P2.64 Wednesday, Jan. 5 Simplified velar ciliation in nonfeeding larvae of Littorina HOFSTEE, J.C.*; PERNET, B.; California State Univ. Long Beach; California State Univ. Long Beach email@example.com
The evolutionary loss of larval feeding has occurred many times in gastropods. In some cases, derived nonfeeding larvae retain feeding structures, but in others these are reduced or lost. The fate of feeding structures in nonfeeding larvae has implications for the potential regain of larval feeding. Littorina, a genus in which larval feeding has been lost at least once, is of interest in this regard. Moran (1999) showed that the nonfeeding larvae of some Littorina spp. retain the velum, which they use for the endocytotic uptake of capsular albumen. Retention of the velum, one of whose main ancestral functions is in capturing particulate food, suggests that re-evolution of larval feeding might be possible in this group. However, it is not clear if these larvae retain the opposed ciliary bands (prototroch, metatroch, and food groove) used to capture particles in feeding larvae. To address this question, we examined velar ciliation of larvae of six Littorina spp. As expected, feeding larvae (of 2 species) bore typical opposed bands. However, the velar lobes of nonfeeding larvae (4 species) bore only a single wide band of short (~10 µm long) simple cilia. Nonfeeding larvae in particle suspensions did not capture particles using velar cilia; particle capture and transport to the mouth were common in feeding larvae in the same conditions. Re-evolution of feeding larvae in this clade thus seems unlikely, as it would require the regain of opposed ciliary bands, or the acquisition of new feeding structures. Reid (1989) hypothesized that the feeding larvae of some members of several related genera (Bembicium, Lacuna, Risselopsis) were derived from ancestors with nonfeeding larvae. Examination of the velar ciliation of closely-related nonfeeding larvae would be useful in understanding these possible reversals.