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International audience; Spicules are mineral-based biocomposites skeletal structures that are widely distributed among phylogenetically distant groups of invertebrates (Porifera, Cnidaria, Mollusca, Echinodermata). Subepidermal spicules are formed under the ectodermal epithelium and are characterized for all groups except mollusks (Aplacophora, Polyplacophora, Bivalvia), their spicules are located on the surface of the body. However, one group of mollusks (Gastropoda: Heterobranchia) have unique subepidermal spicules that have never been detected above the ectodermal epithelium and similarly to those characterized for Porifera, Cnidaria and Echinodermata. Understanding subepidermal spicule formation in mollusks could help solve the question on the origin of spicules. Spicules in nudibranchs have been described for more than 150 years, yet ontogenetic dynamics of spicules have never been studied and the full mechanism of their formation remains unknown. Herein we investigate the spicule formation in different stages of postlarval development of the nudibranch Onchidoris muricata (O.F. Müller, 1776). For the first time, ontogenetic transformations of the spicule complex are described using experiments and different morphological methods. Our studies demonstrate that spicules of O. muricata form in the subepidermal space in early developmental stages immediately after veliger settlement. A single spicule forms inside a huge vacuole within a sclerocyte and remains there throughout the entire life of the specimen. Signs of spicule or sclerocyte migration under the epithelium in postlarval development was not found. Spicules only form during larval settlement, increasing only in size as development furthers. For the first time, spicule mineralization zones were detected at the tips of the spicules as well as the presence of collagen I in the overall composition of the spicules. Thus, our findings suggest that spicules form by an ectodermal cell that emerged under the ectodermal epithelium during the earliest stages of postlarval development.

A total of 13 enzymes, coded by 14 loci, were successfully examined by horizontal starch gel electrophoresis in the intertidal dorid nudibranchs Adalaria proxima (Alder & Hancock) and Onchidoris muricata (Muller). The derived genetic identity and similarity values provided estimates more typical of congeneric, than only confamilial, species. The suggestion is that these two species hold a recent common ancestry, and that Adalaria proxima (with its more advanced, pelagic lecithotrophic larva) is an evolutionary derivative of the Onchidoris muricata stock. At least three loci ( Pgm and Fum, A. proxima; Pep-2, O. muricata ) are all highly polymorphic.

The nudibranch molluscs Adalaria proxima and Onchidoris muricata are semelparous annuals. Although these species are ecologically, morphologically, and taxonomically similar, their reproductive characteristics differ markedly. In the present study, A. proxima individuals generally attained a larger size and commenced spawning later in the spring than O. muricata . The total energy invested in spawn production by A. proxima individuals was ≈ 1.5 times that for O. muricata , however, A. proxima produced this in fewer spawn masses and over a shorter spawning period. Correspondingly, the mean energy content of A. proxima spawn masses was greater (≈ 3 times) than that of O. muricata spawn. Nonetheless, the “average” A. proxima spawn mass contained only an estimated 675 ova in comparison to the 3300 ova in an “average” O. muricata spawn mass. This difference was due to the contrasting larval types exhibited by these species. A. proxima produces large well-provisioned ova which hatch as pelagic lecithotrophic veligers, whereas O. muricata ova are relatively small and develop into long-term planktotrophic larvae. Individuals of both species were observed to undergo degrowth throughout their spawning periods, but total production (soma plus cumulative spawn) continued to rise at a rate equal to, or slightly greater than, prespawning growth rate. Overall production patterns showed considerable intraspecific consistency, but differed between species. Degrowth rate (per day) for A. proxima was in excess ofthat for O. muricata , however, all individuals died after ≈33% of the maximum somatic weight had been lost.