Biomarkers of hypoxia exposure and reproductive function in Atlantic croaker: A review with some preliminary findings from the northern Gulf of Mexico hypoxic zone

The long-term impacts on marine ecosystems and fisheries of the recent worldwide increase in coastal hypoxia cannot be accurately assessed at present due to our limited knowledge of the chronic sublethal effects of hypoxia on marine organisms. Moreover, it is unclear whether many marine fish and other motile species remain in hypoxic bottom waters long enough to trigger adaptive responses to low dissolved oxygen. Therefore, there is an urgent need to develop reliable and sensitive biomarkers of sublethal hypoxia exposure and its deleterious effects on critical functions for maintaining population size such as reproduction. In this paper the molecular and biochemical responses to hypoxia and the role of the hypothalamus–pituitary–gonadal axis in the control of the reproductive cycle in fish are briefly reviewed. The potential use of hypoxia-inducible factors (HIFs) as specific biomarkers of hypoxia exposure and changes in hormone levels and gonadal histology as biomarkers of reproductive function in fish are discussed. Recent field studies with a hypoxic-tolerant estuarine teleost, Atlantic croaker, have provided the first clear evidence in an aquatic species that reproduction and endocrine function are particularly susceptible to interference by environmental hypoxia exposure. Marked impairment of reproductive function and endocrine disruption was observed in individuals collected from hypoxic sites in East Bay, Florida and Mobile Bay, Alabama. The production of mature oocytes and sperm (gametogenesis), as well as sex steroid and vitellogenin levels in the blood, were significantly lower in croaker from the hypoxic sites in East Bay compared to the values in fish collected from the adjoining normoxic Pensacola Bay, whereas gonadal HIF-1α and HIF-2α mRNA expression was significantly elevated in fish from the hypoxic sites. Similar patterns of reproductive and endocrine disturbances and increased HIF-1α and HIF-2α mRNA expression were observed in controlled hypoxia laboratory studies with croaker. Preliminary findings suggest that reproductive and endocrine functions were also impaired in female croaker collected in 2006 from the hypoxic zone off the Louisiana coast. The production of mature oocytes (fecundity) was significantly decreased in fish collected from the hypoxic site compared to that observed at the normoxic site and this was associated with declines in circulating sex steroid and vitellogenin levels and gonadotropin releasing hormone mRNA expression in the hypothalamus. Moreover, tissue expression of HIF-2α mRNA and protein was significantly increased in croaker collected at the hypoxic site. It is concluded from these studies that assessment of HIFα(s) expression and reproductive/endocrine functions are promising as biomarkers of exposure to hypoxia and its potential long-term impacts on fish populations, respectivel