Mouthpart Morphology of Three Sympatric Native and Nonnative Gammaridean Species: Gammarus pulex, G. fossarum, and Echinogammarus berilloni (Crustacea: Amphipoda)
In the last 20 years several nonnative amphipod species have immigrated inland waters of Germany and adjacent central European countries. Some of them have been very successful and could establish stabile populations. In some places, they have even replaced native or earlier established species. The gammarid Echinogammarus berilloni originates from the Atlantic region of France and the north-western part of Spain and coexists in some central European waters with the native Gammarus pulex and G. fossarum. Here, we describe and compare the mouthparts and other structures involved in food acquisition of these three sympatric gammaridean species. Our hypothesis was that differences in the mode of feeding of the three species could be the reason for their coexistence and that these differences would be expressed in differences in mouthpart morphology. The results of our SEM study demonstrate that there are indeed interspecific differences in details of the morphology of the feeding structures. This is especially true for the setation of antennae, maxillulae, gnathopods, and third uropods, which can be interpreted as adaptations to special modes of feeding. Generally, all three species are omnivorous, but specializations in details point to the possibility to use some food resources in a special effective way. 1. Introduction The gammaridean fauna of middle European inland waters has dramatically changed in the last two decades; particularly, Ponto-Caspian gammarideans arrived in middle European rivers, canals, and lakes [1–11]. Immigration has happened and is still ongoing via three main corridors: (i) along the Danube, Main-Donau Canal and Main into the Rhine system; (ii) via the Pipet-Bug connection from the east, and from the north and along the Baltic coast via ships [12]; (iii) also from the Mediterranean region, freshwater gammarideans have enlarged their range of distribution towards western and middle Europe [5, 13–16]. Some of these nonnative gammaridean species could establish stabile populations and occur in high densities, and several species have a severe impact on the ecology of the invaded regions by reducing and even eliminating native and earlier established gammaridean species (therefore, the immigrants are called invasive). One well-examined example of such invasive species is Dikerogammarus villosus (Sowinsky, 1894) [17]. This species is now the dominant gammaridean species in many rivers, canals, and larger lakes all over central Europe, affecting also other members of the macrozoobenthos [1, 6, 18–20]. But not all nonnative species are
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