Growth Rate as a Measure of Food Value in Thaidid Gastropods: Assumptions and Implications for Prey Morphology and Distribution

Rates of body growth were measured for three species of rocky intertidal gastropods: Thais (or Nucella) lumellosa (Gmelin), T. (or N.) canaliculata (Duclos), and T. (or N.) emarginata (Deshayes). Three size classes of each predator species were grown experimentally in cages at two tidal heights on four size classes of each prey species: Semibalanus cariosus (Pallas), Balanus glandula Darwin, and Mytihts edulis L. Growth was also assessed for Thais canaliculata feeding on Mytilus californianus Conrad, and Thais emarginata feeding on Chthamalus dalli Pilsbry. Rates of body growth varied as a function of predator size, prey size and prey species. With one exception (large Thais canaliculatu) intermediate sized Balanus glundulu promoted the most rapid growth for all sizes of all three species of Thais; thus these potentially competing predator species have the same highest ranked prey. Among prey promoting slower growth, those encountered commonly in the normal habitats of the predator promoted more rapid growth than those encountered rarely, suggesting that past evolutionary experience has influenced present food value ( = growth potential) of prey. For some predators, the ranking of prey species changed with predator size; Chthamalus dalli and similar sized Balanus glandula promoted comparable growth in small Thais emarginata but Chthamalus dalli promoted much slower growth than similar sized Balanus glandula in larger Thais emarginata. Rank differences based on prey size in Balanus glandula became more pronounced and in some instances changed with increasing predator size; larger B. glundula promoted relatively faster growth for large snails than for small snails in all Thais species. Finally, growth rates were correlated with two important attributes of fitness in T. canaliculatu and T. emarginata; in general, prey promoting more rapid growth also resulted in an earlier age of first reproduction for initially immature snails and higher rate of egg capsule production for mature individuals. Thus, these growth rates provide a basis from which to examine quantitatively patterns of prey selection from an energyor growth-maximization perspective. These patterns of predator growth also permit inferences about the possible influence of predation on the evolution of prey morphology, life-history, and microhabitat distribution. Size refuges from Thais predation were confirmed for both Semibalanus cariosus and Mytilus californianus. Both Balanus glandula and Mytilus edulis, on the other hand, were vulnerable to predation by all sizes of Thais examined (> 10 mm) regardless of prey size: thus, neither of these species achieves a size refuge. In addition, on rocky shores, vertical distributions of the prey species reflect general preference patterns of their predators: higher value prey species, i.e. those with less well developed defensive morphologies (Balanus glandula, Mytilus edulis), generally occur higher on the shore. This pattern would be expected if preferred prey are consumed first lower on the shore where they are more available.