Origin, morphology and fate of the nonmineralized shell of Coryphella salmonacea, an opisthobranch gastropod

Abstract

Little is known about initial shell formation in molluscs or about the possible functions of the embryonic shell field invagination (SFI). The present paper describes formation and loss of an unusual shell in embryos of the nudibranch Coryphella salmonacea (Couthouy, 1838), collected in February–March, 1981–1984. The first organic shell material was attached to shell field cells just outside the SFI at two weeks after fertilization (5°C). As viewed with transmission electron microscopy, the organic shell in section consisted of two electron dense layers having a total thickness of 16 nm. The shell continued to grow through at least the fifth week of encapsulated development. In the eighth week, the shell still covered the visceral mass; there was no evidence that mantle tissue reflexed and covered the shell or that shell materials were resorbed by the mantle. The shell and sometimes the operculum were discarded within the embryonic capsule prior to hatching at the end of Week 8. The cup-like shell is unusual in that it was shed as a wrinkled organic sheet which apparently lacked mineralization. Fully-developed shells were non-birefringent and did not produce calcium peaks when examined by electron microprobe. Both the location of the first organic shell material outside the SFI and the apparent absence of shell mineralization in the presence of an SFI are consistent with the hypothesis that the SFI is nonsecretory. C. salmonacea is probably in the evolutionary process of losing its shell, a structure of little or no function during encapsulated, nonplanktonic development.