Allometric and trophic effects on shell morphology of Pomacea canaliculata (Caenogastropoda,Ampullariidae) from a geometric morphometrics viewpoint

Pomacea canaliculata, an invasive apple snail native to South America, is a serious pest of aquatic crops in several parts of the world. The origin of inter-population variation in shell shape is thought to be both genetic and environmental but the reaction norms to specific environmental factors are still poorly understood. Our aims were to analyze the existence of direct and indirect (allometric) effects of food availability (FA) on the shape of young adults of P. canaliculata. Full sibling hatchlings were reared under different levels of FA. Nine landmarks and 10 semi-landmarks were determined on photographs of mature shells and analyzed using geometric morphometrics. In both sexes significant allometry was found: a decrease in the spire height in both sexes, and an increase of the aperture size in males and of the last whorl in females. When this allometric component was removed a relationship between size-corrected shape and FA was found only in females, which were more globose and had a larger aperture when grown under high FA. This effect may be explained by the faster growth of the reproductive organs and the thinner shells of the best fed females.     

Morphological Features of Supralittoral Mollusks of the Genus Cecina (Gastropoda: Pomatiopsidae) from Peter the Great Bay, Sea of Japan

Amphibious mollusks of the genus Cecina in the littoral and supralittoral of the Sea of Japan are represented by three species: Cecina manchurica A. Adams, 1861; C. tatarica (Schrenck, 1867); and C. scarlatoi Prozorova, 1996. Additional data on the morphology of the shell and radula are provided. Indices for the identification of adult decollated and corroded shells of these species which retained only 3.0–2.5 last whorls are proposed: (1) the ratio of the width of the ultimate whorl (without aperture) to the length of the ultimate and penultimate whorls and (2) the ratio of the width of the third whorl (from the bottom) to the width of the ultimate whorl (without aperture).     

Buckman's first law of covariation–a case of proportionality

Buckman's first law of covariation states that in ammonoids, coarseness of ribs and other ornamentation correlates negatively with degree of compression (whorl height/width ratio) and involution. This ‘law’ has escaped several attempts at functional or physiological explanation. However, Buckman's first law becomes self‐evident, and almost collapses to a tautology, when we reword it as follows: Size of lateral and ventral ornamentation correlates with size of the aperture (its width and height, respectively). In a shell that is strongly compressed laterally, the diameter of the aperture, and hence the soft parts within, is small in the lateral direction. For such a shell, coarse lateral ribbing would imply impossibly strong relative compression and expansion of the soft body through the formation of a rib. Conversely, if the shell is depressed (compressed dorsoventrally), the diameter of the soft parts is small in the dorsoventral direction, and the ventral ribbing correspondingly small. Buckman's first law is thus simply a statement of proportionality that needs no special explanation. On the contrary, any departure from this ‘law’ would imply strong deformations of the soft body as ornamentation was formed. Measurements on the Triassic ammonite Pseudodanubites halli show that ventral ribs get relatively stronger on compressed specimens, in accordance with this view of Buckman's law. ? Ammonoids, intraspecific variation, morphometry, ornamentation.     

Ontogeny of shell morphology and shell strength of the marine snails Littorina obtusata and Littorina mariae: different defence strategies in a pair of sympatric, sibling species

The sibling marine snails Littorina obtusata (L.) and Littorina mariae Sacchi & Rastelli are sympatrically distributed and the shells of both species are subject to similar breaking forces by predatory crabs. Nevertheless, the two species exhibit rather different growth and defence strategies. To determine growth patterns, we measured changes in five morphological variables with increasing shell length: body whorl thickness at the point of crushing force application, shell height (related to globosity), shell mass, body mass, and apertural lip thickness. We also measured ontogenetic changes in the ability to withstand shell crushing. For most morphological variables, L. mariae showed uniformly allometric growth of juveniles into adults. In contrast, L. obtusata usually exhibited a distinct change in growth pattern upon reaching maturity. As adults, L. mariae showed a more sustained increase in overall shell mass and in body whorl thickness (defence against crushing attacks) and also had proportionally thicker apertural lips (defence against peeling attacks). Littorina obtusata , however, grew to a larger size and their shells could accommodate larger bodies at all sizes. Furthermore, the strength of L. obtusata shells increased faster than could be accounted for by either overall shell mass or thickness at the point of force application, suggesting strengthening by other means such as changes in shell microstructure or shape (other than globosity). These results illustrate the viability of two contrasting antipredator strategies, despite a highly similar phylogenetic history and selective regime.     

Allometric growth and intraspecific variability in the basal Carboniferous ammonoid<Emphasis Type="Italic">Gattendorfia crassa</Emphasis><Emphasis Type="SmallCaps">Schmidt</Emphasis>, 1924

Gattendorfia crassa is an Early Carboniferous (Mississippian) goniatite species with strikingly allometric conch growth. Analysis of 15 high-precision cross-sections of this species demonstrates the small intraspecific variability of some of the conch form characters, but remarkable variability in others. While the whorl expansion rate, umbilical width, and conch thickness vary within narrow limits, the expansion rates of the whorl height and whorl width are remarkably plastic. Variability of most of the characters tends to be smallest in intermediate growth stages, whereas juveniles and adults are more variable. The differences in morphological plasticity are interpreted in terms of the function of the ammonoid conch, especially the orientation of the aperture during life.      

ADAPTATION FROM RESTRICTED GEOMETRIES: THE SHELL INCLINATION OF TERRESTRIAL GASTROPODS

The adaptations that occur for support and protection can be studied with regard to the optimal structure that balances these objectives with any imposed constraints. The shell inclination of terrestrial gastropods is an appropriate model to address this problem. In this study, we examined how gastropods improve shell angles to well‐balanced ones from geometrically constrained shapes. Our geometric analysis and physical analysis showed that constantly coiled shells are constrained from adopting a well‐balanced angle; the shell angle of such basic shells tends to increase as the spire index (shell height/width) increases, although the optimum angle for stability is 90° for flat shells and 0° for tall shells. Furthermore, we estimated the influences of the geometric rule and the functional demands on actual shells by measuring the shell angles of both resting and active snails. We found that terrestrial gastropods have shell angles that are suited for balance. The growth lines of the shells indicated that this adaptation depends on the deflection of the last whorl: the apertures of flat shells are deflected downward, whereas those of tall shells are deflected upward. Our observations of active snails demonstrated that the animals hold their shells at better balanced angles than inactive snails.     

Variation in foot size and shape in some British land snails and its functional significance

Live weight, and the length, breadth and surface area of the extended foot sole were measured in 24 species of British terrestrial snails. Allometric relationships between these variables are described, concentrating on the relationship between foot surface area and weight. Intraspecific foot area/weight relationships deviate from isometric expectation for several species; this may be attributed to density changes, foot-shape variation or the physical constraint imposed by shell aperture size.
The rate of increase of foot sole area with live weight among species is greater than expected under isometry, indicating that larger species partially compensate for the increase in foot loading (weight per unit foot area). The rate of whorl expansion of the shell is related to the deviations from the interspecific foot area/weight relationship, reinforcing the possibility that shell aperture area may limit foot size in some species. Foot ratio (foot length:foot width) is negatively related to live weight and foot loading.
The findings are discussed in relation to the known behaviour and microdistribution patterns of the species.     

Shape variation in the rough periwinkle Littorina saxatilis on the west and south coasts of Britain

Variation in the shape of the shell in Littorina saxatilis Olivi has been shown to be due largely to the same variables on both the west and the south coasts of Britain, and it exhibits various clines. Two important aspects are the size of the aperture, which becomes relatively larger from the Isle of Man southwards to Cornwall and eastwards from Devon to the Isle of Wight, and the jugosity of the shell, which increases with distance from Cornwall both northwards as far as the Isle of Man and eastwards as far as Kent. Superimposed on the clines are domains of shape, notably one in Lewis/Harris, where the shells have a relatively large aperture, which is long and narrow, coupled with a rather globose second whorl. The local and geographical aspects of shell shape variation are discussed.     

Body size variation in the sexually dimorphic scaphopod Rhabdus rectius (Carpenter, 1864) (Dentaliida: Rhabdidae)

ABSTRACT

Male-biased sexual size dimorphism typically evolves via sexual selection for larger males that are favoured by choosy females or are more successful in mate competition with other males. Among marine invertebrates that broadcast their gametes into the ocean for fertilisation, this form of sexual size dimorphism is rare because such species lack direct interactions among males or between the sexes. However, the broadcast-spawning tusk shell Rhabdus rectius was recently reported to show strong male-biased sexual size dimorphism. That pattern might imply interesting and undiscovered sexual selection in this species. We found instead that the distribution of body size variation (weight, shell length) was similar between males and females of R. rectius, and mean sizes were not different between the sexes. However, we noted a male-biased sex ratio (～1:1.3) in our large sample of individuals. Many live scaphopods (and several dead shells) showed partial or complete boreholes drilled by predatory gastropods. Boreholes were observed on males and females in similar proportions. We collected scaphopods along with multiple individuals of one likely scaphopod predator, the small moon snail Euspira pallida, and in the lab we observed successful attacks by moon snails on tusk shells.     

Appearance of morphological novelty in a hybrid zone between two species of land snail

On the small oceanic island of Chichijima, two endemic species of land snails, Mandarina mandarina and M. chichijimana, have discrete distributions separated by a hybrid zone. This study investigates the potential of hybridization as a source of morphological novelty in these snails. Mandarina mandarina possesses a shell with a higher whorl expansion rate and a smaller protoconch than M. chichijimana, relative to shell size. The number of whorls and shell size of M. mandarina do not differ from those of M. chichijimana, because the effect of higher expansion rate on number of whorls and size of the former is compensated for by its smaller protoconch. The whorl expansion rate and protoconch diameter of the individuals from the hybrid populations are intermediate or typical of either of the two species, and their average values show clinal changes along the hybrid zone. However, the hybrid populations include exceptionally high shells with many whorls and flat shells with few whorls, which are never found in the pure populations of either species. In addition, gradual increases in variance in shell height and number of whorls were found from the edges to the center of the hybrid zone. A combination of low expansion rate (typical of M. chichijimana) and a small protoconch (typical of M. mandarina) produces a shell with an extremely large number of whorls because of the geometry of shell coiling. However, the combination of high expansion rate and a large protoconch produces a shell with an extremely small number of whorls. Because of the correlation between the number of whorls and shell height, shells with an exceptional number of whorls possess an extraordinarily high or flat spire. Hybrids can inherit a mosaic of characters that, as they play out during growth, lead to novel adult morphologies. These findings emphasize the importance of hybridization as a source of morphological variation and evolutionary novelty in land snails.     

Shell features, main colonized environments, and fractal analysis of sutures in Late Jurassic ammonites

A precise approach to the quantification of relationships between suture complexity, as measured by fractal analysis (step-line procedure), the architecture of shells, and the main colonized environments, has been made in a set of Late Jurassic ammonites ( N =507). Statistically significant differences between fractal-dimension ( D _f) mean values of evolute and involute shells are interpreted as caused by differences in the surface:volume ( S:V ) ratio. Suture complexity is also related to the shape of whorl section. The lowest D _f values correspond to subcircular whorl sections (low S:V ratio) and the highest ones to acute sections (high S:V ratio). The shape of flanks shows correlation with suture complexity. The highest values of D _f are found in planulate shells and the lowest ones in whorl cross sections with convex flanks. Highly significant differences appear between D _f mean values from unsculptured shells and those from ammonites with ribs and/or tubercles of medium to large size. Multivariate analysis shows a combined variation of shell features and suture complexity, resulting in a heterogeneous distribution of D _f values within the ammonite morphospace, mainly according to structural (shell architecture) and ornamental (sculpture strength rather than density) factors. Finally, the data obtained on relationships between suture complexity and the colonized environments (epicontinental vs. epioceanic inhabitants) suggest that suture complexity is not primarily related to bathymetry, and/or that no major differences in habitat depths existed between epicontinental and epioceanic ammonites.     

Investigation of Shell Characteristics in <Emphasis Type="Italic">Chondrula tridents</Emphasis> (Pulmonata,Enidae) from Highland and Plain Habitats of the Forest–Steppe of the Volga River Basin

Variability of the shell size characteristics was studied in the terrestrial mollusk Chondrula tridens (Müller, 1774) from highland and plain habitats in the forest–steppe of the Volga River basin. The shell height (SH) and width (SW), the height and width of the mouth, and the SH/SW ratio mostly contribute to the species variability. The SH/SW ratio reflects the species–specific constitution of the shell. Analysis of the elementary features and proportions, as well as of the mouth armament of shell structures, indicates significant heterogeneity of the species. The presence of scarps and rock outcrops in some regions studied explain large sizes of snail shells. The decrease in SH, increase in the roundness of shells, and better development of the mouth teeth in mollusks inhabiting the chalk steppe of the southern forest–steppe reflect their adaptation to more xerothermic conditions.     

Dislodgement force and shell morphology vary according to wave exposure in a tropical gastropod (Cittarium pica)

Wave exposure has strong influences on population density, morphology and behaviour of intertidal species in temperate zones, but little is known about how intertidal organisms in tropical regions respond to gradients in wave exposure. We tested whether dislodgement force and shell shape of a tropical gastropod, Cittarium pica, differs among shores that vary in wave exposure. After adjusting for body size, we found that C. pica from exposed shores required greater dislodgement force to remove them from the shore, had slightly larger opercula (the closure to the shell aperture), and were slightly squatter in shape (reduced in shell height relative to shell width) than C. pica from sheltered shores. These morphological adjustments are consistent with those observed in temperate gastropods, which are argued to represent adaptive responses to the risk of mortality associated with dislodgement.     

红眼寄居蟹在实验室和野外条件下对贝壳的利用

占据适宜的贝壳对于寄居蟹的发育、繁殖和存活至关重要。尽管很多研究探讨了蟹类对贝壳的选择,但寄居蟹选择多大的贝壳仍不清楚。在实验条件下,本文作者用巴西Anchieta岛上野生红眼寄居蟹( Pagurusbrevidactylus)最常利用的黑衣蟹守螺( Cerithium atratum)和节桑椹螺( Morula nodulosa)进行了贝壳选择实验,通过回归分析确定目标贝类及其大小。观察到寄居蟹对一种独特贝类的选择具有性别意义,从而验证了野外观察结果。雄性明显地倾向选择黑衣蟹守螺的贝壳,而产卵和非产卵的雌性个体对腹足类贝壳的选择差异不显著。尽管两性之间对适宜贝壳的选择存在差异,贝壳适宜度指数(SAI)表明,种群占据那些足够大的贝壳(SAI =1·20±0·23)。红眼寄居蟹对贝壳利用的这种模式可能是为了避免与体型相似同域物种的竞争,从而在后来的生长中减少频繁地更换贝壳。根据目前的数据可以得到以下结论:红眼寄居蟹对贝壳的选择不仅取决于贝壳的参数,而且还与寄居蟹的个体和性选择有关[动物学报51 (5) : 813 -820 , 2005]。     

Fossil embryos and adult Viviparidae from the Early–Middle Pleistocene of Gesher Benot Ya'aqov, Israel: ecology, longevity and fecundity

Ashkenazi, S., Klass, K., Mienis, H.K., Spiro, B. & Abel, R. 2010: Fossil embryos and adult Viviparidae from the Early–Middle Pleistocene of Gesher Benot Ya'aqov, Israel: ecology, longevity and fecundity. Lethaia, Vol. 43, pp. 116–127.
A unique fossil assemblage of the extinct freshwater viviparids Viviparus apameae galileae and Bellamya sp. with intact embryos enabled the assessment of population ecology parameters applied so far only to recent living populations. The assemblage, from the Early Middle Pleistocene site of Gesher Benot Ya'aqov, occurs in the sedimentary sequence above the Matuyama Brunhes Boundary (MB boundary) at 0.78 Ma. This fossil assemblage is related to a shoreline habitat that was affected by desiccation during a sudden drop in water level. In both species, the youngest females with embryos had five or more whorls, with heights of >24.8 and >16.1 mm in Viviparus and Bellamya respectively. Comparison of shell height and width, aperture height and width and the ratio of shell height/width indicated that females are significantly longer and larger than the shells without embryos. Longevity was estimated as at least 9 years for Viviparus and 8 years for Bellamya and lifetime fecundity of female viviparids was estimated based on longevity and reproductive period. The high number of reproducing females and high number of embryos indicate favourable habitat and climate conditions for both Viviparidae. The fossil Viviparidae from GBY have the characteristics of organisms that are K-selected by natural selection forces. □ Fecundity , fossil embryos , longevity , molluscs , Pleistocene , reproductive output , sexual dimorphism , viviparids.     

江西弯螺属一新种记述(肺螺亚纲,柄眼目,扭轴蜗牛科)

在整理江西地区陆生贝类标本时,经比对鉴定发现1新种,龙潭弯螺Sinoennea longtanensis sp.nov.,隶属肺螺亚纲、柄眼目、扭轴蜗牛科、弯螺属。对新种形态特征、栖息环境作了记述,并与其近似种进行讨论。     

The role of growth,predation, and habitat selection in the population distribution of the crown conch,Melongena corona Gmelin

The intertidal gastropod Melongena corona Gmelin exhibits a size gradient along the shore as a function of habitat. Small, juvenile snails were found on the sand beach and larger adults were found on hard substratum habitats (shell rubble beach and oyster bar). A transplant experiment was performed to test three proximate explanations for this pattern:

• 1.(1) differential growth rate of conchs in different habitats,
• 2.(2) differences in predation intensity between habitats and
• 3.(3) active habitat selection by snails.

Shell scars on the last shell whorl were used as an index of predation pressure. Growth and predation were not significantly different for snails of similar size in different habitats, but snails were found to return to their original habitat when displaced. The ability to home did not differ between sexes or juveniles and adults. Active habitat selection appears to be a significant proximate factor maintaining the population distribution. A number of potential ultimate causes of the size class segregation are suggested.     

广东省福寿螺形态性状变异及其与体质量的关系

【背景】福寿螺是危害极其严重的入侵我国的水生生物,目前利用相关分析、通径分析对福寿螺形态性状变异的研究较少。【方法】随机采集7个地区的福寿螺(雌雄比例差别很大),测量其壳高、壳宽、口宽、层高4个形态性状和体质量,采用相关分析、通径分析方法计算福寿螺的各形态指标变异系数、相关系数和通径系数,剔除对体质量影响不显著的指标,确定每个地区与体质量最相关的指标。【结果】除惠州之外,其余地区福寿螺的壳高和壳宽与体质量的相关性较高,且口宽与壳高共同作用对体质量的直接影响很大。【结论与意义】此结果为研究不同生境、不同温度下福寿螺的形态性状变异程度奠定了基础,同时为福寿螺形态鉴定、分类及其灾害预测预报提供了基础数据。     

Influence of prior experience on shell selection by the white spotwrist hermit crab Pagurus criniticornis (Crustacea: Paguridae)

Individuals of Pagurus criniticornis in a free-choice situation were experimentally tested under different laboratory conditions. In order to assess the effect of recently occupied shells on the size- and type-preference by hermit crabs, individuals were held for 30 days under one of the following two conditions: (1) excess of shells and (2) absence of shells. The crabs were then allowed to select shells from a wide array of empty gastropod shells of the two most-occupied species, as observed previously in the field: Cerithium atratum and Morula nodulosa. Preferred shell type (species) and size (shell aperture width and length) were correlated with hermit-crab size. The crabs showed a strong (100%) preference for C. atratum shells, demonstrating that recent and past experience did not influence either shell-type or shell-size preferences in this pagurid. Handling editor: K. Martens     

Associations with habitat versus geographic cohesiveness: size and shape of Bembicium vittatum Philippi (Gastropoda: Littorinidae) in the Houtman Abrolhos Islands

In the Houtman Abrolhos Islands, Western Australia, the direct-developing littorine snail Bembicium vittatum occupies a wide range of habitats, which are replicated across the three major groups of islands. Earlier studies showed that allozyme similarities followed patterns related to gene flow, independent of habitat, providing an excellent opportunity to test for associations with habitat for traits more likely to be adaptively important. As the first test for adaptive divergence, we examined variation in size and shape of shells among 131 populations in the Abrolhos Islands. Two-fold variations were found in width of adults, the allometric coefficient of height with width, and shell height scaled to a standard width. Quantification of habitat characteristics was summarized by principal components analysis. In contrast with the patterns of divergence for allozymes, shell height, adjusted for width, was strongly associated with habitat: flatter shells are found on exposed, vertical shores, while domed, more globose shells predominate in sheltered sites. This association was stronger for shape of adult-sized snails than for height scaled to an arbitrary size, highlighting the importance of using biologically relevant measures. Even highly isolated and allozymically less variable populations in tidal ponds conformed to this association. Because differences in shape are highly heritable in B. vittatum , this association of shape with habitat, independent of patterns of gene flow, indicates local adaptation. Shell size also varied with habitat, but because growth rate is highly plastic, variation in size cannot be interpreted simply in terms of adaptation. Nevertheless, the pattern of variation indicates that, within realized limits, larger size is generally favourable, but may be constrained by local conditions. Thus, variation in size signals the potential for adaptive divergence of life histories among the many, isolated populations of this species.