Morphology and Development of Odostomia columbiana Dall and Bartsch (Pyramidellidae): Implications for the Evolution...

Although pyramidellid gastropods are a phylogenetically important group of diverse and abundant ectoparasites, little is known about their life histories. Herein, we describe the adult morphology and development of the pyramidellid Odostomia columbiana, which parasitizes the scallops Chlamys hastata and C. rubida in the Northeast Pacific. Anatomically, adult O. columbiana resemble other known pyramidellids although they lack the tentacular pads typical of other Odostomia species. Embryonic development is similar to that described for other pyramidellids: cleavage is unequal, gastrulation is partially by invagination, and considerable growth occurs before hatching. However, embryonic and larval development are much slower than for other described species. The planktotrophic larvae hatch after 19 days of intracapsular development and metamorphose about 2 months later. O. columbiana veligers have a large black pigmented mantle organ to the right of the midline, a distinct metapodial tentacle, and three or four long bristles that project over the operculum from behind the foot. Observations of newly metamorphosed juveniles suggest that previous disagreements regarding the development of heterostrophy are due to variation in the degree of heterostrophy among species. Our observations also generally corroborate certain scenarios explaining the evolution of gastropod cleavage type and larval heterochrony. Unequal cleavage and larvae that hatch without well-developed eyes and tentacles may be characteristic of the common ancestor of pyramidellids and opisthobranchs; however, late development of the larval heart is probably a derived condition of opisthobranchs.