Atypical reproduction in a syllid worm: the stolon of Syllis rosea (Annelida, Syllidae) takes care of its offspring

The family Syllidae, aside from representing the most species-rich family in Annelida, is characterized by a number of sexual and asexual reproductive strategies. With the exception of a few viviparous species, the subfamily Syllinae is characterized by schizogamous reproduction with pelagic larval stages and without parental care. Laboratory rearing of ripe specimens of Syllis rosea showed a different reproductive strategy, hitherto unknown in this subfamily. While male stolons rapidly degenerated after fertilization, female ones released large eggs in a gelatinous cluster attached to the middle-posterior chaetigers. The gel mass progressively compacted as a cocoon wrapped by the stolon body; 7 days after the deposition the larvae hatched out from the cocoon at the metatrochophore stage and the female stolon died after a few days. After hatching the larvae remained associated to the stolon, and young specimens of S. rosea survived up to the 3-chaetiger stage. Until now cocoon brooding by the stolon has only been reported for some Autolytinae. The production of gelatinous egg masses and parental care are known in basally branching clades within Syllidae, suggesting that this reproduction mode might retain some ancestral features. The scarce knowledge about reproductive cycles in Syllinae does not allow clarification whether this strategy is unique for S. rosea, or it occurs in other congeneric species. Further research is needed to understand possible relationships between sexual reproduction and phylogeny, stolon morphology and its adaptation to parental care, and ultimately between reproductive strategies and ecology.