The effect of salinity and temperature on egestion in mud snails (Gastropoda: Hydrobiidae) : A study on niche overlap.

A comparative study of egestion in three species of mud snails under controlled conditions of salinity ranging from 10 to 30‰, and temperatures ranging from 5 to 35°C, shows that Hydrobia ulvae has maximal egestion at the combination of high salinity (30‰) and high temperature (30°C). In Hydrobia neglecta a peak of egestion occurs at the combination of 25‰ salinity and 25°C. Hydrobia ventrosa shows a small maximum at 20‰ and 30°C. However, in contrast to the former two species the egestion is rather uniform over the range of salinities and temperatures tested.Considering egestion as a measure of activity at combined temperature and salinity, there is a moderate degree of overlap of this niche axis. The often observed co-existence of the three species is discussed in relation to the present findings and other evidence illustrating the width of the niches. It is concluded that co-existence, in spite of interspecific competition, is possible provided the environment is sufficiently unpredictable.