VESTIGIAL LARVAL SHELLS IN THE PLANKTONIC VELIGERS OF 2 GYMNODORID NUDIBRANCHS

This paper reports the occurrence of simple, cup-shaped larval shells devoid of whorls in the planktonic veligers of two gymnodorid nudibranchs, Gymnodoris striata Eliot, and an undescribed Gymnodoris sp. from Enewetak, Marshall Islands. Features of the anatomy, developmental history, and larval biology of these unusual veligers suggest that their shells are of reduced importance in functioning to protect them during the larval period, and that the shell type is vestigial. Both species lack an operculum, are unable to retract into the shell, possess a foot and velar lobes which are exceptionally large relative to the shell size, and share the unusual habit of completely enveloping the shell with the foot for extended periods. G. striata may also lack an obligatory planktonic period. Although first reported for G. striata by Risbec (1928), the occurrence of cup-shaped "patelliform" larval shells in nudibranchs with indirect development has been overlooked by subsequent authors. The widely used classifications scheme of Thompson (1961) recognizes only two types of larval shells: type 1, spiral shells forming 3/4 to 1 whorl: and type 2, egg-shaped "inflated" shells. Aside from G. striata, cup-shaped shells have been reliably reported only in the transitory, intracapsular veliger stages of several species which undergo direct development (Soliman, 1977). Recognition of cup-shaped shells as vestigial is consistent with the view of Thompson (1961) that they do not constitute a natural division. The vestigial shell type may be interpreted as an artifact of evolution toward direct development in these species.