Ultrastructure of the gametogenesis of the common Mediterranean starfish, Echinaster (Echinaster) sepositus

Abstract

Echinaster (Echinaster) sepositus is one of the most abundant sea stars in western Mediterranean rocky bottoms, yet its reproductive biology remains virtually unknown. Here we report the ultrastructure of its gametogenesis over 2 consecutive years. It is a gonochoric species with an annual reproductive cycle spawning gametes in late summer and early autumn. Each arm of every individual contained two gonads (dark red in females and yellow in males). In both sexes, the gonad was a single, large sac composed of several smaller sacs. The gonad wall consisted of two multilayered sacs, outer and inner, separated by the genital haemal sinus. The histology of the gonad wall was consistent with that found in other asteroids. Oogenesis was continuous during the year, but eggs were spawned only in late summer. Oocytes were in close relationship with follicular cells that are suggested to transfer nutrients to the oocytes. Spermatogenesis was restricted to 5–6 months in spring-summer. It occurred in columns with an axial interstitial cell supporting each column, and producing processes towards the lumen that remained connected to spermatogenic cells by intercellular junctions. Developing sperm cells were found along the length of the column, while spermatozoa were found free in the testis lumen. Spermatogenesis followed the pattern described for echinoderms, to give rise to an acrosome-bearing, round-shaped spermatozoon. The histology and cytology of the reproductive process in E. sepositus followed the general pattern found for asteroids.