Sodium Regulation in the Amphipod Gammarus Duebeni Lilljeborg from Freshwater Localities in Ireland

The detailed distribution of Gammarus duebeni in freshwater habitats is discussed by Hynes (1954) and Sutcliffe (1967a). It occurs in a number of isolated coastal localities situated on the western sea-boards of Britain, including the Isle of Man, and on the Shetlands, the Faroes, Iceland, Norway and Brittany. In most of these localities the amphipod is restricted to only a few of the streams and rivers apparently available for colonization. In Ireland G. duebeni is not confined to coastal localities; it is widely distributed inland in a variety of freshwater habitats (Reid, 1939; Macan & Lund, 1954). Reid was unable to find any morphological differences between specimens from fresh water in Ireland and specimens obtained from brackish water in the south of England. He tried to breed the latter in fresh water, but without success, and so suggested that there may be two distinct physiological races. Support for this was obtained by Beadle & Cragg (1940), who found that G. duebeni from a freshwater locality survived in distilled water longer than did animals from brackish water. However, more extensive experiments of the same kind carried out by Hynes (1954) using animals obtained from the Isle of Man did not confirm this, and other observations by Hynes on the structure of the antennal glands, and the size and number of eggs, also produced no further support for the suggestion that there are two physiological races. The development of more sensitive methods for determining the uptake and loss of ions has made it possible to reinvestigate the osmoregulatory mechanisms of animals from different habitats. Shaw & Sutcliffe (1961) were able to demonstrate differences in the affinity for sodium ions in the uptake mechanisms of Gammarus pulex from fresh water and G. duebeni from brackish water, and it was suggested that perhaps G. duebeni in freshwater localities has developed an uptake mechanism closer to the type found in G. pulex. G. pulex has a very high affinity for sodium ions at low external concentrations, so that the uptake mechanism situated at the body surface is half-saturated at O-IO-O-IS mM/1. and fully saturated at 1 mM/1. NaCl, whereas in G. duebeni from brackish-water localities the corresponding external concentrations are about ten times higher (Shaw & Sutcliffe, 1961; Sutcliffe, 19676, c). Consequently when G. duebeni is kept in low external concentrations, below about 1 mM/1. NaCl, the rate of sodium uptake is very low and is barely sufficient