No abstract

Broadcasting gametes for external fertilization is a common reproductive strategy of many marine invertebrates; however, little is known about adult behaviors because of the brevity and unpredictability of spawning. We studied the spawning of 2 shallow-water brittle stars -the subtidal Ophiocoma dentata and the intertidal O. scolopendrina -in southern Taiwan by inducing males to spawn after exposure to conspecific ovary suspensions. Laboratory results indicated that male O. dentata preferred to cohabit with females, but that male-male antagonism did not occur. In the field, males with female companions were more inducible to spawn than other males. In contrast, O. scolopendrina did not pair; instead, males rapidly approached released eggs before spawning. Despite this, 53% of simulated egg releases in tide pools did not induce any male to spawn. Male spawning capacities in both species were relatively limited; spawning inducibility rates and discpumping frequencies decreased with repeated inductions. Evidence of sexual competition was observed during the spawning induction of O. scolopendrina in tide pools. Only males that arrived first were likely to spawn, whereas late individuals had low rates of spawning. The fitness of female O. scolopendrina may be limited by the shortage of spawning males; however, at the same time, males face sexual competition in addition to sperm limitation. This is due to the female-biased sex ratio, and the limited spawning capacity of unevenly distributed males.