Observations on the ecology and reproductive biology of the sipunculan wormAspidosiphon muelleriin temperate waters

Abstract

A population of the sipunculan wormAspidosiphon muelleri, located in temperate waters of the western Mediterranean Sea, was monitored monthly for a year. Some aspects related to its ecology and reproductive biology are shown in the present work. The sex-ratio for this population was close to 1:1 (54% females vs 46% males), thus indicating a dioecious reproduction, although showing a lack of sexual dimorphism. Oocytes were detected in females from 4–5 mm; this size might be reached byA. muelleriin a few months. The population density increased notably during the summer (June–August). The spawning event date was indirectly estimated from the average size of the oocytes and the percentage of females with free oocytes in the coelom. Both variables were significantly correlated to shallow water temperature (Pearson correlation;P = 0.003 andP = 0.001, respectively). Oocyte size was also significantly correlated to average irradiance level (Pearson correlation;P = 0.044). Spawning took place between August and September (when the water temperature is getting close to its annual maximum) and may last only a few weeks in these temperate waters. The abundance ofA. muelleridecreased drastically in September, likely as a consequence of the spawning event effort. Bivalves of the speciesEpilepton clarkiaewere collected together with specimens ofA. muelleriliving inside the polychaete tubes used as shelters by the sipunculans, with a prevalence of 11.64%. To a lesser extent some Foraminifera were also reported attached to the body of the sipunculans.