Molluscan Metamorphosis: A Study in Tissue Transformation

Metamorphosis of the pelagic larvae of benthic marine invertebratesis often a cataclysmic event in which a rapid loss of organsspecialized for larval life occurs simultaneously with the renewalor increased rate of development of potential adult organs.In the nudibranch gastropod Phestilla sibogae this change involvesloss of the velum, shell, operculum, larval kidney, some retractormuscles, and some of the pedal mucous glands. Exit from thelarval shell at metamorphosis is rapid and is correlated withthe spread of epidermis from the larval foot over the visceralmass as the visceral mass emerges from the shell aperature.This spreading of epipodial epidermis to cover the entire bodyhas not been previously reported for other nudibranchs. Neithercell proliferation nor active cell motility are responsiblefor this epidermal migration. Rather it appears that the actionof larval muscles pulls the visceral organs out of the shelland simultaneously causes the epipodial epidermis to cover thevisceral mass. This epidermis becomes the definitive adult epidermis.