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The manner in which a gastropod shell coils has long intrigued laypersons and scientists alike. In evolutionary biology, gastropod shells are among the best-studied palaeontological and neontological objects. A gastropod shell generally exhibits logarithmic spiral growth, right-handedness and coils tightly around a single axis. Atypical shell-coiling patterns (e.g. sinistroid growth, uncoiled whorls and multiple coiling axes), however, continue to be uncovered in nature. Here, we report another coiling strategy that is not only puzzling from an evolutionary perspective, but also hitherto unknown among shelled gastropods. The terrestrial gastropod Opisthostoma vermiculum sp. nov. generates a shell with: (i) four discernable coiling axes, (ii) body whorls that thrice detach and twice reattach to preceding whorls without any reference support, and (iii) detached whorls that coil around three secondary axes in addition to their primary teleoconch axis. As the coiling strategies of individuals were found to be generally consistent throughout, this species appears to possess an unorthodox but rigorously defined set of developmental instructions. Although the evolutionary origins of O. vermiculum and its shell's functional significance can be elucidated only once fossil intermediates and live individuals are found, its bewildering morphology suggests that we still lack an understanding of relationships between form and function in certain taxonomic groups.
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Biol. Lett. (2008) 4, 179–182
doi:10.1098/rsbl.2007.0602
Published online 8 January 2008
Evolutionary biology
Further twists in gastropod
shell evolution
Reuben Clement s
1,
*
, Thor-Seng Liew
2
,
Jaap Jan Vermeulen
3
and Menno Schilthuizen
2,4
1
World Wide Fund for Nature-Malaysia, 49, Jalan SS23/15,
47400 Petaling Jaya, Selangor, Malaysia
2
Institute for Tropical Biology and Conservation, Universiti Malaysia
Sabah, Locked Bag 2073, 88999 Kota Kinabalu, Malaysia
3
Nationaal Herbarium Nederland, PO Box 9514, 2300 RA Leiden,
The Netherlands
4
National Museum of Natural History ‘Naturalis’, PO Box 9517,
2300 RA Leiden, The Netherlands
*Author for correspondence (clements@alumni.nus.edu.sg).
The manner in which a gastropod shell coils has
long intrigued laypersons and scientists alike. In
evolutionary biology, gastropod shells are among
the best-studied palaeontological and neontolo-
gical objects. A gastropod shell generally exhibits
logarithmic spiral growth, right-handedness and
coils tightly around a single axis. Atypi cal shell-
coiling patterns (e.g. sinist roid growth, uncoiled
whorls and multiple coiling axes), however,
continue to be uncovered in nature. Here, we
report another coiling strategy that is not only
puzzling from an evolutionary perspective, but
also hitherto unknown among shelled gastro-
pods. The terrestr ial gastropod Opisthostoma
ver miculum sp.nov.generatesashellwith:
(i) four discernable coiling axes, (ii) body whorls
that th rice detach and twice reattach to preced-
ing whorls without any reference support, and
(iii) detached whorls that coil around three
secondaryaxesinadditiontotheirprimary
teleoconch axis. As the co iling strateg ies of
individuals were found to be generally consistent
throughout, this species appears to possess an
unorthodox but rigorously defined set of develop-
mental instructions. Although the evolutionary
origins of O. vermiculum and its shells functional
significance can be elucidated only once fossil
intermediates and live individuals are found, its
bewildering morphology suggests that we still lack
an understanding of relationships between form
and function in certain taxonomic groups.
Keywords: conchology; snail; karst; Malaysia;
Mollusca; morphology
1. INTRODUCTION
Over the past 150 years, evolutionary biology has
benefited from the many qualities of the gastropod shell
(Schilthuizen 2002). It is essentially a complex three-
dimensional structure that acts as the snail’s interface
with the biotic and abiotic environments. Conse-
quently, it is of great importance for survival and the
target of multifarious natural (and possibly sexual;
Schilthuizen 2003) selection pressures. Yet, it answers
to a very limited set of growth parameters, which makes
its evolution reducible to few character-state changes
(Thompson 1942). The gastropod evolutionary history
reveals a dominance of shells resembling helicospiral
cones that monotonically expand according to a logar-
ithmic function ( Thompson 1942 and references
therein). In addition, most shells are right handed and
possess overlapping whorls that coil around a single axis
(figure 1a). Spiral coiling also appears to be dictated by
a set of ‘behavioural’ rules involving shell sculpture. For
example, Hutchinson’s (1989) road-holding model
(RHM) postulates that shell ornamentation (e.g. keels
and low curvature areas) is the vital reference spot for
the attachment of subsequent whorls.
Several groups of gastropods, however, possess
shell-coiling patterns that depart from the above-
mentioned conventions. For instance, shells of
marine vermetids generally deviate from logarithmic
spiral growth late in their ontogeny. In the terrestrial
genus Opisthostoma, shells are not right handed, but
‘sinistroid’ (figure 1c) due to a reversal in coiling
direction in the last half-whorl (Gittenberger 1995).
Furthermore, Opisthostoma shells can possess two
(e.g. Opisthostoma concinnum ; figure 1b)orthree
different coiling axes (e.g. Opisthostoma castor). In
another terrestrial genus, Ditropopsis, the body whorls
can detach to produce an ‘uncoiled’ shell (figure 1d ).
Interestingly, detached whorls of uncoiled gastropods
do not adhere to the sculptural features of previous
whorls; they remain either coiled around the primary
teleoconch axis (figure 1d ) or coil in an irregular
manner (e.g. marine vermetids).
Taking these exceptions into account, we can refine
our earlier generalizations of gastropod shells: (i) they
possess an upper limit of three coiling axes, (ii) detached
body whorls do not reattach to the preceding whorls,
and (iii) whorls once detached either coil around a
primary teleoconch axis or deviate haphazardly. In this
paper, we describe a new species of terrestrial gastropod
with a shell that pushes these evolutionary boundaries
even further. In addition, we discuss its evolutionary
origins, functional significance and coiling strategy.
2. MATERIAL AND METHODS
The specimens upon which the species description was based were
deposited in the Zoological Reference Collection (ZRC), Mollusc
Section (MOL), Raffles Museum of Biodiversity Research ( RMBR)
and National University of Singapore. Using visual inspection and
flotation techniques to extract shells, 38 individuals (including fresh
dead specimens) were obtained from a total of six different 8 m
2
plots at the type locality. Descriptions are based on the shell
characters and nomenclature follows van Benthem-Jutting (1952)
and Vermeulen (1994). Measurements of shells were based on
images obtained from a scanning electron microscope and are in
millimetre. Height refers to the longest dimension of the shell,
while width refers to its perpendicular dimension.
3. SYSTEMATICS
Higher taxon names: Mesogastropoda Thiele 1925;
Diplommatinidae Pfeiffer 1856; and Genus Opisthos-
toma Blanford & Blanford 1860.
(a) Type species
Opisthostoma nilgiricum Blanford & Blanford 1860.
(b) Description
(i) Opisthostoma vermiculum Clements & Vermeulen,
n. sp.
Height:holotype1.5,paratype1.5.Width:holotype0.9,
paratype 1.0. Shell thin, cream or white, not transparent
Electronic supplementary material is available at http://dx.doi.org/
10.1098/rsbl.2007.0602 or via http://journals.royalsociety.org.
Received 1 December 2007
Accepted 7 December 2007
179 This journal is q 2008 The Royal Society
and slightly shiny. First whorl smooth; subsequent
whorls sculptured with fine, white, regularly spaced
radial ribs, becoming widely spaced and flared
nearing aperture before being compressed; spiral striae
absent. Whorls 4.5–5, convex, increasing in radius,
uncoiling at the end of second whorl; detached third
whorl returns to reattach to the base of second whorl,
followed by a phase of detachment, reattachment and
detachment; whorls deviate from primary teleoconch
axis to coil around three additional axes. Suture deep.
No umbilicus. Aperture round, vertical, without
teeth. Peristome duplex, continuous, circular to
rounded triangular.
(c) Remarks
An internal constriction, which qualifies placement
under the genus Opisthostoma (see Vermeulen 1994),
was detected in this species. All 38 specimens demon-
strated four changes in the coiling direction and
underwent similar detachment–reattachment phases,
but displayed slight variations in the angles of each
coiling axis (see electronic supplementary material).
Intraspecific variation (nZ6) among shell dimensions
appeared to be low, with a mean (Gs.d.) height and
width of 1.5G 0.1 and 0.9G0.1, respectively.
(d) Types, locality and distribution
Holotype: ZRC.MOL.002824, Gunung Rapat
(48 33
0
N, 1018 7
0
E), Perak, Peninsular Malaysia,
held in RMBR. Two paratypes: ZRC.MOL.002825
and ZRC.MOL.002826, same data as holotype.
Known only from the type locality.
(e) Etymology
vermiculum’ meaning wormy, as the shell resembles a
worm-like organism. Clements and Vermeulen are
assigned as authors for O. vermiculum sp. nov.
4. DISCUSSION
Evolutionary responses of shell traits to environmental
pressures are among the best-documented microevolu-
tionary processes (Endler 1986). For example, uncoil-
ing in freshwater gastropods appeared to correspond
with periods of high chemical stress during the
Miocene epoch ( Nutzel & Bandel 1993). The impact
of environmentally induced mutations (figure 1e)on
the phenotype of O . vermiculum, however, cannot be
ascertained without fossil records and historical
environmental data. Novel shell characters can also
result from hybridization (Woodruff & Gould 1987),
during which the developmental factors or the gene
(a)
(b)
(c)
(d )
(e)
Figure 1. Different gastropod shell-coiling patterns. (a) Queridomus conulus exhibiting logarithmic helicospiral growth with
tightly coiled whorls around a single axis (black line), (b) O. concinnum and (c) Opisthostoma hailei exhibiting ‘sinistroid’
growth with two different coiling axes (black lines), (d ) Ditropopsis sp. demonstrating uncoiling with detached whorls that
never reattach and (e) a mutant individual of O. hailei. Scale bars, 200 mm.
180 R. Clements et al. Further twists in gastropod shell evolution
Biol. Lett. (2008)
interactions related to morphologies of different species
produce new features (Chiba 2005). However, the
distinctive and invariant coiling patterns of O. vermiculum
specimens examined in our study suggest that its
conchology is under fairly strict developmental–genetic
control and is unlikely to be a product of hybridization
between two other congeners that occur sympatrically on
thesamekarstOpisthostoma megalomphalum and
Opisthostoma paulucciae; obvious differences in their shell
sculpture (e.g. rib spacing and presence of apertural
flares) further reduce the likelihood that O. vermiculum is
an intermediate species.
The gastropod shell sculpture could have evolved
to cope with predation (Palmer 1977), feeding (Illert
1981) or movement (Cain & Cowie 1978). In some
species of Opisthostoma, the evolution of intricate shell
ornamentation may be driven by sexual selection
(Schilthuizen 2003), although empirical evidence thus
far suggests only the role of ‘Red Queen’ coevolu-
tionary interactions with the snails’ predators
(Schilthuizen et al. 2006). The functional significance
of uncoiling (especially in O. vermiculum), however,
remains unclear ( Morton 1965; Nutzel & Bandel
1993). Uncoiled shells may facilitate predator evasion
in some species (i.e. whorl detachment makes snails
effectively larger; Rex & Boss 1976), but appear
energetically disadvantageous to construct (i.e. uncoil-
ing weakens shells and consumes additional shell
material; Rex & Boss 1976) and may even hinder
movement (Clarke 1973). Uncoiling has also been
associated with sessility (e.g. in marine gastropods;
Gould 1968) and gerontic conditions ( Yo c h e l s o n
1971), but the former is precluded in terrestrial snails
and the latter is unlikely because whorl detachment in
O. vermiculum begins early in its ontogeny.
The coiling axis of a gastropod shell is not easily
discernable (Okamoto 1988; Savazzi 1990), but
numerous ‘fixed axes’ mathematical models have
been used to explain shell geometries (Raup 1961;
Raup & Michelson 1965; Løvtrup & Løvtrup 1988;
Illert 1989; Savazzi 1990).Thepresenceoffour
coiling axes in O. vermiculum (figure 2b), which is the
highest number known for a shelled gastropod, poses
a challenge to the development of a model that
accounts for such a morphologically bizarre shell.
Reorientation of shell-coiling axes has been attributed
to a simple rotation of the snail body inside the shell
(Ackerly 1989). A more remarkable and unique
aspect, however, is the departure of the second whorl
from the primary teleoconch axis to revolve around a
secondary (almost perpendicular) axis before latching
back onto the preceding whorl (figure 2c), after
which it proceeds to a detachment–reattachment
(figure 2d )–detachment phase. The importance
of shell ornamentation (e.g. keels and ribs) for
spiral coiling has been corroborated by experimental
(a)
(b)
(c)
(d )
Figure 2. Opisthostoma vermiculum sp. nov. (a) Ventral view, (b) side view indicating four coiling axes (white lines), (c)first
and (d ) second reattachment points of detached whorls (white arrows). Scale bar, 100 mm.
Further twists in gastropod shell evolution R. Clements et al. 181
Biol. Lett. (2008)
tests on the RHM (e.g. Checa et al. 1998), but the
detached whorls in O. vermiculum consistently reat-
tach to preceding whorls without any apparent need
for reference support.
Ultimately, the functional significance and coiling
regulatory mechanism of the shell in O. vermiculum
cannot be investigated without studying live individuals,
but the novelty of its shell-coiling pattern is indisputable.
It is also interesting to note that such phenotypic
peculiarities often occur among microgastropods (less
than 5 mm) such as Opisthostoma. Unfortunately,
Opisthostoma snails are particularly vulnerable to extinc-
tion (IUCN 2004) as most species are restricted to
limestone karsts (Vermeulen 1994), which have become
increasingly threatened by quarrying activities
(Clements et al. 2006). We hope the nding of this
species will not only encourage further research into the
relationship between form and function in gastropods,
but also promote more inventories of limestone karsts
due to their potential for yielding important taxonomic
and evolutionary discoveries (e.g. Morwood et al. 2004).
This project was supported by a research permit (no. 1773,
Economic Planning Unit, Malaysia) and grants from the
Singapore Zoological Gardens and the National University
of Singapore (R-154-000-264-112). We are grateful to
David Bickford, Lahiru Wijedasa, Matthew Lim, Nalini
Puniamoorthy and Stephen Ambu for their comments and
assistance. We also thank two anonymous referees and
Geerat J. Vermeij for greatly improving the manuscript.
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182 R. Clements et al. Further twists in gastropod shell evolution
Biol. Lett. (2008)
... Originally, Opisthostoma vermiculum was placed in the genus Opisthostoma based on the presence of an internal constriction and more than two coilings (Clements et al., 2008). However, a recent molecular phylogenetic study of the Plectostoma, Opisthostoma, and Arinia by Liew et al. (2014) showed that O. vermiculum is unrelated to the other species in the genus (Fig. 2). ...
... Type material of Whittenia vermicula and W. gittenbergeri, is illustrated in Clements et al. (2008) and Vermeulen & Clements (2008). Specimens collected by Foon have been illustrated in Foon et al. (2017). ...
... Type material of Whittenia vermicula and W. gittenbergeri, is illustrated in Clements et al. (2008) and Vermeulen & Clements (2008). Specimens collected by Foon have been illustrated in Foon et al. (2017). ...
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This paper proposes a new genus, Whittenia, for a group of irregularly coiled land snail species hitherto placed in the genus Opisthostoma. Whittenia differs from congeners in Diplommatinidae based on both molecular phylogeny and shell morphology. We reassign two previously described species (Opisthostoma vermiculum and O. gittenbergeri) to Whittenia. Both species are endemic to the limestone karsts of Kinta Valley in Perak, Peninsular Malaysia.
... direction (hereafter termed 'tuba';van Benthem-Jutting, 1952;Vermeulen, 1994;Liew et al., 2014a). Similar morphological transitions during shell ontogeny are known for other extant and fossil molluscs (e.g., Okamoto, 1988;Clements et al., 2008). In addition to this irregular coiling, there is great diversity in the shell radial ribs of Plectostoma in terms of density, shape, and intensity (i.e., amount of shell material in the ribs) (van Benthem-Jutting, 1952;Vermeulen, 1994). ...
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... So far, we have not observed drill holes in Plectostoma shells with no tuba (Schilthuizen et al., 2006;Liew T-S, unpublished data, 2013), and only once the slug was seen attacking a juvenile prey without a tuba by shell-apertural entry (Schilthuizen & Liew, 2008). However, we do not know if the slug has a preference for juvenile or adult prey. ...
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... We selected four Opisthostoma species, namely, Opisthostoma laidlawi Skyes 1902 (Figure 2A Clements andVermeulen, 2008 (in Clements et al., 2008) (Figure 2D), for which the shell forms are, respectively: regularly coiled, slight distortion of the last whorl, strong distortion of the last whorl, and complete distortion of most of the whorls. We retopologised these four shells by following the procedures 1 to 4 ( Supplementary Information Files 12). ...
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Quantitative analysis of organismal form is an important component for almost every branch of biology. Although generally considered an easily-measurable structure, the quantification of gastropod shell form is still a challenge because shells lack homologous structures and have a spiral form that is difficult to capture with linear measurements. In view of this, we adopt the idea of theoretical modelling of shell form, in which the shell form is the product of aperture ontogeny profiles in terms of aperture growth trajectory that is quantified as curvature and torsion, and of aperture form that is represented by size and shape. We develop a workflow for the analysis of shell forms based on the aperture ontogeny profile, starting from the procedure of data preparation (retopologising the shell model), via data acquisition (calculation of aperture growth trajectory, aperture form and ontogeny axis), and data presentation (qualitative comparison between shell forms) and ending with data analysis (quantitative comparison between shell forms). We evaluate our methods on representative shells of the genus Opisthostoma , which exhibit great variability in shell form. The outcome suggests that our method is more robust, reproducible, and versatile than the conventional traditional and geometric morphometric approaches for the analysis of shell form. Finally, we propose several potential applications of our methods in functional morphology, theoretical modelling, taxonomy, and evolutionary biology.
... For those gastropod species with shell coiling geometry fairly approaching a conispiral profile and approximately answering the ideal exponential model (i.e. logarithmic spiral coiling), the contribution, δ i , of whorl number 'i' to the overall shell dimensions ∆ (shell height or shell width) is, according to [15][16][17][18][19][20][21][22]: ...
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Many species among shelled gastropods – either land snails or marine snails – exhibit determinate growth and, therefore, are expected to implement some regulating process aiming at limiting the impact on the final (adult) shell-size of the (possibly excessive) intraspecific variability in the rate of shell-development. Indeed, a usually more or less limited range of variation is allowed for adult shell-size, in those species having determinate growth. Mollusks are expected to be no exception in this respect and, up to now, the occurrence of such a regulating process has actually been reported systematically, despite a still too limited number of investigations. Yet, the question remained of the ability of this regulatory process to finely adjust the end of the final development of both body and shell (specifically in term of the final number of whorls), so as to limit the variations in adult shell-size, despite the amplitude of intraspecific variability in shell development rate. I provide here preliminary empirical evidence for such a “finely tuned” regulating process – the strength of which is mirrored by the degree of “Gouldian” negative covariance between whorl growth-rate and the number of whorls at adulthood. As expected, the strength of the regulating process reveals systematically increasing (i) with the amplitude of the intraspecific variability in shell development-rate and (ii) with the narrowness of the range of variation allowed for the adult shell-size (narrowness which depends upon the identity of the particular species under consideration). In addition to the already reported usual occurrence of this regulating process among shelled gastropods, its “finely tuned” character provides here still further evidence as regards its major importance in controlling the species-specific adult body mass, and this even for typically soft-bodied animals.
... For those gastropod species with shell coiling geometry fairly approaching a conospiral profile and approximately answering the ideal exponential model (i.e. logarithmic spiral coiling), the contribution, δ i , of whorl number 'i' to the overall shell dimensions ∆ (shell height or shell width) is, according to [6][7][8][9][10][11][12][13]: ...
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For most conispirally-coiled Gastropods with determinate growth, the geometry of spirally-winding whorls is usually constrained by a strong negative correlation between whorl growth rate and the number of whorls reached at adulthood, as originally reported by the late S.J. Gould. Yet, beyond the tight control of shell-shape at the species level – resulting from this constraint – what about the amplitude of the intra-specific variability of whorl growth-rate, partly contributing to the variability of the overall shell-size at the species level? I address the issue by designing and implementing a new, indirect method for routinely evaluating whorl growth-rate, thereby aiming at considerably saving measurement time, and making it possible to easily achieve repeated measurements across samples large enough to reach statistical significance. This approach was applied to a series of eight common land snail species. The amplitude of intra-specific variability in whorl growth, evaluated this way, proves: (i) being markedly different among the eight investigated species (by a factor that can exceed 2x); (ii) being, yet, high enough, in all cases, to require compensating variations in the adult number of whorls, so as to limit the resulting consequences on the amplitude of the intra-specific variability of adult shell-size. Despite those marked differences in the amplitudes of intra-specific variability of whorl growth-rate among species, no significant relationship was observed between intraspecific variability of whorl growth rate and species-specific shell-shape types (discoidal/globular/elongate) and only weak positive relationship was observed with species-specific typical shell sizes. However, a rather strong positive correlation was found, as expected, between the degree of intra-specific variability of the whorl growth-rate and the degree of intra-specific variability of the number of whorls reached at adulthood (with the yet unexplained exception of one among the eight investigated species).
... However, the probability of finding a new species is quite high, considering that Diplommatinidae are very small in size, are local endemics and have only been documented in three localities from three regions of Vanua Levu. Clements et al. (2008) mentioned a mutant form, but also that the intraspecific variation among shell dimensions seems to be low. Therefore, we conclude that it is more probable to have a new species rather than an abnormal form, considering the clear differences in shell morphology compared to previously described species in Fiji. ...
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A new species of Diancta of the staircase snail family Diplommatinidae is described from Mt. Savusavu, Vanua Levu Island, Fiji. Due to its left coiling shell and a constriction before the last whorl, it is placed in the genus Diancta . Micro-CT imaging reveals two apertural teeth and an inner lamella that is situated at the zone of constriction. The shell abruptly changes coiling direction by 45 degrees before the last whorl. Up to now, this coiling modus had not yet been documented for any species of Diplommatinidae from the Fiji Islands.
... For those gastropod species with shell coiling geometry fairly approaching a conispiral profile and approximately answering the ideal exponential growth model (i.e. logarithmic spiral coiling), the contribution, δi, of whorl number 'i' to the overall shell dimensions ∆ (shell height or shell width) is, according to [14][15][16][17][18][19][20][21]: ...
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As shown, in particular, by the late S.J. Gould, the involvement of a regulation process, aiming at limiting the range of intraspecific variations in adult shell size, in those land snail species with determinate growth, can be indirectly, but conveniently, diagnosed by highlighting a negative covariance between the whorls growth-rate and the whorls number reached at adulthood. However, up to now, such kind of regulation had only been demonstrated in very few cases among land snails and shelled Gastropods in general. Accordingly, quite more extensive checking is required, across both the taxonomic spectrum and the geometrical range of shell profiles. The present report is a very preliminary contribution addressing these issues, which have been neglected for too long. Considering a still limited number of eight species, yet largely encompassing both taxonomic range and shell profiles, it is shown that strong negative covariances between whorls growth-rate and whorls number are systematically highlighted, thereby supporting the involvement of an efficient regulation process of adult shell size and shape in each eight species. Moreover, the degree of regulation of the adult shell size has been quantified and this regulation proves being remarkably effective as a whole, while yet remaining highly species-specific, with very significant disparities among species – and this, somewhat surprisingly, being quite irrespective of the type of geometrical profiles of shells, among the studied species.
... Several of these hypotheses were discussed in detail by Clements et al. (2008) when detailing the significance of the exaggerated open coiling of the terrestrial microgastropod Opisthostoma vermiculum (Architaenioglossa: Diplommatinidae), but none could be verified without further in vivo study. Liew and Schilthuizen (2014) performed in vivo predator-prey interaction studies for the terrestrial microgastropod genus Plectostoma (Diplommatinidae), and found their results suggested that open coiling may be an anti-predation adaptation that provides a less direct predation path when compared to a typical, tightly coiled gastropod (which counters predator release hypothesis). ...
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This paper describes a new genus and species of subterranean gastropod from a karstic region near Viesca, Coahuila in northern Mexico. Shells of Phreatoviesca spinosa gen. nov. et sp. nov. were found in spring-deposited sediments near the outlet of a cave that dried up in the late 20th century. The new genus can be primarily distinguished conchologically from other phreatic genera by three remarkable characteristics: (i) prominent open coiling of the last whorl, (ii) shovel-shaped spine ornamentations on the teleoconch, and (iii) a coarsely honeycomb-like pitted protoconch structure. Since only dry shells were found, the new species could already be extinct. However, in view of the relative recent drying up of the spring, we consider that Phreatoviesca spinosa is possibly extant in the aquifers in or adjacent to the Viesca region.
... We gained access to lobster stomachs by making a posterior-anterior incision with a scalpel or dissection scissors and pinning the exposed body tissue and integument back to reveal the cardiac stomach, which was removed in one piece with forceps. For a detailed dissection methodology, see We first classified stomach items into 'urchin hard parts', 'other hard parts' and 'non-hard parts' categories before identifying stomach items to at least the Class level, where possible, based on literature research of prey taxonomy and morphometry (Watson 1965;Lawry 1967;Rowan 1989;Clements et al. 2008;Witten et al. 2010;Thuy and Stöh 2011;Haug et al. 2012;Williams 2017). Urchin spine morphometrics are provided in Table S1 and a detailed account of the morphometrics used to classify other stomach items, with an extensive list of the literature used, is provided in Table S2. ...
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Worldwide, lobsters are considered important predators of macroalgae-consuming urchin species, but this has not been tested for Australia’s common lobster, the eastern rock lobster (Sagmariasus verreauxi). We predicted that the abundant urchins, the long-spined urchin (Centrostephanus rodgersii) and the short-spined urchin (Heliocidaris erythrogramma), would form substantial components of lobster diets. To test this hypothesis, we examined 115 lobster stomachs from 9 locations and conducted 14 feeding trials, in which lobsters were offered either urchin species. Dissections revealed various stomach items, with detritus (51%), bivalves (34%), gastropods (28%) and algae (26%) occurring more frequently than urchins (19%). Urchin spines were found in 22 lobsters that ranged in size from 91- to 124-mm carapace length, with all individuals containing H. erythrogramma spines and C. rodgersii spines observed only once. During feeding trials, seven urchins were consumed. Four H. erythrogramma were eaten on Day 1, whereas one was not eaten until Day 11. Two C. rodgersii were eaten on Days 2 and 10. Only three of the six lobsters observed to eat urchins ingested spines. Together, these data show that S. verreauxi is a generalist predator that consumes urchins, although, because urchins could be eaten without ingesting spines, future studies are needed to assess the importance of urchins as dietary items for S. verreauxi.
Article
An unusually well documented case of secondary hybridization and introgression involving two morphologically dissimilar species of land snails is described. In 1915, 55 Bahamian Cerion casablancae were established on Bahia Honda Key, Florida. The introduced snails thrived, bred true to form, and the colony grew until about 1928 when it began to hybridize with C. incanum, the unrelated Florida species. In 1977, morphological hybrids occupied an area of about 3.5 ha. Spatial and temporal aspects of the interaction were characterized morphologically (18 shell characters were studied in source populations, founders, and hybrids sampled in 1933 and 1977) and genetically (variation at 17 allozyme loci in relevant populations sampled in 1977). In addition, special circumstances permitted us to discern the genetics of the founders: C. incanum is isogenic locally, C. casablancae is variable (Po = 0.29-0.35), and the two species are well-differentiated (Nei's D̄ = 0.27). Hybrid intermediacy of form and continued enhanced variation appeared in characters from three covariance sets, while some unique hybrid morphologies resulted from characters in a fourth set. Comparison of 1933 with 1977 samples showed that the hybrids are gradually approaching C. incanum in phenotype. Regardless of phenotype, the snails on Bahia Honda Key were panmictic and outbreeding. There was no evidence for strong selection against hybrids, and by 1977 introduced alleles had spread over 5 ha. However, no pure C. casablancae remain; low (m = 0.026/generation) but persistent gene flow has brought about a 30% diminution of the introduced genome. These observations are useful in interpreting Cerion's remarkable variability as colonization following hurricane dispersal has undoubtedly played a role in the group's complex evolution. More generally, the results are relevant to the problem of interpreting hybrid zones of unknown origin, and the differences in the generally concordant patterns of morphological and genetic introgression reveal constraints on the way components of different coadapted genomes interact.
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Despite an extensive scholarly literature dating back to classical times, seashell geometries have hitherto resisted rigorous theoretical analysis, leaving applied scientists to adopt a directionless empirical approach toward classification. The voluminousness of recent palaeontological literature demonstrates the importance of this problem to applied scientists, but in no way reflects corresponding conceptual or theoretical advances beyond the XIX century thinking which was so ably summarized by Sir D'Arcy Wentworh Thompson in 1917. However, in this foundation paper for the newly emerging science of theoretical conchology, unifying theoretical considerations for the first time, permits a rigorous formulation and a complete solution of the problem of biological shell geometries. Shell coiling about the axis of symmetry can be deduced from first principles using energy considerations associated with incremental growth. The present paper shows that those shell apertures which are incurved («cowrielike»), outflared («stromblike») or even backturned («Opisthostomoidal») are merely special cases of a much broader spectrum of «allowable» energy-efficient growth trajectories (tensile elastic clockspring spirals), many of which were widely used by Cretaceous ammonites. Energy considerations also dictate shell growth along the axis of symmetry, thus seashell spires can be understood in terms of certain special figures of revolution (Möbius elastic conoids), the better-known coeloconoidal and cyrtoconoidal shell spires being only two special cases arising from a whole class of topologically possible, energy efficient and biologically observed geometries. The «wires» and «conoids» of the present paper are instructive conceptual simplifications sufficient for present purposes. A second paper will later deal with generalized tubular surfaces in three dimensions.
Article
Shell coiling is readily visualized as a revolution and expansion of the shell's aperture around the coiling axis. Stereographic projections provide an effective tool for plotting the orientations of the aperture through time/space. In isometric growth, poles to the aperture (eg, perpendiculars to the aperture plane) plot on a small circle on the stereographic projection, and the center of the small circle defines the orientation of the shell's coiling axis. The stereographic analysis requires no a priori determinations of the coiling axis position in the shell. Stereographic analyses consider shell growth in terms of rigid body motions of the entire aperture, as opposed to conventional methods that consider only the form of individual spiral trajectories (single points on the aperture). Stereographic projections offer a novel and instructive approach to the analysis of shell coiling. -Author
Article
In 1915, 55 Bahamian Cerion casablancae were established on Bahia Honda Key, Florida. The introduced snails thrived, bred true to form, and the colony grey until about 1928 when it began to hybridize with C. incanum, the unrelated Florida species. In 1977, morphological hybrids occupied an area of c3.5 ha. Spatial and temporal aspects of the interaction were characterized morphologically and genetically. Comparison of 1933 with 1977 samples showed that the hybrids are gradually approaching C. incanum in phenotype. Regardless of phenotype, the snails on Bahia Honda Key were panmictic and outbreeding. There was no evidence for strong selection against hybrids, and by 1977 introduced alleles had spread over 5 ha. However, no pure C. casablancae remain; low but persistent gene flow has brought about a 30% diminution of the introduced genome. These observations are useful in interpreting Cerion's remarkable variability as colonization following hurricane dispersal has undoubtedly played a role in the group's complex evolution. -from Authors