THE EFFECT OF PREDATION ON PHENOTYPIC PLASTICITY IN THE HOLOCENE POND SNAIL HELISOMA NEWBERRYI

Predation by durophagous arthropods is a well-known and well-documented force driving the evolution of shell morphology in marine gastropods. The evolutionary consequences of such predation in freshwater systems are less well-studied, however. The shores of Bear Lake, on the Utah-Idaho border, are littered with the remains of locally extinct Holocene gastropods that record an unusual pattern of predator-prey interaction. In all regions of the lake, shell repair scars indicate the activity of aperture-entry predators, most likely crayfish. Around most of the lake’s perimeter, these predators show a distinct preference of prey organisms, attacking some species and virtually ignoring others. However in one particularly shallow region of the lake, all species are being predated with roughly equal frequency. Taphonomic experiments conducted in the laboratory strongly suggest that these shells have been subjected to minimal transport and that the patterns of predation most likely reflect real differences in the living populations. This difference in predation rates, for the same prey species, between different regions of the lake created two very different selective regimes for these gastropods. This difference is then in turn reflected in the range of variation in the shell morphology of some of the more phenotypically plastic species.