ROSALES*, A.B.; MARIJNISSEN, S.A.E.; MICHEL, E.; MCINTYRE, P.B.; University of California, Santa Barbara; University of Amsterdam, Netherlands; University of Amsterdam, Netherlands; Cornell University, Ithaca, NY: Snail susceptability to crab predation: A case study of co-evolution from Lake Tanganyika, East Africa

Co-evolution has been invoked as an explanation for Lake Tanganyika�s unusual endemic fauna. The crabs Platytelphusa armata and Potamonautes platynotus are endemic molluscivores, found in the same habitats as the gastropods Lavigeria coronata, L. nassa and L. grandis. As these are the largest snails in the Lavigeria species flock, and live exposed on rocks, we tested whether shell size and sculpture provides protection from predation. We compared the relative susceptibility of six different size classes of snails to predation by P. armata and P. platynotus in laboratory experiments and tested significance with logistic regressions. Our results indicate that P. armata, the more derived of the two crabs, is a more effective predator than P. platynotus. L. coronata, the most sculptured snail, was more frequently damaged but less frequently killed than L. grandis. L. nassa, with intermediate sculpturing, showed intermediate resistance. Finally, all three species of gastropods were killed only in the smallest three size classes, were damaged in the middle classes, and resistant to shell crushing in the largest class. Our results strongly support the idea of an ongoing evolutionary arms race in the endemic fauna of this lake, with large size and sculpture serving as a refuge for the gastropods from crab predation.