Meeting Abstract
53.2 Tuesday, Jan. 5 Comparative morphology and evolution of pilidiophoran larvae (Nemertea) SCHWARTZ, M.L.*; NORENBURG, J.L.; University of Puget Sound; Smithsonian Institution mschwartz@pugetsound.edu
Pilidiophoran life-history has been characterized as possessing indirect development via a planktotrophic larva, the pilidium. Although so-called direct development has also been recorded, only six instances are currently known. This dichotomy in developmental mode is not clear-cut. In pilidiophorans so-called direct development displays a variety of pilidial features, including the adult anlage typical of pilidia, an apical plate, and reduced pilidial lobes. Hence, we refer to it as non-feeding larval development. We present the first extensive and well-supported phylogeny for Pilidiophora and demonstrate that these non-feeding larvae are in fact derived with respect to feeding pilidia, and that they have evolved at least three times. One species pair, Lineus viridis and Lineus ruber, exhibits simple encapsulated, lecithotrophic development in the former, and larval cannibalism in the latter. Encapsulated development among other invertebrates occupying similar high-energy, littoral, rocky shores has been suggested as an adaptation to minimize dispersal and desiccation. The other examples of non-feeding pilidiophorans are sublittoral, and the adaptive significance of encapsulated development or brooding among other sublittoral invertebrates is more controversial. One obvious attribute these pilidiophorans have in common is unpredictable, patchy habitat, which under one hypothesis would favor minimizing dispersal. The relatively few examples among pilidiophorans contribute data but are insufficient for independent tests of competing hypotheses. However, our discovery of three unrelated forms of non-feeding pilidiophoran larvae call into question the accepted expectation that these forms are rare. Although we know pilidia can be abundant and diverse in the plankton, we know the actual developmental mode for only about 30 of the approximately 450 known species of pilidiophorans.
