Meeting Abstract
136.2 Monday, Jan. 7 Long-standing larval mystery solved �”Pilidium recurvatum is the larva of Riserius sp., a basal heteronemertean (Heteronemertea; Pilidiophora; Nemertea) HIEBERT, T. C.*; VON DASSOW, G.; HIEBERT, L. S.; MASLAKOVA, S. A.; Oregon Institute of Marine Biology, University of Oregon; Oregon Institute of Marine Biology, University of Oregon; Oregon Institute of Marine Biology, University of Oregon; Oregon Institute of Marine Biology, University of Oregon terrah@uoregon.edu
The typical pilidium larva of nemertean worms looks like a hat with earflaps. P. recurvatum looks like an athletic sock, swimming around heel first with the toe trailing behind. It was discovered in 1883 in the NW Atlantic off Rhode Island, and has since been reported from Gullmarfjord (Sweden), the Bay of Nha Trang (Vietnam), the Sea of Japan (Russia) and the NE Pacific off Washington and Oregon, but its identity remained mysterious until now. We identified p. recurvatum larvae from Coos Bay, OR based on molecular phylogenetic evidence and the morphology of the metamorphosed juveniles as belonging to the genus Riserius, an unusual mesopsammic heteronemertean. Gösta Jägersten suggested that p. recurvatum may represent an evolutionary intermediate between the planuliform nemertean larva and the typical pilidium. The fact that Riserius is basal within the Pilidiophora supports the evolutionary significance of this larval form. We found two morphologically distinct kinds of p. recurvatum larvae in Coos Bay. Based on the 16S rDNA sequence divergence they represent two separate species, each distinct from the only described species of Riserius, with which they form a monophyletic clade. We have yet to find the adults of these two apparently undescribed species of a previously monotypic genus. We also report on the remarkable choice of prey by the juveniles of Riserius sp. They feed exclusively on the larvae and juveniles of the hoplonemertean Carcinonemertes errans, which itself is an egg predator and parasite of the Dungeness crab, a commercially important species.
