatic vertebrates. Scalloped hammerheads (Sphyrna lewini) are large sharks found in warm temperate and tropical waters. Smaller scalloped hammerheads are generally found in near-shore habitats, but as they grow larger, individuals spend time in deep-water, pelagic habitats. We measured a number of morphological traits of scalloped hammerheads, ranging from 32 to 130 cm, to determine whether there are allometric changes in morphology in association with this shift in habitat. We found that head morphology, caudal fin area and lateral span scaled with negative allometry, whereas the lengths of their pectoral, dorsal and caudal fins, and their pectoral and caudal fin aspect ratios, scaled with positive allometry. Furthermore, the largest shark in our dataset exhibited an optimal body fineness ratio for locomotor efficiency. This suggests that the changes in ecology have profound influences on the functional morphology of scalloped hammerheads. We discuss how these drastic morphological changes relate to potential changes in scalloped hammerhead swimming function and performance.
... megalodon in the family †Otodontidae has been widely explored 22 , the interrelationships between otodontids and other lamniforms remain questionable 25 . This uncertainty, coupled with the fact that sharks of different sizes have been reported as being geometrically similar in body profile 26,27 , suggests that other macropredatory lamniforms, in addition to C. carcharias, could serve as modern analogues open www.nature.com/scientificreports/ of †O. megalodon, thus aiding the reconstruction of body dimensions (i.e. head length, dorsal fin height and width, tail height). ...
... Our results reveal that body dimensions of our analogues isometrically correlate to TL. This finding agrees with previous discoveries of similar relationships in linear body dimensions of several other extant shark species 26,27 . Although the exact phylogenetic relatedness of †O. ...
Article
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Inferring the size of extinct animals is fraught with danger, especially when they were much larger than their modern relatives. Such extrapolations are particularly risky when allometry is present. The extinct giant shark †Otodus megalodon is known almost exclusively from fossilised teeth. Estimates of †O. megalodon body size have been made from its teeth, using the great white shark (Carcharodon carcharias) as the only modern analogue. This can be problematic as the two species likely belong to different families, and the position of the †Otodus lineage within Lamniformes is unclear. Here, we infer †O. megalodon body dimensions based on anatomical measurements of five ecologically and physiologically similar extant lamniforms: Carcharodon carcharias, Isurus oxyrinchus, Isurus paucus, Lamna ditropis and Lamna nasus. We first assessed for allometry in all analogues using linear regressions and geometric morphometric analyses. Finding no evidence of allometry, we made morphological extrapolations to infer body dimensions of †O. megalodon at different sizes. Our results suggest that a 16 m †O. megalodon likely had a head ~ 4.65 m long, a dorsal fin ~ 1.62 m tall and a tail ~ 3.85 m high. Morphometric analyses further suggest that its dorsal and caudal fins were adapted for swift predatory locomotion and long-swimming periods.
... This allowed us to extract a large number of morphological measurements (Table 1), which would not have been possible at fish landing sites due to the fast-paced nature of catch processing. Morphological measurements were selected in line with previous studies (Irschick et al. 2017;Gayford, Godfrey, and Whitehead 2023), and included various measurements of girth, and measurements of the pectoral, dorsal and caudal appendages (Table 1). ...
Article
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The versatility of the shark body form is suggested to be one of the key factors underlying their evolutionary success and persistence. Nevertheless, sharks exhibit a huge diversity of body forms and morphological adaptations. More subtly, it is increasingly evident that in many species, morphology varies through ontogeny. Multiple competing hypotheses exist explaining both the function of specific morphological structures and the interspecific distribution of these ontogenetic morphological shifts. However, existing studies are restricted to a small number of mostly large‐bodied species. In this study, we report allometric scaling relationships from functionally important morphological structures in the spadenose shark (Scoliodon laticaudus). We find that a mosaic of isometric and allometric growth underlies the scaling trends in this species and that cases of allometry are consistent with an ontogenetic shift in diet. Moreover, our results refute suggestions that small‐bodied sharks grow isometrically. Given the small number of existing studies of ontogenetic morphometry in sharks and the life‐history/ecological characteristics of S. laticaudus, this study is a valuable contribution to our understanding of the adaptive value of ontogenetic morphological shifts in elasmobranchs.
... Additionally, we also noted a decrease in body condition from neonates to juveniles (Figure 3.3 & 3.4), which could also be a factor influencing estimates of b than were less than three (Froese 2006). Whole body isometry is likely the rule and allometry the exception in elasmobranchs, where most lengthmass relationships for elasmobranchs report b values between 2.5-3.5 (Irschick et al., 2017;Kishore Kumar et al., 2021;Motta et al., 2014). However, morphological features such as fin proportions do show allometric growth in some species (e.g., Ahnelt et al., 2019;Fu et al., 2016;Irschick and Hammerschlag et al., 2015). ...
... At present, no other studies of denticle scaling are available for comparison to these data, but previous studies of shark body scaling reveal that many aspects of shark body shape scale isometrically, with negative allometry observed only for some aspects of tail shape [84]. Irschick et al. [85] concluded that for eight shark species, large sharks are in general geometrically similar to smaller individuals based on 12 linear measurements that captured overall body and tail shape characteristics. ...
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Centroscymnus coelolepis is a deep-water sleeper shark and, like most sharks, it is covered in placoid scales, or dermal denticles. The morphological diversity of the dermal denticles in this species, however, has not been described in detail, and ontogenetic changes in denticle morphology are poorly understood in sharks. Combining scanning-electron microscopy and micro-CT imaging, we demonstrate the presence of eleven dermal-denticle morphotypes across the ontogeny and different regions of the body of Centroscymnus coelolepis. The snout, interspiracular, and trunk/tail regions have similar changes in denticle morphotype during development. For example, on the trunks and tails of juveniles (~350 mm TL), denticle crowns have two to three longitudinal ridges and three posterior cusps that are gradually replaced by ridgeless and cuspless crowns in adults (>800 mm TL). Sixteen measurements were obtained from the 3D models generated. A principal component analysis demonstrated that the eleven distinct dermal-denticle morphotypes observed were located in different regions of the morphospace. The denticle volume and surface area showed negative allometry with respect to body length throughout the ontogeny. The results reflect the considerable diversity within the denticle multiverse (ontogenetic and intraspecific variations), and much of this diversity remains to be explored to fully understand the role of dermal denticles in shark taxonomy, ecology, and biomechanics.
... Traditional morphometrics, often referred to as linear morphometrics (LM), involves the assessment of body variation using measurable traits such as length, width, angle, and other quantifiable characteristics (Rohlf & Marcus, 1993). LM provides a valuable framework for intra and interspecific comparisons of morphological features of biological and ecological significance (Irschick et al., 2017;Irschick & Hammerschlag, 2015;Klingenberg, 2011;Rolim et al., 2015). LM plays a pivotal role in establishing practical guidelines for fishing, notably in determining permissible capture sizes (Cooke & Cowx, 2006;Santos et al., 2018). ...
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This study aimed to provide data for fisheries management of two threatened guitarfish species, by using linear and geometric morphometric analyses. Linear morphometrics (LM) has limitations in determining minimum catch sizes, whereas geometric morphometrics offers a robust approach to quantifying body form variations with the potential to influence metrics set by legislation. Based on LM analysis, Pseudobatos horkelii were larger than Pseudobatos percellens. Based on geometric morphometric analysis body form differed between the two species, particularly in the head, disk, and posterior region. P. percellens were also sexual dimorphic in body form. Our findings provide valuable insights into permissible catch size limits of these threatened guitarfish and also contribute to improvement of fisheries management strategies where catch is still allowed. The study underscores the necessity of species-specific and sex-specific management directives to effectively ensure the population recovery of both guitarfish species.
... Pectoral fins in tiger sharks measuring 277.9 cm PCL (which translates into 363.3 cm TL following the linear regression provided by Afonso, 2013) are 45.9 cm wide (Irschick et al., 2017), i.e., about 13 % of shark total length. However, since pectoral fins are relatively flexible in younger sharks and can be partially bended in the anterior-posterior direction, it is expectable that the fin would have passed through the strap even if the strap perimeter did not accommodate the whole width of the fin in addition to shark girth. ...
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Plastic waste is currently a major threat to marine ecosystems, and the ever-growing production of plastic materials suggests that this scenario will not change soon. Understanding cryptic effects of plastic debris on keystone marine species is warranted to address ecosystem-level impacts caused by plastic pollution. This study reports on plastic entanglement in top predator tiger sharks, Galeocerdo cuvier, from the western South Atlantic Ocean. Circular plastic straps produced severe trauma in 3 % of all tiger sharks sampled and led to abnormal anatomical development, indicating worrisome incidence and consequences of plastic entanglement in this species. Removing the plastic strap from one shark which had been entangled for at least 9 months was probably the cause for the shark having experienced post-release mortality. This suggests that, in some circumstances, strap removal is not effective to promote the survival of entangled sharks. Eliminating the circular integrity of plastic materials at first use should contribute to mitigate some of the impacts of plastic pollution on marine megafauna since this shape might be determinant in the process of shark entanglement.
... Whole-body isometry is likely the rule and allometry the exception in elasmobranchs, where most length-mass relationships for elasmobranchs report b values between 2.5 and 3.5 (Irschick et al., 2017;Kishore Kumar et al., 2021;Motta et al., 2014). However, morphological features such as fin proportions do show allometric growth in ...
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Body condition is an important proxy for the overall health and energetic status of fishes. The classically used Fulton's condition factor requires length and mass measurements, but mass can be difficult to obtain in large species. Girth measurements can replace mass for wild pelagic sharks. However, girth‐calculated condition has not been validated against Fulton's condition factor intraspecifically, across ontogeny or reproduction, or in a controlled setting. We used the epaulette shark (Hemiscyllium ocellatum), because they are amenable to captive reproduction, to track fine‐scale body condition changes across life stages, oviparous reproduction and between condition indices. We measured four girths, total length and mass of 16 captive epaulette sharks across 1 year and tracked female reproduction daily. We also collected length and mass data from an additional 72 wild‐caught sharks and 155 sharks from five previous studies and two public aquaria to examine the relationship between length and mass for this species. Even though data were derived from a variety of sources, a predictable length–mass relationship (R² = 0.990) was achievable, indicating that combining data from a variety of sources could help overcome knowledge gaps regarding basic life history characteristics. We also found that condition factor decreased during early life stages, then increased again into adulthood, with predictable changes across the female reproductive cycle. Finally, we determined that both Fulton's and girth condition analyses were comparable. Outcomes from this study uniquely provide body condition changes across the complete life history, including fine‐scale female reproductive stages, and validate the use of girths as a nonlethal whole‐organism energetic assessment for fishes.
... The observed growth type seems to confirm that small-bodied sharks allegedly undergo isometric morphological growth and don't change their body shape during their life, as already observed in previous studies (Ahnelt et al., 2020;Fu et al., 2016;Irschick et al., 2017;Irschick & Hammerschlag, 2015). Other studies provided different b values for the species considered, sometimes suggesting an allometric growth type. ...
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Length–weight relationships were observed for six elasmobranch species from the Adriatic Sea. Between 2006 and 2019, data on weight and total length were collected for three shark species Alopias vulpinus, Mustelus mustelus, Mustelus punctulatus; and data on weight and disk width were collected for three batoid species Aetomylaeus bovinus, Myliobatis aquila, Pteroplatytrygon violacea. Data collection comes from monthly observations as part of a monitoring program targeting specifically bycatch on pelagic pair trawlers involved in anchovy and sardine fishing. This fishery operates using pelagic trawls with a minimum mesh size opening of 20 mm. All elasmobranchs were measured to the nearest cm using a measuring board and weighed to the nearest gram using an electronic scale or a dynamometer for the largest specimens. A linear regression analysis performed on log‐transformed data was used to estimate intercept and slope, and to describe the length–weight relationship for each species.
... We confirmed the spiral intestine morphology for species in the majority of shark families. Families within an order do have different spiral structures, though it is common for other morphological features to vary greatly within shark families [42][43][44][45]. There is no clear correlation between shark diet types and spiral intestine morphology. ...
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Looking to nature for inspiration has led to many diverse technological advances. The spiral valve intestine of sharks has provided the opportunity to observe the efficiency of different valve systems. It is supposed that the spiral intestine present in sharks, skates and rays slows the transit rate of digesta through the gut and provides increased surface area for the absorption of nutrients. In this investigation, we use a novel technique—creating three-dimensional reconstructions from CT scans of spiral intestines—to describe the morphology of the spiral intestine of at least one species from 22 different shark families. We discuss the morphological data in an evolutionary, dietary and functional context. The evolutionary analyses suggest that the columnar morphology is the ancestral form of the spiral intestine. Dietary analyses reveal no correlation between diet type and spiral intestine morphology. Flow rate was slowed significantly more when the two funnel-shaped spiral intestines were subjected to flow in the posterior to anterior direction, indicating their success at producing unidirectional flow, similar to a Tesla valve. These data are available to generate additional three-dimensional morphometrics, create computational models of the intestine, as well as to further explore the function of the gastrointestinal tract of sharks in structural and physiological contexts.
... Using a maximum length of 17 m for C. megalodon (after Pimiento et al., 2010), and 11.2 m for C. angustidens based on the specimen ChM PV 7267, we estimated that C. angustidens had a maximum length 65% of the length of C. megalodon and applied a 65% correction to these growth stage bins (Pimiento et al., 2010;Gottfried et al., 1996) and consider C. angustidens neonates to be less than 2.6 m, juveniles between 2.6 m and 6.8 m, and adults more than 6.8 m. Isometric scaling of these growth stage bin boundaries is defensible on the grounds that lamniform and carcharhiniform sharks do not exhibit allometric growth (Irschick and Hammerschlag, 2014;Irschick et al., 2017;Cooper et al., 2020). ...
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Many extant sharks are cosmopolitan as adults but inhabit nursery areas as youngsters-often shallow, dynamic ecosystems with abundant prey for neonates and juveniles. Megatoothed sharks (Otodontidae) were the largest sharks of all time, and nursery areas have been demonstrated for Carcharocles megalodon in the Miocene of Panama, Spain, Florida, and Maryland. An earlier study hypothesized a nursery area for Carcharocles angustidens in the upper Oligocene (23-25 Ma) Chandler Bridge Formation of Charleston, South Carolina. We tested this by reporting and analyzing two collections (n=127) dominated by small teeth of C. angustidens from the Chandler Bridge Formation and some teeth from the underlying lower Oligocene (29-26.57 Ma) Ashley Formation (n=9). Correcting for tooth position, published body length estimation equations yielded body length estimates of 1.5-6.5 m for most individuals. Size-based assignment to age classes (neonates, juveniles, adults) is modified from the larger C. megalodon and scaled based on the largest available specimens of C. angustidens, reported herein. These assemblages are dominated by small individuals (juveniles and neonates) and include few adults. The Oligocene Charleston embayment therefore represents the first documented paleo-nursery area for C. angustidens.
... Furthermore, although all denticles bring up drag-reduction based on similar principles, the morphological features such as the depth, shape, direction, and density of the riblets vary depending on the shark species and even on the part of a shark. 25,26 To date, most works on the fabrication of shark skin inspired low-drag surfaces can be categorized into (1) a simplified structure with a rectangular or triangular line pattern, (2) direct replication of a biological shark skin, and (3) 3D printing of biological shark skins. 17,27−30 While these attempts have successfully proved the mechanism and effectiveness of the structure on drag reduction, they do not provide systematic structural control and high flexibility in morphological change. ...
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Sharks, marine creatures that swim fast and have an antifouling ability, possess dermal denticle structures of micrometer-size. Because the riblet geometries on the denticles reduce the shear stress by inducing the slip of fluid parallel to the stream-wise direction, shark skin has the distinguished features of low drag and antifouling. Although much attention has been given to low-drag surfaces inspired from shark skin, it remains an important challenge to accurately mimic denticle structures in the micrometer scale and to finely control their structural features. This paper presents a novel method to create shark skin-mimetic denticle structures for low drag by exploiting a photoreconfigurable azopolymer. The light-designed denticle structure exhibits superior hydrophobicity and an antifouling effect as sharks do. This work suggests that our novel photoreconfiguration technology, mimicking shark skin and systematically manipulating various structural parameters, can be used in a reliable manner for diverse applications requiring low-drag surfaces.
... To consider how a shark survives without a dorsal fin, we must first consider its function. Dorsal fin size tends to scale allometrically throughout ontogeny in carcharhinid sharks, implying that the fin's function is consistent over time (Irschick et al., 2017;Irschick & Hammerschlag, 2015). Studies of particle velocimetry in a bamboo shark Chiloscyllium plagiosum and spiny dogfish Squalus acanthias suggest that where the forward dorsal fin is located near the centre of mass, as it is in grey reef sharks, then its function is primarily one of stabilisation and the prevention of roll rather than thrust (Maia et al., 2017). ...
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An adult, female grey reef shark Carcharhinus amblyrhnchos was observed missing its first dorsal fin in 2014. The same individual was re‐photographed 4 years later indicating that this numerically dominant reef shark can survive total loss of its first dorsal fin. While this disability may impair the shark's ability to undertake pursuit predation, the species has a diversity of foraging modes that probably facilitates survival.
... This shark grouping (i.e., sandbar, lemon, blacknose, tiger and dusky sharks) is ecologically diverse, but are similar in gross morphology (Irschick et al., 2017). The effect of water temperatures at capture on body surface temperatures influenced most of the similarities observed among this group. ...
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Elasmobranchs (sharks and rays) exhibit a wide range of body forms adapted to various ecological niches. Body form differs not only between species, but between life stages of individual species as a result of ontogenetic allometry. In sharks, it has been proposed that these ontogenetic shifts in body form result from shifts in trophic and/or spatial ecology (the allometric niche shift hypothesis). Alternatively, it has been suggested that ontogenetic allometry may result from intrinsic morphological constraints associated with increasing body size, e.g. to counteract shifts in form-function relationships that occur as a function of size and could compromise locomotory performance. One major limitation affecting our understanding of ontogenetic scaling in sharks is that existing studies focus on postpartum ontogeny, ignoring the period of growth that occurs prior to birth/hatching. In this study, we report ontogenetic growth trajectories from 39 near-term brown smooth hound (Mustelus henlei) embryos taken from manually collected measurements. We found that unlike most other species and later ontogenetic stages of M. henlei, these embryos predominantly grow isometrically, and appear to display relatively high levels of morphological disparity. These results provide rudimentary support for the allometric niche shift hypothesis (as in the absence of ontogenetic niche shifts isometry dominates body-form scaling) and provide important insight into early shark ontogeny and morphological/developmental evolution.
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While the general diet of Mediterranean elasmobranchs has been widely studied, little is known about food partitioning and competition among sympatric species, despite these being important forces structuring marine communities. Using stomach content and stable isotope analyses, we investigated diet and trophic levels and evaluated the diet overlap and partitioning of Scyliorhinus canicula, Mustelus mustelus, and M. punctulatus in the northwestern Adriatic Sea. These shark species were confirmed as opportunistic mesopredators, but significant differences in their diets emerged. The two bentho-demersal Mustelus species had a larger trophic overlap with S. canicula than between each other. Given the pronounced morphological similarity of these two Mustelus species, this is likely a strategy to limit competition. The strictly benthic S. canicula showed a more varied diet compared to the other species. Stable isotope analysis highlighted that despite the smaller size and overlapping diets, S. canicula occupied a slightly higher trophic level. A better characterization of the trophic role of these species in the food web of the basin can be obtained from these data. At an ecosystem level, this information is essential to evaluate the possible consequences of the decline or recovery of the population of these exploited species.
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In nature, the hammerhead shark possesses a special rolled swimming motion to combat the effects of inborn negative buoyancy. Inspired by this natural mechanism, we developed a novel biomimetic robotic hammerhead shark to explore the distinctive rolled swimming motion mode. First, a scaled-down robotic prototype is constructed based on the morphological characteristics of the hammerhead shark. Second, the kinematics and dynamics of fish-like swimming are built, thereafter, model identification and validation are performed to improve the accuracy of the robotic model. Furthermore, the physical effects of the long dorsal fin and the rolling state on swimming performance are investigated indepth by numerically simulating the lift and drag forces over different fin surfaces and the dynamic torque of the body. Finally, extensive aquatic experiments demonstrate the remarkable improvements on locomotion performance and propulsive efficiency of the robotic hammerhead shark by the proposed rolled motion. The obtained results provide a new solution for the long voyage of high-load robotic fish system.
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Many pelagic shark species change body and fin shape isometrically or by positive allometry during ontogeny. But some large apex predators such as the white shark Carcharodon carcharias or the tiger shark Galeocerdo cuvier show distinct negative allometry, especially in traits related to feeding (head) or propulsion (caudal fin). In particular, changes in propulsion are attributed to a shift in swimming mode. The more heterocercal caudal fin of younger individuals with its large caudal fin span seemingly aids in hunting small, agile prey. In contrast, the less heterocercal caudal fin with a larger fin area in larger individuals aids a long-distance slow swimming mode. We were interested if negative allometric effects can be observed in a planktivorous shark, the basking shark Cetorhinus maximus, a large species adapted to long-distance slow swimming. To address this question, we compared three size classes, specifically < 260 cm (juveniles), 299–490 cm (subadults), and from adults > 541 cm total length. Comparing literature data, we found negative allometric growth of the head and of the caudal fin, but a more rapid decrease of relative caudal fin size than of relative head length. Hereby, we provide the first evidence for early negative allometric growth of the caudal fin in a large pelagic filter-feeding shark. Our study further demonstrates that ecomorphological approaches may add valuable insight into the life history of animals that are challenging to study in their natural habitat, including large roving sharks such as the basking shark.
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One of the defining characteristics of Elasmobranchii is the external gill slits, through which water is expelled during exhalation. The morphology of the gill slits is presumed to vary among elasmobranchs, between ram and suction ventilators, and between pelagic and benthic species. However, the evolution of gill slit morphology and the relationship to behavior and ecology is not clear. The relative length of each slit, relative distance between adjacent pairs of slits, and number of slits positioned over the pectoral fin were quantified in 289 preserved specimens representing 185 species in 69 genera, 28 families, and 8 orders of sharks. Six character states of relative gill slit length, five character states of relative inter-gill slit spacing, and five gill slit positions over the pectoral fin were identified. Character mapping onto a phylogeny reveals that the basal state for extant Selachii is to possess slits of similar relative length and spacing with no slits over the pectoral fin. Multiple character states in various combinations evolved within the Carcharhiniformes, which is the most speciose extant group. Ontogenetic differences in gill slit morphology were found in the 12 genera in which adult and juvenile data were collected. Slit spacing in juveniles has an equal tendency to become more similar or more different in adults, whereas slit length changes more often from more similar in juveniles to increasingly different in adults. Exploring the relationships among gill slit morphology and ventilation mode in modern sharks reveals character complexes that are common to four body types. Macropelagic types are ram ventilators with a longer first slit length, similar slit spacing in most, and no slits over the pectoral fin. Most littoral types are ram ventilators, with decreasing slit length and spacing and with one slit over the fin. Benthic types are suction ventilators with decreasing slit length and spacing and up to four slits over the fin. Bathic and micropelagic types are suction ventilators with no slits over the fin and most having similar slit length and spacing. Morphological differences in gill slit morphology may correspond to hydrodynamic differences in exhaled water from the slits by habitat.
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Animal condition typically reflects the accumulation of energy stores (e.g. fatty acids), which can influence an individual's decision to undertake challenging life-history events, such as migration and reproduction. Accordingly, researchers often use measures of animal body size and/or weight as an index of condition. However, values of condition, such as fatty acid levels, may not always reflect the physiological state of animals accurately. While the relationships between condition indi-ces and energy stores have been explored in some species (e.g. birds), they have yet to be examined in top predatory fishes, which often undertake extensive and energetically expensive migrations. We used an apex predatory shark (Galeocerdo cuvier, the tiger shark) as a model species to evaluate the relationship between triglycerides (energy metabolite) and a metric of overall body condition. We captured, blood sampled, measured and released 28 sharks (size range 125–303 cm pre-caudal length). In the laboratory, we assayed each plasma sample for triglyceride values. We detected a positive and significant relationship between condition and triglyceride values (P < 0.02). This result may have conservation implica-tions if the largest and highest-condition sharks are exploited in fisheries, because these individuals are likely to have the highest potential for successful reproduction. Our results suggest that researchers may use either plasma triglyceride val-ues or an appropriate measure of body condition for assessing health in large sharks. Cite as: Gallagher AJ, Wagner DN, Irschick DJ, Hammerschlag N (2014) Body condition predicts energy stores in apex predatory sharks. Conserv Physiol 2: doi:10.1093/conphys/cou022.
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Introduction Although it is widely recognized that sharks and other elasmobranchs often play a role in the transfer of energy between upper trophic levels within marine ecosystems, our understanding of the dynamics of prey consumption and processing of food in elasmobranchs remains rudimentary. To fully comprehend energy flow through elasmobranchs in marine communities it is necessary not only to know what they eat, but also to characterize the rates at which they ingest, digest, and process energy and nutrients contained in prey that is consumed. As with other areas of elasmobranch biology, investigations on dynamics of feeding and processing food lag behind such studies on other marine fishes and vertebrates. By far the most common elasmobranch feeding studies simply describe stomach contents of a particular species in a particular location. Rate of consumption, feeding patterns, and the fate of food once ingested have been examined for very few species of elasmobranchs.
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1. There has been considerable debate over the past decade with respect to wildlife provisioning, especially resultant behavioural changes that may impact the ecological function of an apex predator. The controversy is exemplified by the shark diving industry, where major criticisms based on inference, anecdote and opinion stem from concerns of potential behaviourally mediated ecosystem effects because of ecotourism provisioning (aka‘chumming’ or feeding). 2. There is a general lack of empirical evidence to refute or support associated claims. The few studies that have investigated the behavioural impacts of shark provisioning ecotourism have generated conflicting conclusions, where the confidence in such results may suffer from a narrow spatial and temporal focus given the highly mobile nature of these predators. There is need for studies that examine the potential behavioural consequences of provisioning over ecologically relevant spatial and temporal scales. 3. To advance this debate, we conducted the first satellite telemetry study and movement analysis to explicitly examine the long-range migrations and habitat utilization of tiger sharks (Galeocerdo cuvier) originating in the Bahamas and Florida, two areas that differ significantly with regards to the presence/absence of provisioning ecotourism. 4. Satellite telemetry data rejected the behaviourally mediated effects of provisioning ecotourism at large spatial and temporal scales. In contrast, to the restricted activity space and movement that were hypothesized, geolocation data evidenced previously unknown long-distance migrations and habitat use for both tiger shark populations closely associated with areas of high biological productivity in the Gulf Stream and subtropical western Atlantic Ocean. We speculate that these areas are likely critically important for G. cuvier feeding forays and parturition. 5. We concluded that, in the light of potential conservation and public awareness benefits of ecotourism provisioning, this practice should not be dismissed out of hand by managers. Given the pressing need for improved understanding of the functional ecology of apex predators relative to human disturbance, empirical studies of different species sensitivities to disturbance should be used to guide best-practice ecotourism policies that maximize conservation goals.
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Describes the models and methodology of scaling studies, though various examples (the -3/2 self-thinning law in plants; Cope's Law, that species within a lineage trend towards larger body size with evolutionary time; and scaling of home range and population density in mammals) suggest that they do not give much precision in their predictions for individuals or species. However, they do throw light on a lot of important issues. -S.J.Yates
Article
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Predators can impact ecosystems through trophic cascades such that differential patterns in habitat use can lead to spatiotemporal variation in top down forcing on community dynamics. Thus, improved understanding of predator movements is important for evaluating the potential ecosystem effects of their declines. We satellite-tagged an apex predator (bull sharks, Carcharhinus leucas) and a sympatric mesopredator (Atlantic tarpon, Megalops atlanticus) in southern Florida waters to describe their habitat use, abundance and movement patterns. We asked four questions: (1) How do the seasonal abundance patterns of bull sharks and tarpon compare? (2) How do the movement patterns of bull sharks and tarpon compare, and what proportion of time do their respective primary ranges overlap? (3) Do tarpon movement patterns (e.g., straight versus convoluted paths) and/or their rates of movement (ROM) differ in areas of low versus high bull shark abundance? and (4) Can any general conclusions be reached concerning whether tarpon may mitigate risk of predation by sharks when they are in areas of high bull shark abundance? Despite similarities in diet, bull sharks and tarpon showed little overlap in habitat use. Bull shark abundance was high year-round, but peaked in winter; while tarpon abundance and fishery catches were highest in late spring. However, presence of the largest sharks (>230 cm) coincided with peak tarpon abundance. When moving over deep open waters (areas of high shark abundance and high food availability) tarpon maintained relatively high ROM in directed lines until reaching shallow structurally-complex areas. At such locations, tarpon exhibited slow tortuous movements over relatively long time periods indicative of foraging. Tarpon periodically concentrated up rivers, where tracked bull sharks were absent. We propose that tarpon trade-off energetic costs of both food assimilation and osmoregulation to reduce predation risk by bull sharks.
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The nurse shark is an extremely abundant shallow water species in Florida and the Caribbean, yet its biology is poorly known. Moreover, there is a great deal of misinformation about it in the literature. The maximum size and weight attained by the nurse shark have often been exaggerated. None of the specimens measured in this study exceeded 265cm TL and 114.5kg, and none of the specimens actually measured by other researchers exceeded 280cm. Females reach maturity at a length of 223–231cm, or at 86% of their maximum size. Males reach maturity between 214 and 214.6cm in length or at about 83% of their maximum size. Mating primarily occurs from mid-June to early July. The embryos are enclosed in sturdy egg capsules for the first 12–14 weeks of gestation. In a gravid female, the embryos are at different stages of development during the first four months of gestation. Embryos are lecithotrophic and there is no evidence of any supplemental mode of embryonic nourishment. Embryos measure 28–30.5cm at birth. The gestation period is estimated at about five to six months. Brood sizes are large, ranging from 21 to 50 young, with a median of 34 young. The reproductive cycle of the nurse shark consists of a five to six-month gestation period and a two-year ovarian cycle. Thus, the reproductive cycle is biennial and a female produces a brood every two years. The nurse shark is an opportunistic predator that consumes a wide range of small fishes, primarily grunts (Haemulidae).
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We examined a population of blacktip sharks, Carcharhinus limbatus, within a coastal nursery area to define how individuals use the nursery habitat throughout the summer. We used a series of acoustic hydrophones to passively monitor the movement patterns of sharks for periods up to 167 days. We used passive monitoring data to calculate home range estimates using minimum convex polygon (MCP) and fixed kernel estimators. MCP calculated the extent of an individual's range. Kernel estimates provided information regarding the utilization of space within the home range including core area (50% kernel) and larger excursions outside the core area (95% kernel). Individuals within the nursery area typically used a consistently small core area. All sharks monitored in the study site underwent a home range expansion during the month of July, suggesting a synchronous population-level change in habitat use. This change in habitat use was reflected in all home range calculations. Passive monitoring revealed that young sharks remain within the nursery area for up to 6 months. The long-term use of this nursery area reflects its critical importance to young blacktip sharks.
Chapter
Scaling relationships have been a persistent theme in biology at least since the time of Leonardo da Vinci and Galileo. Because scaling relationships are among the most general empirical patterns in biology, they have stimulated research to develop mechanistic hypotheses and mathematical models. While there have been many excellent empirical and theoretical investigations, there has been little attempt to synthesize this diverse but interrelated area of biology. In an effort to fill this void, Scaling in Biology, the first general treatment of scaling in biology in over 15 years, covers a broad spectrum of the most relevant topics in a series of chapters written by experts in the field. Some of those topics discussed include allometry and fractal structure, branching of vascular systems of mammals and plants, biomechanical and life history of plants, invertebrates and vertebrates, and species-area patterns of biological diversity. Many more examples are included within this text to complete the broader picture. Scaling in Biology conveys the diversity, promise, and excitement of current research in this area, in a format accessible to a wide audience of not only specialists in the various sub-disciplines, but also students and anyone with a serious interest in biology.
Book
Provides complete, integrated coverage of nearly all modes of animal movement, emphasizing general principles in a clear and straightforward fashion. Incorporates background material on the biomechanics and physiology of nervous, skeletal, and muscular components. Adopts a renewed evolutionary approach to locomotion and includes new coverage of non-vertebrate animal locomotor systems. Animals have evolved remarkable biomechanical and physiological systems that enable their rich repertoire of motion. Animal Locomotion offers a fundamental understanding of animal movement through a broad comparative and integrative approach, including basic mathematics and physics, examination of new and enduring literature, consideration of classic and cutting-edge methods, and a strong emphasis on the core concepts that consistently ground the dizzying array of animal movements. Across scales and environments, this book integrates the biomechanics of animal movement with the physiology of animal energetics and the neural control of locomotion.
Article
A new account of the mechanical action of the heterocercal tail in fishes is given, developed from study of the swimming of sharks. The heterocercal tail is capable of delivering a thrust that can be oriented in a wide range of angles in the vertical plane. The orientation results from a balance of forces acting in the tail. Normally the heterocercal tail delivers neither an epibatic nor a hypobatic thrust, but rather one directly through the center of gravity of the fish. On the basis of the new account, predictions concerning the shape and proportions of the heterocercal tail are made and tested.
Conference Paper
One of the most prominent characteristics of early vertebrates is the elongate caudal fin bearing fin rays. The caudal fin represents a fundamental design feature of vertebrates that predates the origin of jaws and is found in both agnathans and gnathostomes. The caudal fin also represents the most posterior region of the vertebrate axis and is the location where fluid, accelerated by movement of the body anteriorly, is shed into the surrounding medium. Despite the extensive fossil record of the caudal fin, the use of caudal characters for systematic studies, and the importance of tail function for understanding locomotor dynamics in fishes, few experimental studies have been undertaken of caudal fin function. In this paper I review two experimental approaches which promise to provide new insights into the function and evolution of the caudal fin: three-dimensional kinematic analysis, and quantitative flow measurements in the wake of freely-swimming fishes using digital particle image velocimetry (DPIV). These methods are then applied to the function of the caudal fin during steady swimming in fishes with heterocercal and homocercal morphologies: chondrichthyians (leopard sharks) and ray-fined fishes (sturgeon and bluegill sunfish). The caudal fin of leopard sharks functions in a manner consistent with the classical model of heterocercal tail function in which the caudal surface moves at an acute angle to the horizontal plane, and hence is expected to generate lift forces and torques which must be counteracted anteriorly by the body and pectoral fins. An alternative model in which the shark tail produces a reactive force that acts through the center of mass Is not supported. The sturgeon heterocercal tail is extremely flexible and the upper tail lobe trails the lower during the fin beat cycle. The sturgeon tail does not function according to the classical model of the heterocercal tail, and is hypothesized to generate reactive forces oriented near the center of mass of the body which is tilted at an angle to the flow during steady locomotion. Functional analysis of the homocercal tail of bluegill shows that the dorsal and ventral lobes do not function symmetrically as expected. Rather, the dorsal lobe undergoes greater lateral excursions and moves at higher velocities than the ventral lobe. The surface of the dorsal lobe also achieves a significantly acute angle to the horizontal plane suggesting that the homocercal tail of bluegill generates lift during steady swimming. These movements are actively generated by the hypochordal longitudinalis muscle within the tail. This result, combined with DPIV flow visualization data, suggest a new hypothesis for the function of the homocercal tail: the homocercal tall generates tilted and linked vortex rings with a central jet inclined postero-ventrally, producing an anterodorsal reactive force on the body which generates lift and torque in the manner expected of a heterocercal tail. These results show that the application of new techniques to the study of caudal fin function in fishes reveals a previously unknown diversity of homocercal and heterocercal tail function, and that morphological characterizations of caudal fins do not accurately reflect in vivo function.
Article
How morphology changes with size can have profound effects on the life history and ecology of an animal. For apex predators that can impact higher level ecosystem processes, such changes may have consequences for other species. Tiger sharks (Galeocerdo cuvier) are an apex predator in tropical seas, and, as adults, are highly migratory. However, little is known about ontogenetic changes in their body form, especially in relation to two aspects of shape that influence locomotion (caudal fin) and feeding (head shape). We captured digital images of the heads and caudal fins of live tiger sharks from Southern Florida and the Bahamas ranging in body size (hence age), and quantified shape of each using elliptical Fourier analysis. This revealed changes in the shape of the head and caudal fin of tiger sharks across ontogeny. Smaller juvenile tiger sharks show an asymmetrical tail with the dorsal (upper) lobe being substantially larger than the ventral (lower) lobe, and transition to more symmetrical tail in larger adults, although the upper lobe remains relatively larger in adults. The heads of juvenile tiger sharks are more conical, which transition to relatively broader heads over ontogeny. We interpret these changes as a result of two ecological transitions. First, adult tiger sharks can undertake extensive migrations and a more symmetrical tail could be more efficient for swimming longer distances, although we did not test this possibility. Second, adult tiger sharks expand their diet to consume larger and more diverse prey with age (turtles, mammals, and elasmobranchs), which requires substantially greater bite area and force to process. In contrast, juvenile tiger sharks consume smaller prey, such as fishes, crustaceans, and invertebrates. Our data reveal significant morphological shifts in an apex predator, which could have effects for other species that tiger sharks consume and interact with. J. Morphol., 2016. © 2016 Wiley Periodicals, Inc.
Article
An adaptationist programme has dominated evolutionary thought in England and the United States during the past 40 years. It is based on faith in the power of natural selection as an optimizing agent. It proceeds by breaking an oragnism into unitary 'traits' and proposing an adaptive story for each considered separately. Trade-offs among competing selective demands exert the only brake upon perfection; non-optimality is thereby rendered as a result of adaptation as well. We criticize this approach and attempt to reassert a competing notion (long popular in continental Europe) that organisms must be analysed as integrated wholes, with Baupläne so constrained by phyletic heritage, pathways of development and general architecture that the constraints themselves become more interesting and more important in delimiting pathways of change than the selective force that may mediate change when it occurs. We fault the adaptationist programme for its failure to distinguish current utility from reasons for origin (male tyrannosaurs may have used their diminutive front legs to titillate female partners, but this will not explain why they got so small); for its unwillingness to consider alternatives to adaptive stories; for its reliance upon plausibility alone as a criterion for accepting speculative tales; and for its failure to consider adequately such competing themes as random fixation of alleles, production of non-adaptive structures by developmental correlation with selected features (allometry, pleiotropy, material compensation, mechanically forced correlation), the separability of adaptation and selection, multiple adaptive peaks, and current utility as an epiphenomenon of non-adaptive structures. We support Darwin's own pluralistic approach to identifying the agents of evolutionary change.
Article
Dorsal fins of actinopterygian fishes are known to function to varying degrees as stabilizers and as propulsive elements that augment thrust from the caudal fin. However, little is known about the ability of elasmobranchs to control three-dimensional conformation of the dorsal fins during swimming, which may alter the force balance during locomotion. In this study, dorsal fin function was investigated in spiny dogfish, Squalus acanthias, swimming steadily at 0.5 and 0.75 BL s-1, using three-dimensional kinematics and electromyography. Points on the dorsal and caudal fins were tracked in dorsal and lateral views with dual high-speed video at 125 f s-1. Electrodes were implanted in three points along each dorsal fin muscle and in the adjacent red epaxial muscle. Conformational changes were detected in both dorsal fins at both speeds. Speed was found to influence lateral displacement of the first dorsal fin relative to trunk undulation, with larger magnitudes at 0.5 BL s-1. The first dorsal fin oscillates at a different phase lag than predicted by position on the body, while the second dorsal fin moves in synchrony with the axial musculature. Muscles of the first dorsal fin show synchronous bilateral activation, while there is no clear pattern in the second dorsal fin. This study provides evidence that spiny dogfish control movements of the first dorsal fin during steady swimming to stabilize body position. In contrast, the second dorsal fin appears to be capable of thrust generation. Thus, there is a dual dorsal fin function in spiny dogfish during steady swimming.
Article
Body form can change across ontogeny, and can influence how animals of different sizes move and feed. Scaling data on live apex predatory sharks are rare and, therefore, we examined patterns of scaling in ontogenetic series of four sympatric shark species exhibiting a range of sizes, ecologies and life histories (tiger, bull, blacktip, and nurse shark). We evaluated 13 linear morphological variables and two areas (caudal and dorsal) that could influence both animal condition and locomotor performance. These measurements included dimensions of the dorsal, pectoral, and caudal fins, as well as several dimensions of body circumference, and of the head. For all four species, the body axis (eye-to-eye, lateral span, frontal span, proximal span) scaled close to isometry (expected slope of 1.0). The two largest sharks (tiger and bull sharks) also showed significant negative allometry for elements of the caudal fin. We found significant negative allometry in the lengths of the upper lobe of the caudal fin (caudal fin 1) and the overall height of the caudal fin (caudal fin 2) in tiger and bull sharks, with slopes ranging from about 0.60 to 0.73. Further, tiger sharks showed negative allometry in caudal fin area. These results suggest that in terms of overall body dimensions, small sharks are roughly geometrically similar to large sharks, at least within the species we examined. However, juvenile tiger (and to a lesser extent bull sharks) are notable in having proportionately larger caudal fins compared to adult sharks. As the caudal fin contributes to generating thrust during forward locomotion, this scaling implies differences among adult and juvenile sharks in locomotor ability. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, ●●, ●●–●●.
Article
Stomach contents from tiger sharks, Galeocerdo cuvier, caught on lines off the central coast of Western Australia were analysed to investigate variations in the diet due to sex, size and geographic location. Stomachs from 84 specimens contained food, while 26 had empty stomachs and 66 had regurgitated. Twelve prey groups were identified, the most common being turtles, sea snakes, teleost fishes, dugongs and sea birds. Dietary overlap was high between males and females. An ontogenetic shift was observed in the diet. Smaller prey (e.g. cephalopods, teleosts and sea snakes) were more common in small individuals, while the occurrence of larger prey (e.g. turtles, dugongs and elasmobranchs) increased with increasing shark size. Differences in the diet were observed between four regions along the central Western Australian coast. The ability to catch and consume large prey, prey availability, prey density, and prey profitability were identified as factors influencing the diet. The high level of occurrence of dugongs and turtles in the diet of G. cuvier, relative to their abundance, suggests that shark predation may play an important role in regulating populations of these species.
Book
This book is about the importance of animal size. We tend to think of animal function in chemical terms and talk of water, salts, proteins, enzymes, oxygen, energy, and so on. We should not forget, however, that physical laws are equally important, for they determine rates of diffusion and heat transfer, transfer of force and momentum, the strength of structures, the dynamics of locomotion, and other aspects of the functioning of animal bodies. Physical laws provide possibilities and opportunities for an organism, yet they also impose constraints, setting limits to what is physically possible. This book aims to give an understanding of these rules because of their profound implications when we deal with animals of widely different size and scale. The reader will find that the book raises many questions. Remarkable and puzzling information makes it read a little like a detective story, but the last chapter, instead of giving the final solution, neither answers all questions nor provides one great unifying principle.
Article
1. Terrestrial ectotherms are likely to face increased periods of heat stress as mean temperatures and temperature variability increase over the next few decades. Here, we consider the extent to which changes in upper thermal limits, through plasticity or evolution, might be constrained, and we survey insect and reptile data to identify groups likely to be particularly susceptible to thermal stress. 2. Plastic changes increase thermal limits in many terrestrial ectotherms, but tend to have less effect on upper limits than lower limits. 3. Although comparisons across insect species have normally not taken into account the potential for plastic responses, mid-latitude species seem most prone to experience heat stress now and into the future, consistent with data from lizards and other groups. 4. Evolutionary adaptive potential has only been measured for some species; there is likely to be genetic variation for heat responses in populations, but selection and heritability experiments suggest that upper thermal limits may not increase much. 5. Although related species can differ by several degrees in their upper thermal limits, there is strong phylogenetic signal for upper limits. If these reflect evolutionary constraints, substantial molecular changes may be required to increase upper thermal limits. 6. Findings point to many terrestrial ectotherms having a limited potential to change their thermal limits particularly within the context of an average predicted temperature increase of 2–4 °C for mid-latitude populations over the next few decades.
Article
Top oceanic predators, especially large predatory sharks (TOPS), appear to be experiencing varying degrees of population declines. Life history data (e.g. diet, reproductive status, age and growth, mortality) are critical for developing effective conservation strategies for TOPS. Presently, lethal sampling remains the most effective and accurate means of gathering these data. To meet such challenges, many scientists have utilized specimens obtained from recreational and commercial fisheries, but have needed to supplement those data with fishery-independent sampling. However, there is growing public and scientific debate as to whether lethal sampling of TOPS is justified for obtaining conservation data. Here we describe the development and use of non-lethal alternatives for collecting data on (1) trophodynamics; (2) maturity state and fecundity; and (3) growth and mortality rates necessary to enact conservation measures for threatened or even data-deficient TOPS.
Article
Stability and procured instability characterize two opposing types of swimming, steady and maneuvering, respectively. Fins can be used to manipulate flow to adjust stability during swimming maneuvers either actively using muscle control or passively by structural control. The function of the dorsal fins during turning maneuvering in two shark species with different swimming modes is investigated here using musculoskeletal anatomy and muscle function. White-spotted bamboo sharks are a benthic species that inhabits complex reef habitats and thus have high requirements for maneuverability. Spiny dogfish occupy a variety of coastal and continental shelf habitats and spend relatively more time cruising in open water. These species differ in dorsal fin morphology and fin position along the body. Bamboo sharks have a larger second dorsal fin area and proportionally more muscle insertion into both dorsal fins. The basal and radial pterygiophores are plate-like structures in spiny dogfish and are nearly indistinguishable from one another. In contrast, bamboo sharks lack basal pterygiophores, while the radial pterygiophores form two rows of elongated rectangular elements that articulate with one another. The dorsal fin muscles are composed of a large muscle mass that extends over the ceratotrichia overlying the radials in spiny dogfish. However, in bamboo sharks, the muscle mass is divided into multiple distinct muscles that insert onto the ceratotrichia. During turning maneuvers, the dorsal fin muscles are active in both species with no differences in onset between fin sides. Spiny dogfish have longer burst durations on the outer fin side, which is consistent with opposing resistance to the medium. In bamboo sharks, bilateral activation of the dorsal in muscles could also be stiffening the fin throughout the turn. Thus, dogfish sharks passively stiffen the dorsal fin structurally and functionally, while bamboo sharks have more flexible dorsal fins, which result from a steady swimming trade off. J. Morphol., 2013. © 2013 Wiley Periodicals, Inc.
Article
A revised interpretation of the mode of action of the heterocercal tail in sharks shows that the upturned tail axis tends to produce a thrust directed downwards behind the centre of balance of the fish and thus gives a moment turning the head upwards. This is countered in two ways—by the rotation of the tail along its longitudinal axis during each lateral beat, and through the action of the ventral hypochordal lobe. The shape of the tail and the mode of action of the tail in all sharks so far considered reflects a balance between these three factors, in all of them the net effect being the production of a forward thrust from the tail that passes directly through the centre of balance of the fiish. There is normally therefore no tendency for the fish to turn around the centre of balance in a sagittal plane but there is a net sinking effect that is countered by the planning effect of the pectoral fins and the ventral surface of the head. A study of 56 species of sharks shows that the tail is constructed according to a remarkably consistent common plan, the extremes being the high angled rather symmetrical tail of pelagic sharks such as hums, Lamna and Rhincodon and the straight tails of benthic sharks such as Ginglymostoma in which a ventral hypochordal lobe is absent. When the general body shape of sharks, including the position of insertion of the median and paired fins and the pattern of growth of fin surface areas is considered, the uniformity of the shark body plan and locomolor function is further emphasised. Four patterns of body form in sharks are recognised: 1) The fast swimming pelagic sharks and the whale sharks have a tail with a high aspect ratio, a conical head, a lateral fluke on the caudal peduncle. 2) The generalised sharks typified by the Carcharhinidae, have lower heterocercal angles, a flattened ventral surface on the head and lack the caudal fluke. 3) The demersal sharks typified by the catsharks (Scyliorhinidae) have a very low, almost straight tail. The ventral hypochordal lobe is absent and the first dorsal fin is posterior in position. 4) The squalomorph sharks are distinct in the absence of the anal fin, presence of a marked epicaudal lobe in the tail and often an elevated insertion of the pectorals. The anal and second dorsal fins are always the smallest fins and the pectorals grow at the fastest rate. In general there is an inverse relationship between size and rale of growth of all fins and the ventral surface of the head. In hammerheads the growth data confirms that the head has a significant planing action in swimming. The pectoral, second dorsal and anal fins show an extreme constancy of position of insertion in all sharks studied. The locomotor mechanism of sharks is adapted for an efficient cruising swimming but at the same time, the potential instability in the sagittal plan allows for the production of turning moments that are used in attack and feeding.
Book
Both a landmark text and reference book, Steven Vogel's "Life in Moving Fluids" has also played a catalytic role in research involving the applications of fluid mechanics to biology. In this revised edition, Vogel continues to combine humor and clear explanations as he addresses biologists and general readers interested in biological fluid mechanics, offering updates on the field over the last dozen years and expanding the coverage of the biological literature. His discussion of the relationship between fluid flow and biological design now includes sections on jet propulsion, biological pumps, swimming, blood flow, and surface waves, and on acceleration reaction and Murray's law. This edition contains an extensive bibliography for readers interested in designing their own experiments.
Article
We implement a landmark-free method for the quantification of shape variation in evolutionary biology. The method is referred to as ‘geodesic distance analysis’ (GDA). It represents shapes as continuous boundaries that are invariant to translation, rotation and uniform scaling. Boundaries are represented as functions in an infinite-dimensional nonlinear space. The arc length of the curve is used as a parameter to model the shape. The angle between the tangent vector to the curve and the positive x axis is defined as a function of the arc length. In this way, shapes are compared and represented using their angle functions. Shape differences are calculated by finding geodesics (or shortest paths) between shapes of curves on the shape space. GDA offers an environment for the performance of shape statistics that are intrinsic to the space of planar continuous curves. The method becomes especially appropriate when the objects of study are continuous curves for which landmarks cannot be identified with certainty. We present applications of this method to the analysis of organismal morphology. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 104, 217–233.
Article
Sharks are marine consumers believed to occupy top positions in marine food webs. But surprisingly, trophic level estimates for these predators are almost non-existent. With the hope of helping better define the ecological role of sharks in marine communities, this paper presents standardized diet compositions and trophic levels calculated for a suite of species. Dietary composition for each species was derived from published quantitative studies using a weighted average index that takes into account sample size in each study. The trophic level (TL) values of the 11 food types used to characterize the diet (obtained from published accounts) were then used to calculate fractional trophic levels for 149 species representing eight orders and 23 families. Sharks as a group are tertiary consumers (TL>4), and significant differences were found among the six orders compared, which were attributable to differences between orectolobiforms (TL<4) and all other orders, and between hexanchiforms and both carcharhiniforms and squatiniforms. Among four families of carcharhiniform sharks, carcharhinids (TL=4.1, n=39) had a significantly higher TL than triakids (TL=3.8, n=19) and scyliorhinids (TL=3.9, n=21), but not sphyrnids (TL=3.9, n=6). When compared to trophic levels for other top predators of marine communities obtained from the literature, mean TL for sharks was significantly higher than for seabirds (n=28), but not for marine mammals (n=97). Trophic level and body size were positively correlated (r s =0.33), with the fit increasing (r s =0.41) when the three predominantly zooplanktivorous sharks were omitted, and especially when considering only carcharhinid sharks (r s =0.55).
Article
Bamboo sharks (Chiloscyllium plagiosum) are primarily benthic and use their relatively flexible pectoral and pelvic fins to rest on and move about the substrate. We examined the morphology of the pectoral fins and investigated their locomotory function to determine if pectoral fin function during both benthic station-holding and pelagic swimming differs from fin function described previously in leopard sharks, Triakis semifasciata. We used three-dimensional kinematics and digital particle image velocimetry (DPIV) to quantify pectoral fin function in five white-spotted bamboo sharks, C. plagiosum, during four behaviors: holding station on the substrate, steady horizontal swimming, and rising and sinking during swimming. During benthic station-holding in current flow, bamboo sharks decrease body angle and adjust pectoral fin angle to shed a clockwise fluid vortex. This vortex generates negative lift more than eight times that produced during open water vertical maneuvering and also results in an upstream flow that pushes against the posterior surface of the pectoral fin to oppose drag. In contrast, there is no evidence of significant lift force in the wake of the pectoral fin during steady horizontal swimming. The pectoral fin is held concave downward and at a negative dihedral angle during steady horizontal swimming, promoting maneuverability rather than stability, although this negative dihedral angle is much less than that observed previously in sturgeon and leopard sharks. During sinking, the pectoral fins are held concave upward and shed a clockwise vortex with a negative lift force, while in rising the pectoral fin is held concave downward and sheds a counterclockwise vortex with a positive lift force. Bamboo sharks appear to sacrifice maneuverability for stability when locomoting in the water column and use their relatively flexible fins to generate strong negative lift forces when holding position on the substrate and to enhance stability when swimming in the water column. J. Morphol. 249:195–209, 2001. © 2001 Wiley-Liss, Inc.
Article
Tiger sharks, Galeocerdo cuvier, are apex predators in a variety of nearshore ecosystems throughout the world. This study investigates the biology of tiger sharks in the shallow seagrass ecosystem of Shark Bay, Western Australia. Tiger sharks (n = 252) were the most commonly caught species (94%) compared to other large sharks. Tiger sharks ranged from 148–407cm TL. The overall sex ratio was biased towards females (1.8:1), but the sex ratio of mature animals (> 300cm TL) did not differ from 1:1. Contrary to previous accounts, tiger sharks were caught more often in all habitats during daylight hours than at night. Tiger shark catch rates were highly correlated with water temperature and were highest when water temperatures were above 19C. The seasonal abundance of tiger sharks is correlated to both water temperature and the occurrence of their main prey: sea snakes and dugongs, Dugong dugon. Stomach contents analysis indicated that sea turtles and smaller elasmobranchs were also common prey. The importance of major seagrass grazers (dugongs and green sea turtles, Chelonia mydas) in the diet of tiger sharks suggests the possibility that these sharks are keystone predators in this ecosystem.
Article
Comparative studies of the relationship between 2 phenotypes, or between a phenotype and an environment, are frequently carried out by invalid statistical methods. Most regression, correlation, and contingency table methods, including nonparametric methods, assume that the points are drawn independently from a common distribution. When species are taken from a branching phylogeny, they are manifestly nonindependent. Use of a statistical method that assumes independence will cause overstatement of the significance in hypothesis tests. Some illustrative examples of these phenomena are given, and limitations of previous proposals of ways to correct for the nonindependence discussed. A method of correcting for the phylogeny is proposed. It requires that we know both the tree topology and the branch lengths, and that we be willing to allow the characters to be modeled by Brownian motion on a linear scale. Given these conditions, the phylogeny specifies a set of contrasts among species, contrasts that are statistically independent and can be used in regression or correlation studies. -from Author