THE RELATIONSHIP OF LARVAL SHELL MORPHOLOGY TO MODE OF DEVELOPMENT IN MARINE PROSOBRANCH GASTROPODS

The larval shell of molluscs contains a preserved record of the organism's growth and developmental history. Many investigators have utilized the information recorded in the larval shell morphology to infer mode of development. Marine prosobranch gastropods were collected from intertidal sites from Maine to Florida and were cultured in the laboratory. Scanning electron microscopy was used to document the relationship of larval shell morphology to development. Four species with planktotrophic larvae, Crepidula fornicata, Crepidula plana, Cerithium atratum and Ilyanassa obsoleta, and four species with non-planktotrophic development, Crepidula convexa, Littorina obtusata, Busycon canaliculatum and Urosalpinx cinerea, were successfully reared through metamorphosis. The size of the protoconch I was significantly correlated with egg diameter. Planktotrophic larvae developed from smaller eggs with a smaller protoconch I than did larvae of species with non-planktotrophic development. The boundary between the protoconch I and the protoconch II at hatching was clearly demarcated in shells of planktotrophic larvae. No change in shell morphology was observed at metamorphosis in larvae of Crepidula fornicata and Crepidula plana. However, a clearly demarcated boundary between the protoconch II and the teleoconch was visible in specimens of Ilyanassa obsoleta. In species with non-planktotrophic development, Crepidula convexa, Busycon canaliculatum and Urosalpinx cinerea, the transition from the protoconch I to the teleoconch occurred before hatching. However, in specimens of Littorina obtusata the teleoconch phase of shell growth was not evident before hatching. In some specimens of Littorina obtusata, breaks in the pattern of the surface shell morphology of the protoconch I were observed. The variability observed in the surface shell morphology of the prosobranchs examined in this study stresses the need for investigators to make comparisons with confamilial or congeneric species of known development when inferring mode of development. © 1990, Marine Biological Association of the United Kingdom. All rights reserved.