The involvement of sodium transport in the volume regulation of the amphipod crustacean, Gammarus duebeni.

The maintenance of body volume forms an integral part of the overall osmoregulation of all metazoan animals. The fact that such regulation occurs in various invertebrates has long been experimentally established and it has been shown that the capacity to regulate the body-water content is particularly well developed in euryhaline species (Beadle, 1937; Ellis, 1937, 1939; Jergensen & Dales, 1967; Schwabe, 1933; Nagel, 1934). The constancy of the fluid content of various marine animals (Vinogradov, 1953) implies that they too regulate their volume accurately in their normal environment though when placed in dilute media many stenohaline species show a sustained increase in volume (Dakin & Edmonds, 1931; Bethe, 1934; Krishnamoorthi, 1962). The presence of the exoskeleton in arthropods limits the extent to which the volume can be changed but this does not absolve these animals from the need to regulate their fluid intake and output actively. A clear illustration of this is provided by the isopod Asellus aquaticus, where failure of volume regulation in unfavourable circumstances results in an uptake of water which can be sufficiently severe to cause the body to become cylindrical instead of dorso-ventrally flattened (Needham, 1947). Size increase at moult might also be expected to vary with the magnitude of the gradient between blood and medium in the absence of adequate volume-regulating mechanisms, but it has been shown that there is no such variation in the euryhaline crab Callinectes sapidus (Haefner & Shuster, 1964). The most obvious responses of Crustacea to changes in internal volume are variations in the volume of urine produced and in the amount of water taken in. Thus the urine volume of Carcinus maenas (Nagel, 1934; Shaw, 1961), Pachygrapsus crassipes (Gross & Marshall, i960) and Gammarus fasciatus (Werntz, 1963) increases with the gradient maintained between blood and medium, and Kamemoto & Ono (1969) have demonstrated that such variations in the rate of urine flow in the crayfish Procambarus clarkii are in part controlled by a hormone emanating from the eyestalk. Water intake by drinking can also be varied according to need, at least in some forms. Thus Neomysis integer adjusts its drinking rate in different external salinities (Ralph,