Year-round reproduction in a seasonal sea: biological cycle of the introduced ascidian Styela plicata in the Western Mediterranean

The widely introduced ascidian Styela plicata is very common in the Western Mediterranean, an area that can act as a source for secondary introductions due to its high shipping activity. In order to understand the potential of this species to colonize new habitats, its reproductive features were assessed in the Western Mediterranean by means of monthly monitoring of two populations (Vilanova i la Geltrú 41°12′53″N, 1°44′11″E; Blanes 41°40′29″N, 2°47′56″E) from January 2009 to December 2010. The reproductive activity of this species was assessed through gonad histology and a gonad index. Population size-structure was measured monthly in order to study recruitment dynamics. No clear seasonal pattern was observed, and mature gametes and recruits were present all year long. Spawning was potentially continuous, although it seemed punctuated with pulses of gamete release, particularly in spring. A prolonged reproductive period is likely to confer a competitive advantage on S. plicata in temperate seas, where most species reproduce seasonally, and may promote recurrent introductions as larvae are available for settlement on transport vectors over much of the year.