LESOWAY, M.P.*; PAGE, L.R.; University of Victoria, Canada; University of Victoria, Canada: Comparing gastropods during delay of metamorphosis

The evolutionary origins of feeding in spiralian larvae are controversial. Despite contradictory claims regarding single or multiple origins of larval feeding, there have been few detailed morphological studies to provide evidence for either argument. Recent phylogenetic work in gastropods suggests that feeding larvae may have originated independently in the caenogastropods and in the opisthobranchs. I investigated morphological evidence for this independent origin by comparing growth of the dorsal stomach, larval shell, and radula during the larval phase and the delay period following metamorphic competence in the caenogastropod Euspira lewisii and the opisthobranchs Onchidoris bilamellata and Haminoea vesicula. Measurements from transmission electronmicrographs showed that the style sac of E. lewisii increased in area and cell number during both the larval and possibly the delay period. The area of the style sac of H. vesicula did not significantly increase during the delay period, and most intriguingly, showed no significant increase in cell number from hatching through the delay period. This suggests that the increase in size of the style sac results from enlargement rather than proliferation of cells in this opisthobranch. Both the larval shell and the radula of E. lewisii continued to grow after competence, whereas the larval shell and radula of the opisthobranchs showed no increase in size during the delay period. These data not only show differences in ability of caenogastropod and opisthobranch larvae to grow after metamorphic competence, but also suggest that larvae of these two groups may be using different mechanisms to enlarge. These previously unrecognized differences may be evidence of multiple origins of larval feeding in the gastropods and contribute much needed data to the ongoing discussion of the origins of larval feeding.