Seasonal patterns of gametogenesis in a North Atlantic brooding asteroid, Leptasterias tenera.

Abstract

The reproductive system of Leptasterias tenera, a five-rayed, North Atlantic sea star, was studied throughout an annual cycle. In general, the gross anatomy and histological structure of the gonads were comparable to descriptions for other species of Leptasterias (L. hexactis, L. pusilla, L. groenlandica). Gametogenesis in L. tenera followed the characteristic pattern of brooding Leptasterias, but certain specific variations were noted.In L. tenera, spermatogenesis was completed prior to shedding, whereas in L. pusilla, differentiation of spermatocytes continued for at least 14 days after sperm release. Relict sperm remaining in the testis following shedding in both species were ultimately removed by phagocytic action of amoeboid lymphocytes. Two types of lymphocytes, amoeboid lymphocytes and petaloid cells, were present in the haemal sinus of male L. tenera.In the female reproductive cycle of L. tenera, developing oocytes remained close to the basal germinal membrane and haemal sinus throughout the entire period of growth and vitellogenesis, unless squeezed out due to crowding. It was suggested that oocytes which were pushed into the lumen failed to complete development and subsequently were broken down by amoeboid accessory cells derived from the germinal epithelium. The need to maintain contact with the basal membrane was called the "position effect." Other accessory cells in the germinal epithelium become follicle cells.In L. tenera, the timing of the period of greatest activity in both male and female reproductive cycles was positively correlated with the period of active feeding.The time and place of fertilization and maturation are unknown in L. tenera. Sometime following spawning, the ova were placed in the pyloric stomach where the early stages of development occurred. The mass of embryos was then transferred to the brood chamber where development was completed in about three months.Certain features of the brooding mode of reproduction apparently constitute a genetically fixed pattern within Leptasterias. These include a cycle of oogenesis requiring at least two years, resorption of excess oocytes, production of a small number of large, yolky ripe ova, and an extended period of brood-protection within a brood chamber. However, a comparison of reproductive patterns in three species (L. tenera, L. hexactis and L. pusilla) indicates that modifications of the genetic pattern have occurred.