Although some species of fish, crustaceans, and molluscs may behaviorally avoid hypoxic masses of small size and limited duration, others cannot. In a series of crustaceans, tolerance of hypoxia over 28 days at 30C, decreases as follows: Eurypanopeus depressus (38 Torr = LC 50 ) > Palae monetes pugio > Rhithropanopeus harrisii > Penaeus aztecus > Callinectes sapidus (121 Torr = LC 50 ). Callinectes sapidus and E. depressus die during 1 2-h exposure to anoxia and their heat dissipation rates (quantified by microcalorimetry) are depressed in seawater at 25% air saturation (normoxia) to only 32 and 47% of their metabolic rate at normoxia. In contrast, starved Crassostrea virginica and Thais haemastorna are anoxia tolerant; their metabolic rates are depressed under anoxia to 75% and 9% of the normoxic rate. Hypoxia tolerance is greater at 20C than at 30C for Penaeus aztecus and Crassostrea virginica, but no temperature effect on tolerance exists for Callinectes sapidus. Hypoxia tolerance varies inversely with salinity for Penaeus aztecus at 20 and 30C and for Callinectes sapidus at 30C, but it varies directly with salinity at 20C in Callinectes sapidus. Greater depression of metabolic rate occurs in molluscs during anoxia exposure (and is correlated with greater hypoxia tolerance) than occurs in Callinectes sapidus and Penaeus aztecus, which are not anoxia tolerant. Heavy mortality probably occurs in young Callinectes sapidus and Penaeus aztecus and in

Hypoxia can have profound effects on individual organisms. This chapter focuses on the mechanisms different kinds of animals possess to avoid, tolerate, and adapt to low levels of oxygen in water; selected examples illustrate these mechanisms. While some organisms can detect and avoid hypoxic water, avoidance is not always possible, especially in the case of sessile organisms. When an organism cannot avoid hypoxia, its response may depend on the intensity and thz duration of the bout of low oxygen. Examples of responses to hypoxia include a depression in feeding as well as a decrease in molting and growth rates. During acute exposures to hypoxia some organisms can maintain aerobic metabolism by making effective use of a respiratory pigment, or increasing ventilation rates, or increasing the flow of blood past the respiratory surfaces or combinations of all three. Responses to chronic hypoxia are different and include the production of greater quantities of respiratory pigment and changing the structure of the pigment to one with an adaptive higher oxygen affinity. Many organisms respond to hypoxia by switching fiom aerobic to anaerobic metabolism and some simply reduce their overall metabolism. Hypoxia is often accompanied by hypercapnia (an elevation in water C02), which produces an acidification of the body tissues, including the blood, and has physiological implications that can also be profound and separate from the effects of low oxygen. Finally, there is evidence that hypoxia can inhibit immune responses, causing greater mortality than would otherwise occur when organisms are challenged with a pathogen.

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