THE ROLE OF THE FOLLICULAR EPITHELIUM IN THE OOSORPTION PROCESS IN EUPERA-PLATENSIS DOELLO JURADO, 1921 (BIVALVIA, SPHAERIIDAE) - A LIGHT-MICROSCOPIC APPROACH

Eupera platensis Doello Jurado, 1921, is a simultaneously hermaphrodit ic freshwater bivalve. The gonad produces a small number of large, yol ky eggs and few sperm simultaneously. Eggs are incubated within both i nner demibranchs, and embryos (45 to 90) are retained until released a s fully developed juveniles. At the light microscope level, the oosorp tion process implies: 1) breakage of the egg cell membrane and consequ ent liberation of vitellinic material to the alveolar lumen, 2) phagoc ytosis of the vitellinic droplets by the follicule cells, and 3) react ion of blood amoebocytes that invade the acinus lumen to phagocytose t he remnants of the degenerated oocytes. Oosorption is frequently found in gonads which have recently spawned; however, it has been also dete cted in unspawned ones. Phagocytosis by the follicule cells affects oo cytes in a late stage of vitellogenesis as well as ripe and unspawned ova. Although in other mollusks (mainly in gastropods), the follicule cells are involved in the phagocytosis of degenerative oocytes, the pr ocess is not as extensive as reported here in E. platensis. Assuming t hat parent energy devoted to reproduction is a finite and limited reso urce, and taking into account the reduced number of large ova suscepti ble to being retained within inner demibranchs, the oocyte breakdown a nd subsequent phagocytosis of degenerated oocytes by follicule cells a re here considered as processes which, integrated with the set of life history traits that constitutes the reproductive strategy of the spec ies, leads to prevention of important energy loss that would imply the development of surplus gametes with no chance of success.