Reproductive cycle and spawning induction in the gregarious brittle star Ophiothrix fragilis (Echinodermata) in the Oosterschelde (Netherlands)

Abstract

Summary The reproductive biology of the gregarious brittle star Ophiothrix fragilis is described using the gonad index (GI) and the maturity index (MI) in conjunction with the photoperiod and water temperature. The ophiuroid has a typical annual reproductive cycle going through five distinct reproductive stages. After a resting period during the winter season, the growth and maturation period begins in parallel with a rise in seawater temperature. Spawning takes place during summer after which the gonads regress and the next cycle begins. The rise in temperature seems to influence gonadal growth and more specifically the maturation period of the gametogenic cycle, the indices each attaining higher values earlier in 1999 than in 2000, temperature having reached a maximum a month earlier that year. Representation of the MI in polar coordinates shows a relatively homogenous population during the winter and spring season (resting and maturation periods respectively) while the summer season is more heterogeneous due to the presence of individuals in various reproductive stages. Elevated water temperatures (above 16°C,) mechanical stress believed to be due to strong hydrodynamics and the release of gametes by conspecifics seem to induce spawning in males and females with the former being more sensitive to these cues.