Effect of neighboring prey species on antipredator response of the marine gastropod <i>Nassarius hypolius</i>

Prey often share predation risk with their neighbors. The presence of neighbors can dilute the probability of attack on any given individual, thereby decreasing predation risk. If the identity of a neighboring prey species affects predation risk, prey individuals might alter their antipredator response accordingly. This study tested the hypothesis that the intertidal gastropod Nassarius hypolius responds differently to the carnivorous whelk Neptunea arthritica in the presence of neighboring conspecifics, Gigahomalopoma sangarense, or Cantharidus japonicus. Both Na. hypolius and G. sangarense are preferred over C. japonicus by the whelk, but the handling time for G. sangarense is approximately 10 times longer than that of the other two species. This suggests that the predation risk to Na. hypolius is more strongly reduced by the presence of neighboring G. sangarense than by conspecifics, but less strongly reduced by neighboring C. japonicus. The frequency of antipredator response (i.e., burrowing in the sand) before and after exposure to a predator stimulus were compared among three groups in which a focal Na. hypolius was placed among five (1) conspecifics, (2) G. sangarense, or (3) C. japonicus. A significant increase in the frequency of antipredator response was observed in groups 2 and 3 following the predator stimulus, but not in group 1. Although this suggests that the antipredator response of Na. hypolius is influenced by the identity of neighboring prey species, the direction of response did not align with our initial hypothesis. Alternatively, Na. hypolius may tailor its antipredator response to the density of nearby conspecifics.