Geographical variation in shell shape of the pod razor shell Ensis siliqua (Bivalvia: Pharidae)

The present study assessed the existence of variation in the shell shape of the pod razor shell (Ensis siliqua) throughout its distributional range in the north- eastern Atlantic. Shells of E. siliqua caught at seven collecting sites (three in Portugal, three in Spain and one in Ireland) were studied by geometric morphometric methods, using both landmark- and contour-based methods. Both approaches (landmarks inside the valves and shell outline) discriminated the shells from Aveiro (centre of Portugal) and Strangford Lough (Ireland) from those caught in the nearby localities (remaining Portuguese and Spanish sites,maximum distance of 550 km by sea). Landmark analysis revealed that shells from Aveiro were more similar to shells from Ireland (*1,500 km far away). Contour anal- ysis revealed that shells from Aveiro had a shape with a comparatively larger height-to-width ratio, whereas shells from Ireland showed a slightly more curved outline than in the remaining sites. Landmark- and contour-based methods provided coherent complementary information, confirming the usefulness of geometric morphometric analyses for discerning differences in shell shape among populations of E. siliqua. A brief review of previous applications of geometric morphometric methods to modern bivalve spe- cies is also provided.The authors would like to thank Dr. Dai Roberts and Adele Cromie for providing samples of pod razor shells from Ireland. This study was funded by Community Initiative Programmes (INTERREG-IIIB, Atlantic Area) Sustainable HARvesting of Ensis (090–SHARE) and Towards Integrated Management of Ensis Stocks (206–TIMES) from the European Community. Marta M. Rufino and Paulo Vasconcelos benefited from postdoctoral grants (SFRH/BPD/14935/2004 and SFRH/BPD/26348/2006, respectively) awarded by the Fundação para a Ciência e Tecnologia (FCT—Portugal). Finally, the authors acknowledge three anonymous referees for valuable comments and suggestions that greatly improved the revised manuscript.publishe

Generic aspects of molluscan shell morphogenesis: theoretical, experimental and comparative approaches

How an embryo develops its particular form during ontogeny and how shape changes through evolutionary time are two closely linked questions. An approach to these issues, mainly inspired by D'Arcy Thompson's work, is to highlight the 'laws of form', that is how developmental systems determine the variation of organismal forms on short and long time scales. During the last decades, theoretical models of morphogenesis have allowed the identification of some of these rules from experimentally well-studied developmental systems. Molluscs are also well suited to address the relationships between evolution and development. The preservation of the ontogeny of the shell due to its accretionary growth and the excellent fossil record in this group are undeniable advantages. Also, molluscan shell shape and growth have been the focus of extensive theoretical work, revealing the regularity of accretionary growth which often conforms well to logarithmic spiral coiling. The study of evolutionary changes occurring in mollusc lineages relies nearly exclusively on the interpretation of shell morphologies. Important taxonomic features of molluscs include the shape of the aperture, the degree of coiling of the shell tube, the ornamentation (ribs, tubercles, spines, keels) and growth features (growth halts, constrictions, varices). The evolution of the molluscan shell is characterized by frequent convergences in form and ornamentation. As a consequence, the recognition of transformation of one shape into another crucially depends on the knowledge of how these shell shapes are generated. The comparison between different clades of molluscs can be informative with regards to the basic rules of accretionary growth. In particular, it has been pointed out that common rules of accretionary growth could underlie the morphogenesis of the shell and its evolution in ammonoids and gastropods. Evidences come from the comparison of intraspecific patterns of covariation between shell characters, from the examination of growth changes occurring at maturity and from the analysis of teratological shells with malformations caused by injuries or change in living conditions in both clades. In some highly variable ammonoids species, it has been shown that simple growth rules could underlie the evolutionary recurrent covariation of aperture shape, degree of coiling and intensity of the ornamentation (Buckman's law of covariation). Similarly, these characters covary with the spacing between growth halts during the ontogeny of some ammonoids species. A central objective of this thesis is to investigate what kinds of generic rules could produce the patterns of variation of molluscan shell shape. In a first part, it is discussed how generic models can inform us about the generation and evolution of structures of particular size and shape. In a second part, a null hypothesis model of shell growth is proposed. The intricate relationships between growth rate and allometry are described. The kind of morphological variation expected given these basic growth rules is compared to experimental evidence in developmentally plastic shells of intertidal gastropods. A population of recent gastropods (Hexaplex trunculus, Muricidae), originated from a single egg mass and bred in laboratory for about a year and a half is used to describe the ontogenetic patterns of covariation between shell characters and the dynamics of growth. This study highlights a covariation between growth rhythm (frequency and amplitude of pulses of growth), growth halts spacing, aperture allometry and intensity of ornamentation. In particular, variation in growth rhythm is regarded as critical in generating the observed covariation between growth halts spacing and ornamentation. A simple growth model is proposed to account for the integration of the covariation of these shell characters. Some recurrent patterns of variation in ammonoids species could result from similar rules tied to basic constraints of accretionary growth. The theoretical and empirical framework developed here can assist in formulating and testing new hypotheses of growth of molluscan shells. It paves the way toward the development of data-driven mathematical models which could facilitate the comparison of theoretical and empirical data in the future, and perhaps helps interpreting them in a developmental, ecological or evolutionary context. More generally, this dissertation argues that the time parameter is mandatory to the study of allometry, if one seeks to understand the relationships between size and shape and how they vary in populations

Snail shell coiling (re-)evolution and the evo-devo revolution

International audienceDuring the last two decades evolutionary developmental biology has become a major research programme whose findings put into question some concepts lying at the core of the _Synthetic Theory_. However, some authors are waiting for a _revolution_ in biology, one in which the existing genetic determinism will give way to a new conceptual understanding of the complexity of living organisms. This _revolution_ should necessarily pass through the elaboration of an appropriate theoretical framework integrating the non-linear dynamics of development as its fundamental basis. This objective implies a drastic shift in the way causality is generally understood as well as a purge of numerous convenient but misleading metaphors such as genetic or developmental programmes. Although most authors do not take these metaphors too literally, some persist in employing such _instructionist_ notions in a more literal perspective, and, in doing so, deny some concepts at the core of evolutionary developmental biology. We critically review two recent studies suggesting that shell coiling has re-evolved in a family of limpets (Calyptraeidae, Gastropoda). We stress that this putative re-evolution of snail shell coiling results only from an arbitrary scoring procedure leading us to consider shell coiling as a binary discrete character. We show that the way in which these authors connect this case study to evolutionary theories stems from the unwarranted premise of a linear mapping of genes onto phenotypes where particulate inheritance of morphological characters seems implicitly assumed. We illustrate how the persisting unclear role of genes in morphogenesis allows the maintenance of the adaptationist programme

Growth-dependent phenotypic variation of molluscan shells: implications for allometric data interpretation

In recent years, developmental plasticity has received increasing attention. Specifically, some studies highlighted a possible association between shell shape and growth rates in intertidal gastropods. We use a growth vector model to study how hypothetical growth processes could underlie developmental plasticity in molluscs. It illustrates that variation in instantaneous shell growth rate can induce variability in allometric curves. Consequently, morphological variation is time-dependent. Basing our model parameters on a study documenting the results of transplants experiments of three gastropods ecomorphs, we reproduce the main aspects of the variation in size, shape, and growth rates among populations when bred in their own habitat or transplanted to another ecotype habitat. In agreement with empirical results, our simulation shows that a flatter growth profile corresponds to conditions of rapid growth. The model also allows the comparison of allometric slopes using different subdata sets that correspond to static and ontogenetic allometry. Our model highlights that depending on subdata sets, the main effects could be attributed to source population or environment. In addition, convergence or divergence of allometric slopes is observed depending on the subdata sets. Although there is evidence that shell shape in gastropods is to some extent growth rate dependent, gaining a general overview of the issue is challenging, in particular because of the scarcity of studies referring to allometry. We argue that the dynamics of development at the phenotypic level constitute a non-reducible level of investigation if one seeks to relate the observed amount of phenotypic variation to variability in the underlying factors

Synchrotron-aided reconstruction of the conodont feeding apparatus and implications for the mouth of the first vertebrates

The origin of jaws remains largely an enigma that is best addressed by studying fossil and living jawless vertebrates. Conodonts were eel-shaped jawless animals, whose vertebrate affinity is still disputed. The geometrical analysis of exceptional three-dimensionally preserved clusters of oro-pharyngeal elements of the Early Triassic Novispathodus, imaged using propagation phase-contrast X-ray synchrotron microtomography, suggests the presence of a pulley-shaped lingual cartilage similar to that of extant cyclostomes within the feeding apparatus of euconodonts (“true” conodonts). This would lend strong support to their interpretation as vertebrates and demonstrates that the presence of such cartilage is a plesiomorphic condition of crown vertebrates