Mouthpart Morphology of Gammarus Roeselii Compared to a Successful Invader, Dikerogammarus Villosus (Amphipoda)

Abstract

Until about 20 years ago, the well-established gammarid amphipod Gammarus roeselii, originating from the Balkan Peninsula, was widespread and often the dominant amphipod species in rivers, canals, and big lakes of Germany and adjacent countries. Since 1989 Dikerogammarus villosus, a Ponto-Caspian pontogammarid, has successfully invaded the aquatic systems of mid and western Europe reducing and even eliminating native and earlier established gammarideans including G. roeselii. Field observations and laboratory experiments indicate that these two gammaridean species have different feeding habits. In our SEM study we wanted to test the hypothesis that different modes of food acquisition are expressed in differences in the morphology of the mouthparts and other structures involved in feeding, such as the antennae and gnathopods, possibly are indicative of the success of D. villosus over G. roeselii. Although both gammarideans have tools for omnivorous feeding, comparisons indeed revealed obvious differences in appendages involved in food acquisition. In G. roeselii, the molar surfaces of the mandibles are structured like rasps, suggesting they are well suited for grinding firm plant material, whereas those of D. villosus are only weakly structured. In G. roeselii, the basipodal endites of the maxillulae are armed with chisel-like cuspidate setae, and the maxillipeds bear flattened cuspidate setae, possibly a specialization of these mouthparts for scraping off adherent food from the substrate. We did not find such modifications on the maxillulae and maxillipeds of D. villosus. Compared to D. villosus, the setation of the antennae and the gnathopods in G. roeselii is sparse and short. Therefore, filter feeding, one of the feeding modes of D. villosus, seems to play a minor role in the nutrition of G. roeselii.