Annual Changes in the Ovarian Activity of the Catfish, Mystus tengara (Ham.) (Teleostei)

抄録

Teleosts ovary undergoes regular changes in many species, but the breeding season and spawning reflex are varied. The ovarian histomorphology and annual changes and the spawning periodicity of this catfish were studied on the total of 78 individuals collected monthly in one year period of 1963-64. By gross morphology and its seasonal changes, especially on the changes in cell structure in this species, nine separate stages were distinguished to categorise the developing oocytes. As to the annual ovarian changes, seven phases of ovarian cycle was described based on the percentage and duration of different stages of developing oocytes, average diameter of oocytes and nature of ovarian wall-Immature Virgin (October to February), Preparatory Virgin (March-April), Maturing Virgin (April-May), Prespawning Virgin (June), Spawning (late June through August), Depletion (late August-September), and Recoupment (October-November). The studies revealed a close correspondence among gonosomatic index, ova diameter and water temperature. Discussion was made on some interpretations as to the origin of new crops of oocytes in teleosts ovary. In Mystus the new oocytes were found to be formed from dividing oogonia, which were transformed from stroma cells; the fish lacked a well defined germinal epithelium. As to the spawning periodicities, of which examples in teleosts were introduced, the present species revealed to produce two or more batches of eggs during the same season (April to August or early September) by finding that only two types of ova are present in ripe ovaries, immature and mature, the latter containing oocytes in late yolk stage and mature stage, and by examining gonosomatic index, ova diameter and histological nature of ova. Finally, relative and total fecundity in bony fish were discussed, and the present study suggested that the unshed ripe oocytes undergoing atresia must be counted in computing the total fecundity. It was estimated that in Mystus nearly 93% oocytes finally ripen and are shed.