Overripe eggs and postovulatory corpora lutea in the threespine stickleback, Gasterosteus aculeatus L., form trachurus

Abstract

Threespine sticklebacks (Gasterosteus aculeatus L., form trachurus) with overripe, ovulated eggs have a hard 'berried' abdomen, easily distinguishable from the soft, smoothly distended abdomen of fish with normal ovulated eggs. The 'berried' fish weigh less and have a smaller GSI than 'nonberried' ovulated fish but the percentage water of their whole ovaries is the same. In contrast with 'nonberried' fish, 'berried' fish do not respond to male courtship. Overripe eggs are typical of those in other species with aggregation of cytoplasm and oil droplets at the animal pole. However, in contrast with normal ovulated eggs, they are larger, more transparent, and have a higher percentage water content although surrounded by much less fluid within the ovarian cavity. Overripe eggs do not appear to form a perivitelline space in fresh water. In 'berried' fish both the postovulatory corpora lutea (follicles) and ovarian epithelium (lining of the ovarian cavity) show histological signs of regression, in contrast with the histologically active condition of these tissues in fish with normal ovulated eggs. It is suggested that postovulatory corpora lutea play a role in maintenance of ovulated eggs in the ovarian cavity through secretion of steroid(s) which in turn stimulate the ovarian epithelium to secrete the bathing fluid of the eggs.