Reproductive cycles and recruitment of the two co-existing tropical brittle-stars from the barrier reef of Toliara (Madagascar), Ophiocoma scolopendrina and Ophiomastix venosa

The reproductive cycles and recruitment of two co-existing brittlestar species, Ophiocoma scolopendrina and Ophiomastix venosa, were investigated on the barrier reef of Toliara (Madagascar) from February 2000 to April 2001. Both species inhabit the boulder tract of the barrier reef and are highly abondant. O. scolopendrina has planktotrophic larvae, reproduces continuously, and new recruits are seen year-round. Conversely O. venosa has lecitotrophic larvae and has a clear annual reproductive cycle. In the latter species, spawning occurs during the austral summer (from November to February) and sexual recruits were seen mostly from February to May. It is hypothesized that the differences in reproductive strategies shown by the two species are linked to local environnemental conditions and species adaptations. The tolerance to emersion shown by O. scolopendrina allows it to sucessfully colonize frequently emerged areas. In O. venosa, adults live in permanently immersed tidal channels and juveniles were often seen as symbiotes of O. scolopendrina. This facultative commensalism allows them both to resist immersion and to increase the species recruitment area.