Article

Diet of spiny lobsters from Mahé Island reefs, Seychelles inferred by trophic tracers

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Abstract

Spiny lobsters (Panulirus longipes, P. penicillatus and P. versicolor) are an important resource in Seychelles, where they inhabit coastal carbonate and granite reefs that have been impacted by multiple coral bleaching events over the past two decades. Little is known about their biology and ecology in this region. Interspecific competition for food resources was previously suggested, but no quantitative data on the diets of spiny lobsters were available. Using carbon and nitrogen stable isotope compositions and fatty acid profiles of three spiny lobster species and their potential prey, a Bayesian mixing model for diet estimation was applied to compare the diet proportions of spiny lobsters among species and between reef types (carbonate and granite reefs). Model outputs suggested the three lobster species consume mainly crustaceans (Anomoura hermit crabs; half of the diet), then Echinoidea (sea urchins), algae and molluscs. P. versicolor was found to consume slightly more molluscs and algae than the two other studied species, which was consistent with its lower trophic level (2.4 vs 2.8 for the two other species). Trophic level did not increase with carapace length of spiny lobsters, but large individuals had higher carbon isotopic values suggesting that they might feed closer to the coast or more on detritus feeders than their smaller congeners. Diets of spiny lobsters were fairly similar between carbonate and granite reefs, except that lobster inhabiting granite reefs consumed more sea urchins. While our overall findings were consistent with gut contents of Panulirus spp. from other world regions, they should be confirmed, as the discrimination of several prey based on trophic tracers was low, which increased mixing model uncertainty.

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... As the biochemical composition of consumers largely reflects that of their prey (Iverson 2009), similar FA and SI trophic niches characterise similar dietary patterns. Our study found high habitat use and dietary similarities between the three Seychelles spiny lobster species, consistently with previous reported findings (Sardenne et al., 2021). The δ 13 C value ranges and FA profiles of the three species were distinctive of benthic consumers (high in 20:4n-6 with low-to-moderate levels of 20:5n-3 and 22:6n-3). ...
Article
Spiny lobsters (P. penicillatus, P. longipes and P. versicolor) are heavily dependent on habitats like coral reefs, known to be highly vulnerable to climate change-driven degradation. Yet, little is known about their trophic ecology and their adaptive capacity to a changing environment. In this study, we used fatty acids (FA) analysed in the hepatopancreas and δ13C and δ15N stable isotopes analysed in the tail muscle of three spiny lobster species from the Seychelles coastal waters to (1) infer habitat use, dietary patterns and potential for resource competition and (2) investigate the effects of reef type and coral bleaching on their trophic niche metrics. We found that there was a potential for interspecific competition between the three species, shown by their high dietary overlap (mean FA niche overlap ranging from 71.2% to 99.5% for P. longipes and P. versicolor in P. penicillatus) and similar habitat use (δ13C value ranges). P. penicillatus, the largest of the three species, was more a generalist than the two other species (i.e., had a larger FA niche) and P. versicolor seemed to feed on smaller/earlier life stage prey than P. longipes (based on differences in δ15N values). The potential for resource competition of Seychelles spiny lobsters appeared higher in granite than carbonate reefs, and in post-2016 coral bleaching reefs. Our results suggest that P. penicillatus could have a greater adaptive capacity to climate change due to its higher dietary plasticity and that competition between Seychelles spiny lobsters may increase in the future as the frequency and severity of bleaching events is predicted to increase with climate change.
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Persistence of subpopulations dispersed among spatial management units, such as marine reserves, relies heavily upon the quality of local habitat, as well as size and connectivity of habitat patches. We examined the relative abundance of the red rock lobster Jasus edwardsii and its primary bivalve food resources within 3 marine reserves in the Doubtful-Bradshaw Sound complex in Fiordland, New Zealand. We used stable isotope analysis (delta C-13, delta N-15 and delta S-34) to resolve differences in the carbon sources to food webs supporting J. edwardsii inhabiting these regions, Furthermore, we examined patterns in the relative concentration and delta C-13 signature of specific fatty acid biomarkers as an independent test of the carbon Sources to each population. We found distinctive patterns in the relative abundance of red rock lobsters, with significantly more animals in the marine reserves at Te Awaatu Channel and Kutu Parera than in the surrounding open fishing areas and within the reserve at Taipari Roa. Taipari Roa Reserve is distinctive in that bivalve abundance is extremely low due to freshwater input from the Manapouri hydroelectric power plant. Analysis of delta C-13, delta N-15 and delta S-34 of red rock lobster muscle tissue as well as delta C-13 of 16: 1 omega 7 and 18: 1 omega 7 indicated that, in those areas where heterotrophic bivalves are rare, red rock lobsters rely more on recycled carbon made available by chemoautotrophs. These findings suggest that efficacy of the new marine reserves is influenced by habitat quality in terms of the availability and abundance of food resources for red rock lobsters. We highlight the importance of considering habitat quality for effective implementation of marine reserves.
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Tides are mixed semidiurnal, with a maximum range of 1.8 m. Sites exposed to the dominant S. E. Trades experience the highest degree of wave action. Fringing coral reefs are developed around the island, on the eastern side they are generally wide and more or less continuous, but on the western side they are confined to narrow bay reefs. Submarine topography, wind direction and strength have influenced the development and resultant shape of the reefs. The most obvious feature of the flora and fauna occurring on and in the fringing reef is that there is a zonation parallel to the shore. This zonation may be related to that occurring upon rocky intertidal shores throughout the world: most of the reef lies in the sublittoral zone but with projections upwards into the eulittoral and littoral fringe zones. The upper limit of abundant coral corresponds with the appearance of Laminaria in temperate regions. A number of distinct communities of organisms can be recognized forming and inhabiting these zones. The area above highwater mark but with maritime influence is the supralittoral zone and around Mahé is colonized by salt-resistant plants such as Ipomoea, Scaevola, Cocos and Calophyllum . In creeks and sheltered areas mangrove patches occur, commonly Rhizophora and Avicennia and inhabited by many crabs such as Uca, Cardisoma and Scylla , and the gastropod Terebralia . Where the shores are rocky, either granite or beachrock they are colonized by a typical rocky shore biota; in the littoral fringe blue green algae and Littorina are common, below in the eulittoral zone green algae, barnacles, limpets, several species of Nerita, Isognomon and oysters occur. The area between the tide marks on sandy beaches is inhabited by the crabs Ocypode and Coenobita and the burrowing bivalves Donax and Atactodea . On the reef flat proper, communities are almost completely sublittoral and only partly emersed at lowest spring tides. In sheltered areas around Port Victoria, where the once extensive mangrove swamps have now been cleared, the remaining sediment is blackened, has a high organic content and smells of hydrogen sulphide; the overlying water is turbid with generally reduced salinities. The area is inhabited by a large population of ‘fiddler’ crabs ( Uca ), green algae and the burrowing bivalves Gafrarium, Quidnipagus and Psammotea . On reef flats which are wide enough beds of marine angiosperms are developed which extend up to 300 m seawards from the base of the beach. They are mainly formed by the genera Thalassia, Cymodocea and Syringodium which by means of rhizome and root systems accumulate and fix sediment. These ‘grass’ beds support a large population of holothurians and burrowing bivalves, principally Codakia, Quidnipagus, Tellinella and Gafrarium . On windward reefs immediately seawards of the grass beds are open sandy areas which interdigitate with lower ridges of loosely bound calcareous algal cobbles and coral fragments arranged at right angles to the algal ridge with which they are in continuity. Brown algae such as Sargassum and Turbinaria are abundant and attached to hard substrates; Halimeda is common in the sandy areas. A great number of species of burrowing gastropods and bivalves inhabit the sandy areas, the fauna on the cobble ridges is very similar to that occurring on the algal ridge. The algal ridge is well developed only on windward reefs, it consists of a low ridge at the seaward edge of the reef flat made up of a cavernous growth of calcareous red algae such as Lithophyllum and Porolithon . This substrate is clothed in Sargassum, Turbinaria and many other algae. The associated fauna shows an extraordinary diversity with many species of gastropods, particularly Cypraea, Conus, Triphora, Turbo and Trochus . Many crabs, echinoids, ascidians, sponges and cemented foraminifera are present. The extreme seaward edges of the reef flat, the reef front and the reef edge are the only environments where corals are the dominant organism in the community. The reef edge marks the seaward extent of the reef flat and slopes shallowly seawards from the algal ridge to the change in slope at the reef front and is the area where the waves break. The reef front is generally steep but around Mahé is only about 20 m high, descending to the shallow base platform outside the reefs. Two distinct reef edge and front communities can be recognized, the Acropora-Millepora-Stylophora community characteristic of windward edges with good water circulation and dominated by branching corals, and the Porites-Favia-Leptoria community is characteristic of reef edges in sheltered areas with massive, rounded, growth forms. 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1. The use of stable isotopic techniques to study animal diets and trophic levels requires a priori estimates of discrimination factors (Δ 13 C and Δ 15 N, also called fractionation factors), which are the differences in isotopic composition between an animal and its diet. Previous studies have shown that these parameters depend on several sources of variation (e.g. taxon, environment, tissue) but diet as a source of variation still needs assessment. 2. We conducted an extensive review of the literature (66 publications) concerning estimates of animal-diet Δ 13 C (n = 290) and Δ 15 N (n = 268). We analysed this data set to test the effect of diet isotopic ratio on the discrimination factor, taking into account taxa, tissues, environments and lipid extraction treatments. Our results showed differences among taxonomic classes for Δ 13 C, but not for Δ 15 N, and significant differences among tissues for both Δ 13 C and Δ 15 N. We found a significant negative relationship between both, Δ 13 C and Δ 15 N, with their corresponding diet isotopic ratios. This relationship was found also within taxonomic classes for mammals (Δ 13 C and Δ 15 N), birds (Δ 13 C), fishes (Δ 13 C and Δ 15 N) and invertebrates (Δ 13 C and Δ 15 N). From these relationships, we propose a method to calculate discrimination factors based on data on diet isotope ratios (termed the 'Diet-Dependent Discrimination Factor', DDDF). 3. To investigate current practice in the use of discrimination factors, we reviewed studies that used multi-resource isotopic models. More than 60% of models used a discrimination factor coming from a different species or tissues, and in more than 70% of models, only one Δ 13 C or Δ 15 N was used for all resources, even if resources had very different isotopic ratios. Also, we estimated DDDFs for the studies that used isotopic models. More than 40% used Δ 15 N values and more than 33% used Δ 13 C values differing > 2‰ from estimated DDDFs. 4. Synthesis and applications . Over the last decade, applied ecologists have discovered the potential of stable isotopes for animal diet reconstruction, but the successful adoption of the method relies on a good estimation of discrimination factors. We draw attention to the high variability in discrimination factors, advise caution in the use of single discrimination factors in isotopic models, and point to a method for obtaining adequate values for this parameter when discrimination factors cannot be measured experimentally. Future studies should focus on understanding why dis-crimination factors vary as a function of the isotopic value of the diet.
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Modern approaches to Ecosystem-Based Management and sustainable use of marine resources must account for the myriad of pressures (interspecies, human and environmental) affecting marine ecosystems. The network of feeding interactions between co-existing species and populations (food webs) are an important aspect of all marine ecosystems and biodiversity. Here we describe and discuss a process to evaluate the selection of operational food-web indicators for use in evaluating marine ecosystem status. This process brought together experts in food-web ecology, marine ecology, and resource management, to identify available indicators that can be used to inform marine management. Standard evaluation criteria (availability and quality of data, conceptual basis, communicability, relevancy to management) were implemented to identify practical food-web indicators ready for operational use and indicators that hold promise for future use in policy and management. The major attributes of the final suite of operational food-web indicators were structure and functioning. Indicators that represent resilience of the marine ecosystem were less developed. Over 60 potential food-web indicators were evaluated and the final selection of operational food-web indicators includes: the primary production required to sustain a fishery, the productivity of seabirds (or charismatic megafauna), zooplankton indicators, primary productivity, integrated trophic indicators, and the biomass of trophic guilds. More efforts should be made to develop thresholds-based reference points for achieving Good Environmental Status. There is also a need for international collaborations to develop indicators that will facilitate management in marine ecosystems used by multiple countries.
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This study investigated the influence of the spatial arrangement of habitat patches on the diet and nutrition of a common reef-associated generalist consumer, the western rock lobster Panulirus cygnus. Stable isotopes (C-13/C-12 and N-15/N-14) and gut contents were used to assess diet and nutrition of lobsters collected from 8 shallow patch-reef sites on the lower west coast of Australia in April and October 2005. A distance-based linear model indicated that the predominant benthic habitat surrounding a reef (seagrass or macroalgae/sand-dominated) was an important source of variation in diet and nutrition, explaining significant (p < 0.01) variation in isotope signatures and gut contents of 52.7 and 7.0%, respectively. Mobile invertebrates, sessile filter feeders, coralline algae and seagrass were consumed by lobsters from all sites, but sessile filter feeders (sponges and colonial ascidians) were consumed in significantly greater volumes (p < 0.05) at macroalgae/sand-dominated sites (21.16 +/- 3.0%) than at seagrass-dominated sites (<= 6.1 +/- 1.08%). A modified mass balance mixing model (IsoSource), which factored in C:N ratios of food sources and lobster-specific delta C-13 and delta N-15 discrimination values, was used to determine the contribution of food sources to lobster nutrition. Articulated coralline red algae were an important source, especially from sites dominated by macroalgae and sand contributing 22-72% to nutrition. Bait potentially contributes up to similar to 30% of lobster muscle nutrition and therefore may also play an important nutritional role for lobsters in areas where lobster potting occurs. Macroalgae, rather than seagrass, appears the most likely autochthonous energy source driving P. cygnus production in shallow coastal waters; however, seagrass plays an important role as habitat for lobster prey.
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The size structure and taxonomic composition of coral communities in the inner (Granitic) Seychelles were studied 10 years after a thermal stress-induced mass mortality event.A survey of the abundance, population size structure and community composition of hard corals across 21 sites from three different geomorphological reef types on the Seychelles Bank provided high resolution data for discriminating coral communities based on diversity, taxonomic composition, colony abundance, surface area and size frequency distributions.Results emphasize the severely impoverished and depauperate nature of inner Seychelles hard coral communities, which had lower generic diversity (40 genera recorded), and lower abundance (3.3 colonies m-2) of hard corals (excluding juveniles) than other coral reef regions of the Indian Ocean for which comparable data are currently available.Analysis of coral communities indicated that management had no appreciable effect on juvenile or adult coral abundance at this point in time, and that low juvenile density (9.9 colonies m-2) may severely limit recovery of many individual reefs in the inner Seychelles.While some sites were found to have appreciable coral cover (>20%), others, including long-standing protected areas with no fishing, are now in an advanced state of erosion and framework collapse with very low juvenile coral replenishment and negligible available hard substratum suitable for coral settlement.Some of these reefs may have passed the threshold of viable recovery, now being in a self-reinforcing, non-coral dominated erosional phase.These findings indicate variable coral community condition, with many sites showing little sign of recovery. If persistence of live hard coral is a management goal, the existing protected areas within the Seychelles Bank may require review to ensure protection of sites with high recovery potential, while a suite of other management tools should be implemented in the remaining areas.
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Human activities have serious impacts on marine apex predators. Inadequate knowledge of the spatial and trophic ecology of these marine animals ultimately compromises the viability of their populations and impedes our ability to use them as environmental biomonitors. Intrinsic biogeochemical markers, such as stable isotopes, fatty acids, trace elements, and chemical pollutants, are increasingly being used to trace the spatial and trophic ecology of marine top predators. Notable advances include the emergence of the first océanographie "isoscapes" (isotopic geographic gradients), the advent of compound-specific isotopic analyses, improvements in diet reconstruction through Bayesian statistics, and tissue analysis of tracked animals to ground-truth biogeochemical profiles. However, most researchers still focus on only a few tracers. Moreover, insufficient knowledge of the biogeochemical integration in tissues, fractionation and routing processes, and geographic and temporal variability in baseline levels continue to hamper the resolution and potential of these markers in studying the spatial and feeding ecology of top predators.
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Omnivores play an important role in the routing and distribution of organic matter across food webs. We demonstrate a novel approach to quantifying the coincidence of landscape-scale nutritional gradients with niche breadth in terms of variability in trophic level and use of basal organic matter sources. We provide an example of the links between individual variability in resource use and habitat of a broad-spectrum omnivore, the red rock lobster Jasus edwardsii. Information on the co-occurrence of J. edwardsii with kelp bed habitats Ecklonia radiata and with their preferred prey Mytilus edulis galloprovinciallis were collected at 60 sites across Fiordland, southwest New Zealand. Analysis of distance-based linear models (DISTLM) indicated that the presence of mussels was the best predictor of lobster occurrence in the model set. At a subset of sites, we collected lobster muscle for stable isotope analysis and measured 3 demographic parameters from the lobsters: relative abundance, sex and carapace lengths. We characterised habitats with surveys of common kelp and mussels. Using stable isotope signatures (delta C-13 and delta N-15), we calculated individual-based estimates of trophic level and the mixture of organic matter sources, i.e. phytoplankton and macroalgae. Using DISTLM, lobster and mussel densities best explained variability in lobster diet. Variability in resource use was distinct inside and outside of kelp bed habitat. In kelp beds, lobsters fed at a higher average trophic level, with low variability among individuals in trophic level and use of organic matter sources. Outside kelp beds, individual variability indicated broad trophic diversification. These patterns indicate a strong influence of the nutritional landscape at the scale of the metapopulation, which has important implications for understanding dietary influences on population structure.
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Western rock lobsters, Panulirus cygnus are an abundant benthic consumer distributed along the temperate west coast of Australia and constitute the largest single species fishery in Australia. As a dominant consumer, it is important to understand their predator–prey interactions as they can potentially exert strong trophic effects, and may influence ecosystem function as seen in other spiny lobster species. While previous field studies have focused on the diet composition of P. cygnus, this study investigated their preference for various benthic invertebrate prey to better understand the likely predator–prey interactions of P. cygnus. Prey preferences of small sub-legal juvenile lobsters, as well as medium and large legal-sized mature lobsters were investigated using laboratory feeding trials to identify size-associated differences in lobster prey preference. Handling time and diet quality were investigated to estimate energetic cost and gain from consuming different prey which may explain prey choice by lobsters. It was found that large lobsters preferred crabs and mussels while medium and small lobsters preferred crabs over mussels, gastropods, and sea urchins. This suggests that strong predator–prey interactions between P. cygnus and crabs may occur in the wild.
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The natural diet of juvenile western rock lobsters (Panulirus cygnus George) at two sites, one of high and one of low growth rate, was examined to determine if there was a relationship between natural diet and growth rate. The diet of P. cygnus at the high growth rate site (Cliff Head in Western Australia) encompassed a wide range of animal and plant material, but was dominated by molluscs. There were significant variations in both the proportions of the different dietary items and in the degree of foregut filling with time of year. At the low growth rate site (Seven Mile Beach) the diet covered a similar range of animal and plant material as at Cliff Head, but was dominated by two species of foliose coralline algae. There was little variation in the proportions of the various dietary items with time of year, but the degree of foregut filling showed significant seasonal variation.The dietary data did not provide any evidence to support the hypothesis of Chittleborough that limited food supply is the cause of differences in growth between the sex/age-class groups, although there was an obvious association between the animal-rich diet at Cliff Head and the plant-rich diet at Seven Mile Beach and annual growth at the two sites. The nutritional role of the various dietary items, particularly the plant material, requires further evaluation. The allocation of nutritional resources to competitors also requires examination. The broad dietary spectrum of P. cygnus and their high densities in the coastal reef ecosystem suggest a significant role of grazing and predation by this species in the control of shallow reef communities of Western Australia.
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Knowledge regarding differences in evacuation rates of diet items from a consumer's stomach is important when using gut content analysis to quantify consumer diet. Evacuation rates of three diet items (pilchards, crabs and coralline algae) from the foreguts of western rock lobsters (Panulirus cygnus) were compared in aquaria. To determine evacuation rates, lobsters were allowed to consume offered food over a 90-min feeding period before being killed at 4, 6, 8, 10 and 12 h after the feeding period concluded. Diet items differed in their rate of evacuation from lobster foreguts with coralline algae evacuated most rapidly, followed by crabs, then pilchards. The evacuation of crabs and pilchards was still not complete 12 h after the feeding period concluded. Food not evacuated after 12 h predominantly consisted of hard components of the lobster diet, indicating that it is these components that account for slower evacuation. Observed variation in evacuation rates between diet items may skew the results of studies that use gut content analysis to quantify the diet of western rock lobsters.
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Confirms the hypothesis that carbon fixation in aquatic plants having thicker, stagnant boundary layers, will result in more positive δ13C values due to greater diffusion resistance and subsequent assimilation of otherwise normally discriminated 13C. The average δ13C value found in the literature for benthic algae in lakes was 26‰, whereas that for riverine benthic algae was -29‰. The greater water turbulence to which planktonic algae are exposed is known to dramatically reduce boundary layer thickness and was found to cause even more severe 13C depletion, resulting in an average value of -32‰. This same effect also operates in coastal environments where the average δ13C value for marine phytoplankton was -22‰ compared to -17‰ for marine benthic algae. -from Author
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Stomach contents of 382 Palinurus elephas collected in the Columbretes Islands Marine Reserve (north-western Mediterranean) were examined to study the diet and to assess ontogenetic and sex related differences in feeding regime. Molluscs, crustaceans, and sea urchins were the most common prey. Diet composition varied with lobster size but not with sex. Ontogenetic changes in diet were reflected in the progressive reduction of the contribution of gastropods and crustaceans and in the increased importance of other prey such as fish.
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In this paper we review recent advances in Stable Isotope Mixing Models (SIMMs) and place them into an over-arching Bayesian statistical framework which allows for several useful extensions. SIMMs are used to quantify the proportional contributions of various sources to a mixture. The most widely used application is quantifying the diet of organisms based on the food sources they have been observed to consume. At the centre of the multivariate statistical model we propose is a compositional mixture of the food sources corrected for various metabolic factors. The compositional component of our model is based on the isometric log ratio (ilr) transform of Egozcue (2003). Through this transform we can apply a range of time series and non-parametric smoothing relationships. We illustrate our models with 3 case studies based on real animal dietary behaviour.
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The western rock lobster (Panurilus cygnus George.) is a conspicuous consumer in the coastal ecosystems of temperate Western Australia. We used stable isotope analysis and gut content analysis to determine the diet and trophic position of western rock lobsters from mid-shelf coastal ecosystems (35–60 m depth) at three locations. Lobsters were primarily carnivorous, and no consistent differences in diet were detected with varying lobster size, sex or among locations. The main components of the diet were bait (from the fishery) and small crustaceans – crabs and amphipods/isopods. Foliose red algae, bivalves/gastropods and sponges were minor contributors to diet. The diet of lobsters in deep coastal ecosystems differed from the results of previous studies of diets of lobsters from shallow coastal ecosystems. In particular, coralline algae and molluscs – important prey in studies of lobsters from shallow coastal ecosystems – were minor components of the diet. These differences are likely to reflect differences in food availability between these systems and potentially, differences in choice of prey by lobsters that inhabit deeper water. Given the high contribution of bait to lobster diet, bait is likely to be subsidizing lobster production in deep coastal ecosystems during the fishing season.
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The natural diet and mode of feeding of the rock lobster Jasus lalandii (H. Milne Edwards) was determined in a rock-lobster sanctuary near Cape Town, South Africa. Field observations were tested and confirmed by means of aquarium studies. Rock lobsters feed mainly upon ribbed mussels Aulacomya ater (Molina), which comprise the largest component of the sessile benthic fauna. Mussel remains were found as the major constituent in 97% of the rock-lobster stomachs examined. The density of rock lobsters averaged 8,100 per hectare (0.81 m-2), while mussel biomass averaged more than 5 kg (wet whole weight) m-2 within the same depth range (12 to 30 m). More than 80% of the mussel biomass comprised large individuals between 60 and 90 mm in length. Large rock lobsters (mainly males) were capable of feeding on all sizes of mussels, although many of these were inaccessible to predation. Smaller rock lobsters became progressively more limited in the size range of mussels which they could crack open and consume. Competition between rock lobsters for small mussels appeared to be intense, as mussels of suitable size for feeding were generally in short supply to most of the rock-lobster population. Hence, feeding and growth rates of rock lobsters are likely to be affected by the relative population densities of predator and prey. Growth rates of rock lobsters could be limited by food supplies even in areas where mussel biomass is comparatively large.
Article
Fundamental to the accuracy of stable isotope analysis in trophodynamic studies is the ability to predict discrimination between a consumer and its diet. Despite the widespread use of stable isotope analysis in trophic ecology, uncertainty still surrounds the factors affecting consumer-diet discrimination. Here we present evidence that diet quality and location of muscle tissue analysed affects the consumer-diet discrimination for the western rock lobster, Panulirus cygnus. Consumer-diet δ15N and δ13C discrimination for western rock lobster tail tissue were 1.67–2.97 and 2.92–3.60‰, respectively, with δ13C discrimination differing to values reported in the literature. Differences in nitrogen and carbon discrimination were observed between tail and leg tissue of lobsters of 1.22 and 1.13‰, respectively. Diet quality was also found to affect consumer-diet discrimination, with high protein pilchard diet leading to lower δ15N and higher δ13C discrimination. Diet quality should be considered as a factor that has the potential to affect consumer-diet discrimination when interpreting results from stable isotope studies.
Article
Stable isotopes are often used as natural labels to quantify the contributions of multiple sources to a mixture. For example, C and N isotopic signatures can be used to determine the fraction of three food sources in a consumer's diet. The standard dual isotope, three source linear mixing model assumes that the proportional contribution of a source to a mixture is the same for both elements (e.g., C, N). This may be a reasonable assumption if the concentrations are similar among all sources. However, one source is often particularly rich or poor in one element (e.g., N), which logically leads to a proportionate increase or decrease in the contribution of that source to the mixture for that element relative to the other element (e.g., C). We have developed a concentration-weighted linear mixing model, which assumes that for each element, a source's contribution is proportional to the contributed mass times the elemental concentration in that source. The model is outlined for two elements and three sources, but can be generalized to n elements and n+1 sources. Sensitivity analyses for C and N in three sources indicated that varying the N concentration of just one source had large and differing effects on the estimated source contributions of mass, C, and N. The same was true for a case study of bears feeding on salmon, moose, and N-poor plants. In this example, the estimated biomass contribution of salmon from the concentration-weighted model was markedly less than the standard model estimate. Application of the model to a captive feeding study of captive mink fed on salmon, lean beef, and C-rich, N-poor beef fat reproduced very closely the known dietary proportions, whereas the standard model failed to yield a set of positive source proportions. Use of this concentration-weighted model is recommended whenever the elemental concentrations vary substantially among the sources, which may occur in a variety of ecological and geochemical applications of stable isotope analysis. Possible examples besides dietary and food web studies include stable isotope analysis of water sources in soils, plants, or water bodies; geological sources for soils or marine systems; decomposition and soil organic matter dynamics, and tracing animal migration patterns. A spreadsheet for performing the calculations for this model is available at http://www.epa.gov/wed/pages/models.htm.
Article
Juvenile spiny lobsters Panulirus argus (Latreille) from three behaviorally and ecologically distinct ontogenetic groups (algal, 5–15 mm carapace length; transitional, 16–25 mm CL; and post-algal, 26–35 mm CL) were tethered in their characteristic shelters and on open substratum to evaluate size related differences in predation risk. Field experiments performed at two sites near Long Key, Florida Bay nursery habitat indicate that juveniles attain a partial size refuge from a suite of abundant algal lobster predators at about the time they emerge from settling habitat. Algal lobsters experience significantly decreased mortality by sheltering at night, thereby attaining a survival rate comparable to that of larger, older juveniles that forage nocturnally in the open. Diver surveying and limited net sampling revealed an array of lobster predators including octopus, portunid crabs, bonnethead sharks, nurse sharks, sting rays, gray snapper and toadfish, as well as general crustacean predators including bonefish and permit. High relative mortality of the smallest juveniles suggests that predation on the algal and early transitional phases is a potential bottleneck to population recruitment.
Article
Use of stable isotope ratios to trace pathways of organic matter among consumers requires knowledge of the isotopic shift between diet and consumer. Variation in trophic shift among consumers can be substantial. For data from the published literature and supplementary original data (excluding fluid-feeding consumers), the mean isotopic shift for C was +0.5±0.13‰ rather than 0.0‰, as commonly assumed. The shift for C was higher for consumers analyzed as muscle (+1.3±0.30‰) than for consumers analyzed whole (+0.3±0.14‰). Among consumers analyzed whole, the trophic shift for C was lower for consumers acidified prior to analysis (−0.2±0.21‰) than for unacidified samples (+0.5±0.17‰). For N, trophic shift was lower for consumers raised on invertebrate diets (+1.4±0.21‰) than for consumers raised on other high-protein diets (+3.3±0.26‰) and was intermediate for consumers raised on plant and algal diets (+2.2±0.30‰). The trophic shift for S differed between high-protein (+2.0±0.65‰) and low-protein diets (-0.5±0.56‰). Thus, methods of analysis and dietary differences can affect trophic shift for consumers; the utility of stable isotope methods can be improved if this information is incorporated into studies of trophic relationships. Although few studies of stable isotope ratios have considered variation in the trophic shift, such variation is important because small errors in estimates of trophic shift can result in large errors in estimates of the contribution of sources to consumers or in estimates of trophic position.