Divide and broadcast: sexual reproduction in the West Indian brittle star Ophiocomella ophiactoides and its relationship to fissiparity

Abstract

It is known that asexual reproduction by fission is the dominant mode of propagation in the small (disc diameter <5.0 mm) ophiocomid brittle star Ophiocomella ophiactoides (H. L. Clark). This species is, however, able to reproduce sexually as well. Sexual reproduction occurs throughout much or all of the year in a population of O. ophiactoides at Jamaica. Gonads are often present in recently split individuals, indicating that sexual and asexual reproduction could occur simultaneously. Regeneration of new gonads in the new disc half is underway by the time an individual is competent to split again. Unsexable individuals are significantly smaller than males, which in turn are significantly smaller than females. The unsexable individuals appear to be below a threshold size for gonad production. Fecundity is low, with the largest females producing a maximum of about 7 400 eggs of small (mean diameter 80 μm) size. Fertilized eggs develop into planktotrophic ophioplutei which are typical of the larvae of other species of ophiocomid brittle stars in morphology, rate of development and duration of planktonic period. Egg size and larval type for O. ophiactoides are characteristic of brittle stars with the planktotrophic mode of development, but egg number is uncharacteristically low. Fissiparity in O. ophiactoides may have evolved in association with small body size, concomitant low fecundity and a high-risk planktotrophic larva as a means of ensuring reliable recruitment. Available data, although sparse, suggest that this hypothesis may be applicable to fissiparous brittle stars in general. Fission in brittle stars has a broad systematic incidence at the familial level which suggests that it has evolved recurrently. The ability both to divide and broadcast appears to be a useful combination for O. ophiactoides: it can recruit reliably at large size to occupy favourable habitats quickly, but, it can still maintain low level dispersal of larvae which could provide escape from deteriorating habitats and the potential to colonize new habitats.