Adaptations of Crustaceans to Land: A Summary and Analysis of New Findings

Abstract

Crustaceans have adapted to land through various morphological, physiological, biochemical, and behavioral modifications, of which some are shared by all land-dwelling crustaceans and others are unique to animals within a particular habitat. Among the three groups of crustaceans having truly terrestrial members, the amphipods have achieved their success on land primarily by behavioral means, while the isopods and the decapods have developed many morphological, physiological, and biochemical adaptations as well. In all three groups, behavioral modifications ensure that loss of water is minimal, that the animals are exposed to favorable rather than extreme environmental conditions, and that the fine line between evaporative cooling and excessive dehydration is maintained. In most crustaceans the excretion of nitrogenous wastes requires that copious supplies of water be available for washing away the soluble end-products. Yet terrestrial isopods are able to excrete ammonia as a gas, without being subject to toxic side-effects. In decapods, either ammonia or insoluble uric acid may be excreted, with ammonia the more likely product when water is available, uric acid when water is scarce. In adult land crabs water balance is maintained through the concerted action of gills, pericardial sacs, and gut. These organs may take up, store, and redistribute salts and water in response to control exerted by the central nervous system through its secretory products. In larvae of land crabs these organs are not known to function in this way. Rather, the larvae are adapted to cope with osmotic problems of their planktonic existence. Gaseous exchange in adult land crabs is carried on not only by the gills but also by the highly vascularized lining of the branchial chambers, and the hemocyanin ot these crabs is adapted to function in the environment peculiar to each species. Terrestrial crabs seem unable to withstand low temperatures, but their high rate of cytochrome c oxidase activity may help them to survive when temperatures are high. Modifications in behavior must have occurred quite early in the transition of crustaceans from sea to land. Then, as now, appropriate behavioral responses to light, temperatuie, humidity, tidal cycles, and so on. were crucial if a terrestrial animal was to survive. Social interactions, both for courtship and for aggression, required the sending and receiving of appropriate visual and acoustic signals and were promoted by the ritualization of potentially injurious patterns of behavior.