CILIARY CURRENTS OF NONFEEDING VELIGERS IN PUTATIVE BASAL CLADES OF GASTROPODS

The most basal clades of gastropods may lack feeding larval stages in all surviving species, although many species have planktonic veliger l arvae with a preoral (prototrochal) band of cilia for locomotion. We e xamined subvelar cilia and currents for comparison with feeding larvae of other gastropods. The larvae represented three major groups of arc haeogastropod grade: the lottiid limpets Lottia pelta and Tectura scut um, the fissurellid limper Diodora aspera, and the trochid Calliostoma ligatum. Subvelar cilia immediately adjacent to the prototroch were l onger than those farther posterior. Subvelar currents were from poster ior to anterior along dorsal areas of the velum and from anterior to p osterior along ventral areas of the velum. These observed particle pat hs and inferred directions of beat of subvelar cilia suggest a cleansi ng function, with currents exiting the mantle cavity in an anterior di rection dorsally and laterally and moving posteriorly to join ciliary currents along the foot ventrally. Only in C. ligatum were particles s ometimes transported in the dorsal to ventral direction just below the prototrochal band and then only when the prototrochal cilia were arre sted. Particles were not captured between bands of cilia with opposed ciliary beat, transported toward the mouth in a ciliated food groove, or ingested by the archaeogastropod veligers, whereas these events wer e observed easily in other gastropod veligers. Fluorescence of algae w as absent in guts of the archaeogastropod veligers exposed to phytopla nkton but present in guts of other gastropod veligers. These observati ons support the hypothesis that veligers of the Patellogastropoda and Vetigastropoda do not feed and that the subvelar cilia serve other fun ctions. If capacity to feed has been lost, subvelar ciliation has been modified for other functions, perhaps cleansing. If the ciliation rep resents the ancestral condition, then ciliation for cleansing may have been modified for feeding.