Exogonadal oogenesis in a temperate holothurian.

Unusual structures were detected on the visceral peritoneum of the ovarian tubules in about 5%-10% of female sea cucumbers (Cucumaria frondosa) collected off Newfoundland, eastern Canada. The condition varied from mild to severe, with localized castration observed in the most heavily affected tubule sections. Investigation of the structures using histology, transmission electron microscopy (TEM), and gene analysis revealed that they were oocytes at different stages of development, growing singly or in groups of up to six. Their size and composition were consistent with those of oocytes found in the lumen of the ovaries, although "exogonadal" oocytes were devoid of a vitelline coat and presented few cortical granules. TEM sections suggest that the atypical oocytes emerged from the peritoneum and grew toward the coelomic cavity, and that they were not in direct contact with the basal lamina or the inner germinal layers. Similar masses have been observed in C. frondosa from the Gulf of St. Lawrence (Québec, Canada) and the Barents Sea (Russia), and in C. japonica from Russia and Psolus fabricii from Canada. The possibility that exogonadal oogenesis is attributable to anthropogenic disturbances should be investigated even though some of the affected specimens originate from presumably pristine locations.