Hypoxia in fish
Fish are exposed to large oxygen fluctuations in their aquatic environment since the inherent properties of water can result in marked spatial and temporal differences in the concentration of oxygen (see oxygenation and underwater). Fish respond to hypoxia with varied behavioral, physiological, and cellular responses in order to maintain homeostasis and organism function in an oxygen-depleted environment. The biggest challenge fish face when exposed to low oxygen conditions is maintaining metabolic energy balance, as 95% of the oxygen consumed by fish is used for ATP production through the electron transport chain. Therefore, hypoxia survival requires a coordinated response to secure more oxygen from the depleted environment and counteract the metabolic consequences of decreased ATP production at the mitochondria. This article is a review of the effects of hypoxia on all aspects of fish, ranging from behavior down to genes.
Hypoxia tolerance in fish can often be determined by assessing the value of critical O2 tension, often referred as Pcrit. Pcrit is a point where an oxy-regulator becomes an oxy-conformer. The oxygen consumption rate (MO2) of oxy-regulators are unaffected by the changes in environmental oxygen tension (PO2), whereas the MO2 of oxy-conformers are affected by the changes in PO2, showing decreased MO2 with decreasing PO2. Individuals with low Pcrit values are usually associated with high hypoxia tolerance while individuals with high Pcrit values tend to be more hypoxia sensitive. However, some species or individuals can be complete oxy-conformers and may not show distinctive Pcrit points. Therefore, experimentally defining hypoxia in such species can be difficult.
In mammals there are several structures that have been implicated as oxygen sensing structures; however, all of these structures are situated to detect aortic or internal hypoxia since mammals rarely run into environmental hypoxia. These structures include the type I cells of the carotid body, the neuroepithelial bodies of the lungs as well as some central and peripheral neurons and vascular smooth muscle cells. In fish, the neuroepithelial cells (NEC) have been implicated as the major oxygen sensing cells. NEC have been found in all teleost fish studied to date, and are likely a highly conserved structure within many taxa of fish. NEC are also found in all four gill arches within several different structures, such as along the filaments, at the ends of the gill rakers and throughout the lamellae. Two separate neural pathways have been identified within the zebrafish gill arches both the motor and sensory nerve fibre pathways. Since neuroepithelial cells are distributed throughout the gills, they are often ideally situated to detect both arterial as well as environmental oxygen.
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