Size selection by a gape-limited predator of a marine snail: insights into magic traits for speciation

The intertidal snail Littorina saxatilis has repeatedly evolved two parallel ecotypes assumed to be wave adapted and predatory shore crab adapted, but the magnitude and targets of predator-driven selection are unknown. In Spain, a small, wave ecotype with a large aperture from the lower shore and a large, thick-shelled crab ecotype from the upper shore meet in the mid-shore and show partial size-assortative mating. We performed complementary field tethering and laboratory predation experiments; the first set compared the survival of two different size-classes of the crab ecotype while the second compared the same size-class of the two ecotypes. In the first set, the large size-class of the crab ecotype survived significantly better than the small size-class both on the upper shore and in the laboratory. In the second set, the small size-class of the crab ecotype survived substantially better than that of the wave ecotype both on the upper shore and in the laboratory. Shell-breaking pred...