Abstract

We report the presence of nocturnal pelagic aggregations of adult individuals of the polychaete Platynereis australis (heteronereids) associated with Macrocystis pyrifera kelp beds in several locations (7) along Fueguian and Magellan channels, Chile (53°-55°S, 67°-73°W), in 2006 and 2007. Adult specimens showed morphological changes in 2/3 of the body, with modified parapodia and large coalescent black eyes. Pre-natatory region consists of 19 to 22 segments, while the epitoke region comprises from 60 to 90 segments. Specimens were sexually mature and mean egg size between 135 and 155 μm in diameter, having a similar size along the entire body, indicating a simultaneous development in the coelomic cavity. Pelagic swarms were observed in all sites in spring. Observations made in autumn reported the presence of heteronereids in tubes attached to Macrocystis fronds. This may constitute the existence of an intermediate phase between the benthic and pelagic phase. However, further studies are required to clarify this behavior. We discuss the close association between Platynereis and forests of M. pyrifera at different stages of their life cycle.