Reproductive biology and ontogenesis in the polychaete genus Autolytus (Annelida: Syllidae): Observations on laboratory-cultured individuals

Abstract

Specimens from a presumably hybrid population of Autolytus spp., with A. prolifer and A. brachycephalus as extreme phenotypes, and including all intermediates inbetween, were used throughout this investigation on reproductive biology and ontogenesis. Laboratory culture of larvae, juvenile and adult individuals was successful, using laboratory-bred Eirene viridula (Hydrozoa) as food instead of Dynanema pumila on which this genus feeds in its natural habitat. Schizogamous reproduction of the asexual nurses by formation of sexual stolons was studied in detail. The observed mode of stolon formation was not consistent with earlier descriptions: it could neither be termed strictly scissiparous nor gemmiparous. Not only stock setigers but also segments added by the prepygidial proliferation zone contribute to the very first stolon and also to some of the subsequent stolons. Incorporation of stock setigers into stolons led to size-reduction of the nurses. Gemmiparous stolons were formed intermittently. Contrary to earlier reports, a proliferative segment which produces all stolons except for the first of a stolon chain was never observed. A separate proliferation zone was generated for each new gemmiparous stolon. The further differentiation of stolons was observed, setiger formulae for stolons of both sexes were established, and a sex-specific pattern of development of stolonial head appendages was detected. It could be confirmed that oocytes are produced in the terminal setigers of the nurse and are transferred to the adjacent stolon after its head has begun to develop. Testes and sperm were found exclusively in the anteriormost setigers of the stolons. Mating and egg-laying of stolons was observed and ontogenesis of 99 larvae, hatched from the egg sac of one single female, was studied. No larval or deutometameres were simultaneously formed, but setigers were produced one by one. No setigerous segment was ulteriorly integrated into the peristomium as in nereid larvae. Stolons were formed at the earliest 80 days after hatching by 50 individuals. The median survival value was 214 days, and a maximum life span of 550 days was recorded.