Biology and distribution of Gammarus aequicauda and G. insensibilis (Crustacea, Amphipoda) in the lagoon system of Bages-Sigean (France)

1. Gammarus aequicauda (Martynov, 1931) and Gammarus insensibilis Stock, 1966 have been the subject of a quantitative ecological research project in the lagoon system of Bages-Sigean (France, dépt. Aude). From April 1976 till August 1977 samples have been taken at 31 different stations all over the lagoon at intervals of about 2 weeks. With the samples thus obtained we studied the biology and distribution of the two species in relation to environmental factors, the most important being chlorinity and temperature. 2. Both species reproduce throughout the year and have a regular reproductive pattern with maxima in spring and autumn. In G. aequicauda. as compared to the other species, this peak is earlier in spring and later in autumn. This is caused by a different influence of the temperature on the reproductive activity of each species. 3. The reproductive success of G. insensibilis is greater than that of G. aequicauda and during most of the time G. insensibilis has a wider distribution over the lagoon than G. aequicauda. 4. The tolerance spectrum of G. aequicauda is much broader than that of G. insensibilis. In stable environments G. insensibilis is the dominant species and G. aequicauda is scarcely found. In more unstable environments both species occur but G. aequicauda has advantage over G. insensibilis, especially at higher and at lower chlorinities. 5. In localities where both species coexist competition is avoided by a spatial separation based on differences in preferences for substrate and food. Moreover competition is avoided through a rapid succession of changes in the environment which intermittently work in favour of one of the two species.