Physiological adaptations of crayfish to the hypoxic environment

SYNOPSIS. Crayfish routinely encounter waters of reduced oxygen tension, resulting in a broad array of behavioral and physiological responses. Many animals when faced with this stress will simply remove themselves from the irritating environment through voluntary migration. When an animal, either by choice or through physical constraints remains in a hypoxic environment it must compensate for the reduction in O₂ availability. Many crayfish have the ability to maintain oxygen consumption independent of water Po₂ down to some critical level; below this the animal can no longer maintain normoxic levels of aerobic metabolism. Regulation of oxygen uptake is thought to be due to a hypoxia-induced hyperventilation along with an increase in hemocyanin O₂ affinity and an improvement in the ability of the respiratory surfaces to transfer O₂. Crayfish exposed to a reduction in water oxygen also show a strong bradycardia, which is compensated for by an increase in stroke volume, resulting in a maintenance of cardiac output. The adaptive advantage of this response is uncertain. As water Po₂ drops crayfish have been shown to redistribute cardiac output, presumably through the action of the cardioarterial valves. Hemolymph is shunted to the anterior end of the animal, resulting in a greater perfusion of nervous tissue. The animals' ability to detect changes in water Po₂ appear to result from O₂ sensitive receptors located on the gills or in the branchiocardiac veins. The integrated physiological response toward environmental hypoxia allows the crayfish to not only deal with the stress but to maintain activity.