Meeting Abstract
91.10 Thursday, Jan. 7 Comparative Anatomy of Planuliform Nemertean Larvae GAVELIS, Gregory*; MASLAKOVA, Svetlana; Oregon Institute of Marine Biology gavelis@uoregon.edu
Nemertean developmental studies have focused primarily on the pilidium larva of the pilidiophoran group, with little attention to the planuliform larvae of the palaeonemertans or hoplonemerteans. Morphogenesis of the proboscis, cerebral organs, and digestive system have conflicting descriptions in the light-microscopy-based literature for hoplonemerteans. We reassessed larval development of the hoplonemertean Emplectonema gracile, and also described for the first time early development of the palaeonemertean Procephalothrix spiralis using confocal microscopy. The proboscis in E. gracile developed from an internal mass (as in Paranemertes peregrina) rather than a distinct invagination (found in Pantinonemertes californiensis). Paired antero-lateral invaginations described in development of other species (and homologized with either rudiments of the nervous system or cerebral organs) were observed in early development of both P. spiralis and in E. gracile, however, their ultimate fate is yet to be determined. As in many hoplonemerteans, E. gracile was found to possess a “transitory epidermis” of large cleavage-arrested cells which were gradually replaced by the definitive epidermis. In P. spiralis, the epidermis of newly hatched larvae of P. spiralis was composed of relatively few large cells, similar to the other studied palaeonemertean Carinoma, larvae of many hoplonemerteans, and pericalymma and test larvae of some mollusks. In Carinoma these large cells are derived from the trochoblast lineage and form a vestigial prototroch, which suggests that nemerteans ancestrally possessed a trochophore type larva. Because palaeonemerteans are paraphyletic and Procephalothrix is basal to Carinoma (Thollesson and Norenburg 2003) the fate of these cells in P. spiralis is of particular interest to hypotheses concerning the evolution of larval development in nemerteans.
