Turbellaria (Archoophora: Acoela) from Antarctic sea ice endofauna – examination of their micromorphology

Abstract

Acoel Turbellaria constitute a regular component of the metazoa populating Antarctic sea ice (sea-ice endofauna). Two species were collected, which differ in colour, size, shape and egg spawning season. They do not resemble any known pelagic species. Their small body diameter of less than 300 μm allows them to penetrate deeply into the network of brine channels. Their vertical distribution within one ice floe was positively correlated with the accumulation of algal biomass; maxima for both parameters were found in the bottom 5 cm of the floe. The method by which the Turbellaria invade the sea ice is not clear. At present we have no indication that they pass through a pelagic or benthic stage in their life-cycle. As the Turbellaria were found to populate sea ice in areas with water depths ranging from 370 to 4450 m, the presence of benthic phases in their life-cycle, either free-living or epizooic, is not very probable. We suggest that the Turbellaria either use migrating invertebrates as a vector for their propagation or pass through a pelagic stage in their life-cycle.