Visual and chemical cues from aquatic snails reduce chironomid oviposition

Chironomus oppositus sensu lato and the freshwater gastropod Physa acuta are distantly related taxa inhabiting temporary ponds. Although their distributions overlap, their peak abundances do not coincide in time and space. This study used field-based mesocosm experiments to examine the effect of P. acuta on the distribution and abundance of C. oppositus. Results indicated that chironomid larval abundance was substantially higher in mesocosms without P. acuta. The abundance of C. oppositus decreased as snail density increased, from one snail per litre. To investigate whether this result was due to differential survival by chironomid larvae or preferential oviposition by chironomid adults, both factors were manipulated separately and the mechanisms involved investigated. Direct-interaction experiments demonstrated that direct (mechanical) and indirect (water-borne) interference by snails had limited impact on the survivorship of third- or fourth-instar C. oppositus. Oviposition trials indicated that chironomid egg strings were five times less abundant in mesocosms containing both visual and water-borne snail cues. However, the presence of each of these cues separately did not result in a similar reduction. This study demonstrates that negative covariance occurred between larval chironomids and snails, with chironomids avoiding snails through oviposition site selection. This has important implications for understanding the mechanisms structuring freshwater communities.