After the introduction of Gammarus tigrinus in The Netherlands some 25 years ago and of other recently invading amphipods (Crangonyx pseudogracilis and Corophium curvispinum) the native species decreased, the invaders increased. The success of these invaders and its impact on the local amphipod fauna is discussed. Electrophoretic tests give evidence that a second invasion of Gammarus tigrinus, this time from Germany, is taking place. Some predictions about future developments are made.
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.
Genetic differentiation of morphologically variable members of the G. pulex-group in northwestern Europe was investigated by electrophoresisat 20 enzyme loci. Five recently described related forms were examined with reference to the “classical” species G. pulex, G. wautieri and G. fossarum. G. pulex and G. wautieri were shown to be genetically homogeneous and clearly distinct species. In the other species, morphologicallysimilar forms were shown to be genetically distant, and inter-areal intraspecific genetic differentiation may warrant recognition of subspecies or sibling species. In some cases, levels of reproductive isolation of the forms involved could be assessed by cross-breeding experiments. To estimate dispersal capabilities, gene flow levels were indirectly estimated from gene frequency data. Gene flow levels are generally low and fluctuating population sizes may enhance the occurrence of stochastic processes. Dendrograms derived from genetic distances were compared with zoogeographical and paleoclimatological evidence. The distribution areas of the species involved correspond to patterns predicted by these data.
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