Predator-prey interactions in opisthobranch gastropods: effects of prey body size and habitat complexity

Abstract

ABSTRACT: This study examines size-specific predation by navanax Aglaja inerxnis (Gastropoda: Opisthobranchia), upon the sea hare Aplysia californica (hereafter Aplysia) (Gastropoda: Opisthobran- chia). Aplysia reach a refuge in size from navanax predation at ca 10 cm in length, but very few individuals reach this size. Most Aplysia in the field were 5 3 cm long. Navanax diet in the field was disproportionately composed of larger Aplysia (> 2 cm long). Laboratory experiments with 0.5, 1.0 and 2.0 cm Aplysia and 2 levels of habitat complexity revealed that encounter rates, which rose wth Aplysia size, were overwhelmingly important in determining size-selectivities. Attack probabilities (given an encounter) were always equal to 1, and capture success probabhties (gven an attack) rose slightly with Aplysia size. Increasing habitat complexity reduced encounter rates disproportionately for 0.5 cm Aplysia as compared with 2.0 cm Aplysia. The results suggest that the effects of body size and habitat complexity on predation may interact strongly in a way which depends upon the specific foraging biology of the species involved and upon the predator-prey ratios used. INTRODUCTION