1. Caves are often assumed to be predator-free environments for cave fishes. This has been proposed to be a potential benefit of colonising these otherwise harsh environments. In order to test this hypothesis, the predator – prey interaction of a belostomatid (predator) and a cave fish (prey) occurring in the Cueva del Azufre
(Tabasco, Mexico) was investigated with two separate experiments.
2. In one experiment, individual Belostoma were given a chance to prey on a cave fish, the cave form of the Atlantic molly ( Poecilia mexicana ), to estimate feeding rates and size-specific prey preferences of the predator. In the other experiment, population density of Belostoma was estimated using a mark – recapture analysis in one of the cave chambers.
3. Belostomatids were found to heavily prey on cave mollies and to exhibit a prey preference for large fish. The mark – recapture analysis revealed a high population density of the heteropterans in the cave.
4. The absence of predators in caves is not a general habitat feature for cave fishes. None the less predation regimes differ strikingly between epigean and hypogean habitats. The prey preference of Belostoma indicates that cave-dwelling P. mexicana experience size-specific predation pressure comparable with surface populations, which may have implications for life-history evolution in this cave fish.