Evolution of shell loss in Opisthobranch gastropods: sea hares (Opisthobranchia, Anaspidea) as a model system

Due to the advantages of the hard, calcifying shell, the Mollusca are one of the most successful animal phyla. The shell forms during embryonic and larval development; however, many molluscan groups have a highly reduced shell or have lost it completely as development and maturation proceeds. These major developmental transitions in shell morphology frequently correlate with ecological transitions (e.g. diet change/change from planktonic to bethic existence pre- and post-metamorphosis, respectively). While shell loss may leave an organism vulnerable to predation, many have evolved alternative means to deter predators. Here we compare and contrast the post-hatching larval development and shell growth through the use of the life cycle staging of Bursatella leachii and Aplysia californica in laboratory settings. The larval developmental sequence of B. leachii is indistinguishable to other previously described plankotrophic aplysiids. However, the growth rate and size of B. leachii larvae differ from A. californica larvae substantially, growing relatively faster and larger by an average of 10 μm. We also describe the life cycle of B. leachii in context of the development of the larval shell and its subsequent loss in post-metamorphic stages. Comparison of the Stage 6 shells, both whole and cross-sections, of A. californica and B. leachii through the use of SEM showed little difference in morphology. These data indicate that we have established a reliable culturing technique for B. leachii in the laboratory which makes this species can be easily amendable to experimentation at all developmental stages. Metamorphosis and shell loss/reduction in A. californica and B. leachii highlight the differences of the developmental program of both species, which reflects its complexity at a molecular, cellular and organismal level. The comparison of sea hares is an ideal evolutionary comparative model system for the loss of acquired features. Molluscan biomineralization has been of broad scientific interest ranging from paleontological (molluscan shells provide one of the best fossil records for a metazoan phylum), to material science (perl and nacre formation) research. Although the properties (i.e. evolutionary origins, construction, patterning, physical) of the molluscan shell have been studied for decades, the underlying molecular and cellular mechanisms of how shell formation occurs are just recently surfacing with the identification of a handful of shell forming proteins. It is now known that one of the main components involved in the control of shell synthesis are the proteinaceous constituents of the shell matrix with in different kinds of functions (i.e. cell signaling, enzymatic activities), which are contributing to the diversity of different shell types in gastropod, bivalve and scaphopod molluscs. However, the differential gene expression and regulation within the mantle still remains unknown. Here we relate the developmental expression of eleven genes present in the mantle, the organ responsible for the secretion of the shell, in the sea hare Aplysia californica (Opisthobranchia, Anaspidea). Six genes that show very little changes in expression levels (Cluster 1). Three genes shows increased levels of expression during trochophore and veliger stages which then decrease in metamorphic stages (Cluster 3). Two genes had peptide-like profiles, genes that low expression during early development but have high expression levels late in development (Cluster 4). All eleven genes display dynamic spatial and temporal expression profiles within the larval shell field and mantle for the construction of the larval shell. The expression data from these eleven genes reflect the regulatory complexity that underlies the molluscan shell construction during larval stages. While the fabrication of the shell is taking place, the incorporation of both ancient and novel genes during also suggest that there is a core set of mantle-secreting genes for shell construction was provided by a shared metazoan ancestor to produce the range of molluscan shell types we see today