Complexity of the prey spectrum ofAgaronia propatula(Caenogastropoda: Olividae), a dominant predator in sandy beach ecosystems of Pacific Central America

Abstract

Olivid gastropods of the genusAgaroniaare dominant predators within invertebrate communities on sandy beaches throughout Pacific Central America. At Playa Grande, on the Pacific Coast of Costa Rica, we observed 327 natural predation events byAgaronia propatula. For each predation event, we documented prey taxa and body size of both predator and prey. The relationship between predator and prey size differed for each of the four main prey taxa: bivalves, crustaceans, heterospecific gastropods, and conspecific gastropods (representing cannibalism). For bivalve prey, there was increased variance in prey size with increasing predator size. Crustaceans were likely subdued only if injured or otherwise incapacitated. Heterospecific gastropods (mostlyOlivella semistriata) constituted half of all prey items, but were only captured by small and intermediately sizedA. propatula. LargeO. semistriataappeared capable of avoiding predation byA. propatula. Cannibalism was more prevalent among largeA. propatulathan previously estimated. Our findings suggested ontogenetic niche shifts inA. propatulaand a significant role of cannibalism in its population dynamics. Also indicated were size-dependent defensive behavior in some prey taxa and a dynamic, fine-scale zonation of the beach. The unexpected complexity of the trophic relations ofA. propatulawas only revealed though analysis of individual predation events. This highlights the need for detailed investigations into the trophic ecology of marine invertebrates to understand the factors driving ecosystem structuring in sandy beaches.