Three examples of recent findings illustrating the intricacy of inter‐relationships between mechanisms responsible for gas exchange and transport and salt and water balance are described: 1) Divalent cations raise the oxygen affinity of crustacean hemocyanins (Hcs). At low salinity the respiratory consequences of a loss of Ca++from the blood of an estuarine crab are offset by a concomitant and compensatory rise in blood pH and a shift in intrinsic oxygen binding properties, possibly brought about by a change in composition of the constituent subunits of the He molecule. The net result is a return to the original oxygen affinity during normoxia but not hypoxia, when anaerobic metabolism results in acidosis due to the accumulation of acidic end products. The acidosis requires compensation by allosteric actions of L‐lactate, the chief end product, and Ca++, a product of acid dissolution of the exoskeleton. The combined actions of these two effectors restores oxygen affinity to the original level. 2) Oxygen carrying capacity of He‐containing bloods of the arthropods are low because of the low blood pressures that accompany the replacement of a fluid with a solid skeleton. Hydrostatic pressures in the sinuses surrounding the excretory organ, the site of extracellular volume regulation, now barely exceed the colloid osmotic pressures of the extracellular oxygen carrier. Natural selection has promoted the formation of high molecular weight He polymers having 6–48 active sites/‐osmotically active particle. Nonetheless the excess of hydrostatic over colloid osmotic pressure, which permits primary urine formation and net fluid output from a hyperosmotic animal, is very small. Colloid osmotic pressure, and thus oxygen carrying capacity, could be no higher and the degree of polymerization could be no smaller without impairing fluid balance with the medium. 3) Divalent cations lower the oxygen affinity of gastropod He. Within the ecological range the loss of Ca++and Mg++from the blood of an osmoconforming conch is not great enough to influence oxygen affinity but it does lower cooperativity and thus the oxygenation of the blood at the gill. Oxygen uptake goes down at low salinity and the animal does not survive mesohaline waters.