Life cycle of Marphysa sanguinea (Polychaeta: Eunicidae) in the Venice Lagoon (Italy).

The life cycle of a brackish water population of Marphysa sanguinea(Polychaeta Eunicidae), from the Venice Lagoon (Italy), was investigated fromApril 1993 to August 1994. Marphysa sanguinea is a large-sized gonochoricspecies with annual iteroparous strategy and synchronous spawning at populationlevel. The sex-ratio was close to 1:1. There were no morphological differencesbetween males and females and spawning occurred without epitokalmetamorphosis. The gonadial activity was maximum during summer periodin both sexes. From May to September in the females, the greater number ofimmature oocytes could be observed; starting from November immatureoocytes decreased and progressively those with a wider diameter increased innumber. Spawning took place in April\u2013May and generally not all eggs werespawned, a small proportion being kept as a reserve material for the followinggamete production. The pelagic phase (trochophore and metatrochophore stages)was short (2\u20133 days). The reduction in the dispersal phase, together withthe development of a strong anterior musculature, brought to an early acquisitionof the benthic habit, which was attained with the construction of themucous tube inside of which the larva lived. The results highlighted that colonizationof brackish environments took place thanks to morphological andphysiological features that allow this species to live in a sediment layer characterizedby the stability of the physical and chemical characteristics of thesubstratum