Tolerance differences and interspecific competition in three members of the amphipod genusGammarus

Abstract

1. In a series of experiments withG. pulex, G. duebeni celticus andG. d. duebeni populations, pure or mixed, their tolerances for several temperature/salinity conditions have been determined. Tested were 100%, 10%, 1% and 0.1% seawater in combination with temperatures of 5, 10 or 15°C. In general all species tolerated the 5°C conditions better than those with higher temperatures, and the middle salinity range (1% forG. pulex, 1% and 10% forG. d. celticus andG. d. duebeni) better than the lower or higher salinities. 2. In nearly all situations tested.G. pulex had a lower survival rate thanG. d. celticus andG. d. duebeni. 3. Notwithstanding the sometimes unfavourable conditions forG. pulex, this species can compete succesfully withG. d. celticus under most of the experimental conditions, but the interspecific relations betweenG. pulex andG. d. duebeni end in the competetive exclusion of the first species. These results support two theories: (1)G. pulex is able to compete succesfully withG. d. celticus and to expel the latter of its niche, and (2)G. d. duebeni is capable to invade freshwater and to establish inland populations. 4. The differences demonstrated betweenG. d. celticus andG. d. duebeni are an argument for a different development and different capabilities of both subspecies. 5. Sterile interspecific matings between these (sub) species are temperature dependent and do not play a major role in the interspecific relations connected with competition.