REPRODUCTION SEXUÉE D'UNE ASTÉRIE FISSIPARE, SCLERASTERIAS RICHARDI (PERRIER, 1882) / SEXUAL REPRODUCTION IN A FISSIPAROUS ASTEROID, SCLERASTERIAS RICHARDI (PERRIER, 1882)

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Abstract
Sclerasterias richardi, a relatively deep sea asteroid (140–200 m) from the border of the Mediterranean continental shelf, is characterized by an asexual reproduction by fissiparity concomitant with a functional sexuality. A monthly sampling of a population from Calvi (Corsica) has allowed a study of the complete sexual cycle from 354 histologically-treated specimens. The 218 sexually defined animals (62% males, 38% females) show strict gonochorism. In males, spermatogenesis is cyclic and sexual maturity seems to be reached before that of the females. In females, the different stages of oogenesis are well marked: oogonia and parietal oocytes disappear only at maturity. Oligolecithic oocytes (120–150 μn) show a synchronous growth. The annual reproductive cycle is well defined in both sexes with one spawning period from mid-September to mid-October. After spawning, a resting period (from mid-October to mid-January) occurs during which unspawned oocytes are phagocytized by more or less isolated accessory cells. These phagocytic cells have never been found in male specimens. Each month the presence of specimens without gonads or unsexable individuals is one of the characteristics of this cycle. Their high proportion during the organization stage and after spawning can be easily explained. In March they are frequent too, owing to the infestation of gonads by Ciliates. As shown by our samples, the bottom water temperature is nearly the same during the whole year and cannot be directly involved as the dominant exogenous variable stimulating spawning. As a consequence of fissiparity which affects the main part of the population there is a great inter- and intra-individual variability. The reproductive potentiality is low: as a female emits approximatly 400–500 ova whose development produces planktotrophic larvae with a long pelagic life, it is clear that sexual reproduction is accessory in comparison with asexual reproduction by fission.

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