Potential for passive internal dispersal: eggs of an aquatic leaf beetle survive passage through the digestive system of mallards

Long-distance dispersal of aquatic invertebrates is in many cases facilitated by resistant life stages surviving the passage through the gut of water birds. While this is well known for taxa like crustaceans and bryozoans, it has only very rarely been documented for aquatic insects. The fully aquatic leaf beetle Macroplea mutica shows a remarkably wide distribution throughout the whole Palearctic region, despite showing very little potential for active dispersal. The aim of this study was to test the potential for endozoochorous dispersal in Macroplea mutica by feeding beetle eggs to mallards (Anas platyrhynchos). It was hypothesised that eggs of these water beetles might survive internal transport within birds. Approximately 1% of ingested M. mutica eggs could be retrieved from faeces of mallards 2-8h after ingestion. Larvae hatched from 20% of the retrieved eggs after gut passage. These results represent one of the first documented cases of insects surviving the gut passage through a water bird, and the first experimental evidence of the potential for bird-mediated endozoochorous dispersal of aquatic beetles. The possibility of internal transport could help to explain the remarkably wide distribution of an otherwise strikingly immobile beetle species.