Evolutionary Simplification of Velar Ciliation in the Nonfeeding Larvae of Periwinkles (Littorina spp.)

Abstract

The evolutionary loss of larval feeding has occurred multiple times in gastropods (e.g., 1, 2), with variable consequences for larval structures used to capture partic ulate food: in some cases, derived nonfeeding larvae retain structures used for food capture, but in others these are reduced or lost (2, 3). The fate of these structures in non feeding larvae has implications for the potential re-evolu tion of larval feeding (4, 5). Larval feeding has been lost at least once in the genus Littorina, but nonfeeding larvae of some species in the genus retain the velum, using its mar ginal cells for endocytotic uptake of capsular albumen (3). Retention of the velum, one of whose main ancestral func tions is particle capture, suggests that re-evolution of larval feeding might be possible in this group (5). We show that nonfeeding larvae of four Littorina spp. have lost the ciliary bands presumably used by their ancestors to concentrate planktonic food particles from suspension. Re-evolution of feeding in these larvae thus would require regaining op posed ciliary bands or other feeding structures. Acquisition of similar comparative data from other gastropod clades may help elucidate the conditions favoring loss of ancestral feeding structures in derived nonfeeding larvae. The larvae of marine invertebrates are diverse in form