The mechanism of adaptation to varying salinity the common eel and the general problem of osmotic regulation in fishes

Abstract

The teleost fishes live in two different environments, the fresh water of lakes and rivers, and the salt water of the sea. With few and unimportant exceptions, these habitats are, respectively, very much hypotonic and very much hypertonic to the fish blood. In both cases continual performance of osmotic work is required for the maintenance of the normal blood concentration. The vast majority of fishes are able to exist only in one type of environment, although some variation in that environment is tolerated. The most pronounced exceptions are various species of the eel, Anguilla ; other species showing similar but generally less ability include the salmon, the stickleback (genus Gasterosteus ), and some of the Cyprinodonidæ, notably species of Fundulus . All species of teleosts, apparently, show adaptive responses in blood concentration to slight changes in the salinity of the environment, and the work of many investigators, particularly Duval (1925) and Smith (1930), indicates that the adaptive responses and mechanisms in the eel differ only in degree from those in the osmotically less resistant fishes. The “chloride cells” of the eel gills discovered recently (Keys and Willmer, 1932), were found in all species of fish examined, and were present merely in greatest numbers in the eel.