Environmental hypoxia: A threat to the gonadal development and reproduction in bony fishes

Abstract

Vast stretches of open water bodies are gradually becoming hypoxic as a result of depletion of oxygen levels mainly due to various human anthropogenic activities. This problem of hypoxic stress on the fish population is likely to be exacerbated soon since the aquatic hypoxic environment is continuously spreading over vast areas worldwide. In recent years, various harmful effects of hypoxia to bony fishes have been reported, such as the restriction of energy-consuming metabolic processes, arrest of the growth of ovary and testes that are associated with endocrine disruption, loss of sperm and egg quality, inhibition of fertilization, hatching success, and also the reduction of larval survivability, thereby impairment of overall reproductive and developmental processes in fish. Disruption of the brain-pituitary-gonad axis, and certain enzymes related to steroidogenesis and vitellogenesis in fish have also been reported as the primary targets for an endocrine malfunction during hypoxia. Hypoxia-sensitive downregulation of key genes responsible for controlling sex hormones' synthesis has been documented in certain bony fishes. Further, continuous exposure to hypoxia was reported to induce early expression of pro-apoptotic/tumor suppressor p53 genes, thereby causing immense cell death in hypoxic embryos. However, the cellular responses to long-term hypoxia exposure and the degree of reproductive impairments in bony fishes are still not adequate to figure out the actual underlying mechanisms. The present review intends to highlight the current knowledge about the detrimental impact of chronic/acute hypoxia at different stages of fish reproduction and the associated underlying molecular mechanisms.

Keywords

• Sex hormones
• Endocrine disruption
• Gametogenesis
• Gonadal development
• Fish reproduction
• HPG-Axis