Aplexa hypnorum(Gastropoda: Physidae) exerts competition on two lymnaeid species in periodically dried ditches

Abstract

-Samples of adult Aplexa hypnorum were experimentally introduced into periodically dried ditches colonized by Galba truncatula or Omphiscola glabra to monitor the distribution and density of these snail species from 2002 to 2008, and to compare these values with those noted in control sites only frequented by either lymnaeid. The introduction of A. hypnorum into each ditch was followed by the progressive colonization of the entire habitat by the physid and progressive reduction of the portion occupied by the lymnaeid towards the upstream extremity of the ditch. Moreover, the size of the lymnaeid population decreased significantly over the 7-year period, with values noted in 2008 that were significantly lower than those recorded in 2002. In contrast, the mean densities were relatively stable in the sites only occupied by G. truncatula or O. glabra. Laboratory investigations were also carried out by placing juvenile, intermediate or adult physids in aquaria in the presence of juvenile, intermediate or adult G. truncatula (or O. glabra) for 30 days. The life stage of A. hypnorum had a significant influence on the survival of each lymnaeid. In snail combinations, this survival was significantly lower for adult G. truncatula (or O. glabra) than for intermediate snails. In contrast, the survival of juveniles was similar to that noted in the corresponding controls. This interspecific competition between the physid and either lymnaeid may not be due to food present in their habitat, but might be due to the toxicity of mucus secreted by intermediate and adult A. hypnorum.