CRAYFISH PREDATION ON THE COMMON POND SNAIL (LYMNAEA-STAGNALIS) - THEEFFECT OF HABITAT COMPLEXITY AND SNAIL SIZE ON FORAGING EFFICIENCY

Optimal foraging theory was used to explain selective foraging by the introduced signal crayfish (Pacifastacus leniusculus) on the thin-shel led common pond snail (Lymnaea stagnalis). Crayfish predation efficien cy was studied in relation to habitat complexity and snail size. In a pool experiment(area 1.3 m(2)) single adult crayfish were allowed to f eed on four size classes of snails for one week. A pair-wise preferenc e trial (aquarium experiment) tested if adult crayfish selectively pre dated on particular size classes of snail and if prey value (expressed as snail dry mass per handling time) could explain the size range of snails chosen. Crayfish preferred the smallest size classes of snails in both pool and aquaria experiments. In the pool experiment crayfish had a strong effect on snail survival. Habitat complexity did not affe ct overall snail survival, but resulted in reduced predation pressure on the smallest size classes of snails. Handling time and shell-thickn ess increased exponentially with increasing snail size, and the two sm allest size classes had the highest prey values. The results suggest t hat crayfish can structure the abundance and size distribution of thin -shelled snails, through size-selective predation and reduction of mac rophytes. The mechanisms behind the choice of snails may be based on p rey value and reduced exposure time to predators and conspecifics. Cra yfish effects on snail size distribution may be less pronounced in com plex habitats such as macrophyte beds.