Consequences of Dissimilar Defenses Against Predation in a Subtidal Marine Community

Abstract

The interaction between the preferences of predators and defenses of prey and the role it plays in shaping distribution and abundance patterns of two gastropod species in a subtidal marine community were investigated. The snails, Tegula eiseni and T. aureopincta, are patchily distributed on horizontal rocky substrata at depths of 2—10 m off Santa Catalina Island, California, USA. The dominant invertebrate predators in these habitats are the sea stars Pisaster giganteus and Astrometis sertulifera, the whelk Kelletia kellettii, the cephalopod Octopus bimaculatus, and the lobster Panulirus interruptus. Their preferred prey include sear urchins, crabs, and sessile bivalves; the Tegula species are less—favored prey. A positive correlation was found between the abundance of the five predators and that of their preferred nongastropod prey. A negative association was detected between predator density and abundance of all motlile gastropods, prey that tend to rank lower on the preference hierarchies of the predators. The two most common snail species were the Tegula congeners, which comprise 83% of the gastropod group by number. The spatial distributions of the two Tegula species are not identical. T. eiseni, but not T. aureotincta, is abundant in areas supporting moderate densities of predators. Both species are rare in habitats with high predator densities, and are common where few predators occur. The two Tegula species are known to have defenses against predation that differ from another. T. aureotincta displays a flight behavior in the presence of Pisaster, Astrometis, and Kelletia. No such reaction is elicited when the snail is exposed to rapidly moving _unavoidable" predators such as Octopus and Panulirus. T. eiseni does not employ flight defenses, but instead has morphological features that reduce its acceptability as food. Such features are not characteristic of T. aureotincta. These differences in defense probably account for the marked preference on T. aureotincta over T. eiseni by invertebrate predators. Field experiments involving predator and prey manipulations on patch reefs indicate that both emigration and death by predation account for the lack of T. aureotincta in areas with high and intermediate densities of predators. As a consequence of its relatively more effective defense, T. eiseni can persist habitats that contain moderate numbers of predators. The absence of T. eiseni in high predator density areas is the result of direct predation. The dissimilarity in defense mechanisms of the snails accounts for the observed difference in distribution and abundance of the Tegula species. Further, the distribution of predators is correlated with that of their preferred prey. The defenses of the favored prey appear superior to those of motile gastropods, and, as a consequence, the less—preferred gastropod prey are heavily consumed when they occur in the same areas. Although there might be little direct interaction between prey groups, the spatial distribution of preferred pry can thus influence the distribution and abundance of motile grastropods