Ovarian development and alteration in demersal and pelagic fishes of Pranburi River estuary, Thailand

The stressful environment in Pranburi River estuary of Thailand severely impacts the aquatic ecosystem and may be causing reproductive alteration in the estuarine fishes living in this area, although there is little evidence of such deteriorated reproductive system. Thus, we aim to investigate the reproductive system, in particular the ovarian alteration using histological techniques in two groups of fishes: (1) demersal fishes, viz. long-whisker catfish (Mystus gulio), dark sleeper (Butis humeralis) and maned goby (Oxyurichthys microlepis), and (2) pelagic fishes, viz. greenback mullet (Planiliza subviridis), green puffer (Tetraodon fluviatilis) and Singapore glassy perchlet (Ambassis kopsii). Ten samples of each fish were collected during January - March of 2016-2018. Histological observation demonstrates that both demersal and pelagic fishes have an asynchronous oocyte development and maturation, which proceeds through two successive phases: primary growth phase and secondary growth phase. The fishes exhibit significant histopathological alterations in ovaries, including the atretic oocytes in both phases, with irregular shapes and degeneration of yolk granules. Interestingly, all collected demersal fishes possess a significantly higher number of atretic oocytes compared to the pelagic fishes. The observed abnormalities seem to be associated with the environmental stress, which may be more severe in the benthic region of the estuary.