MORPHOLOGY AND DEVELOPMENT OF ODOSTOMIA-COLUMBIANA DALL AND BARTSCH (PYRAMIDELLIDAE) - IMPLICATIONS FOR THE EVOLUTION OF GASTROPOD DEVELOPMENT

Although pyramidellid gastropods are a phylogenetically important grou p of diverse and abundant ectoparasites, little is known about their l ife histories. Herein, we describe the adult morphology and developmen t of the pyramidellid Odostomia columbiana, which parasitizes the scal lops Chlamys hastata and C. rubida in the Northeast Pacific. Anatomica lly, adult O. columbiana resemble other known pyramidellids although t hey lack the tentacular pads typical of other Odostomia species. Embry onic development is similar to that described for other pyramidellids: cleavage is unequal, gastrulation is partially by invagination, and c onsiderable growth occurs before hatching. However, embryonic and larv al development are much slower than for other described species. The p lanktotrophic larvae hatch after 19 days of intracapsular development and metamorphose about 2 months later. O. columbiana veligers have a l arge black pigmented mantle organ to the right of the midline, a disti nct metapodial tentacle, and three or four long bristles that project over the operculum from behind the foot. Observations of newly metamor phosed juveniles suggest that previous disagreements regarding the dev elopment of heterostrophy are due to variation in the degree of hetero strophy among species. Our observations also generally corroborate cer tain scenarios explaining the evolution of gastropod cleavage type and larval heterochrony. Unequal cleavage and larvae that hatch without w ell-developed eyes and tentacles may be characteristic of the common a ncestor of pyramidellids and opisthobranchs; however, late development of the larval heart is probably a derived condition of opisthobranchs .