Sodium Regulation in the Fresh-Water Amphipod, Gammarus Pulex (L.)

Abstract

ABSTRACT Sodium influx and loss rates in Gammarus pulex were measured at constant temperatures. The sodium loss rate was immediately influenced by a change in temperature, with a Q10 of 1·5 to 2·0 at temperatures between 0·3 and 21·5°C. The sodium influx rate is apparently influenced in the same way. The sodium uptake mechanism in G. pulex from three localities was half-saturated at an external concentration of 0·10−0·15 mM/1. sodium. The total sodium loss rate remained approximately constant in animals acclimatized to the range of external concentrations from 2 to about 0·2 mM/1. sodium. 18 % of the sodium was lost in urine with a sodium concentration estimated at 30−50 mM/1. The remainder of the sodium loss was due to diffusion across the body surface. In animals acclimatized to concentrations below about 0·2 mM/1. sodium the sodium loss rate was reduced, due to (a) a lower diffusion rate following a fall in the blood sodium concentration, and (b) the elaboration of a more dilute urine. There was a very close association between changes in the blood sodium concentration, the elaboration of a very dilute urine, and the rate of sodium uptake at the body surface. The results indicate that a fall in the blood sodium concentration leads to simultaneous activation of the sodium uptake mechanisms at the body surface and in the antennary glands. It is estimated that, by producing a dilute urine, total sodium uptake in G. pulex is shared equally between the renal uptake mechanism and the mechanism situated at the body surface. In sea-water media G. pulex drinks and expels fluid from the gut. In a medium slightly hyperosmotic to the normal blood concentration the amount imbibed was equal to the normal rate of urine flow when in fresh water.