Foraging Activity and Prey-size Selection in Nucella freycineti(Deshayes)(Neogastropod: Muricidae)

Abstract: The seasonal preferences for prey species and prey size by the dogwhelk Nucella freycineti(Deshayes)were investigated on an intertidal rocky shore in Hokkaido, northern Japan. Adult dogwhelks(>25 mm shell length)mainly preyed upon the mussels Septifer virgatus and Mytilus sp. in the field throughout the year. In the laboratory, they prefbrred Mytilus sp. over S. virgatus in June, and this preference increased in August. The prey values of the two mussels were compared using the formula(energy amount of one prey)/(drilling time). I assumed that the dry tissue weight of the prey reflected the energy content and that the valve thickness of the prey reflected the drilling time. Although the dry weights of Septifer and Mytilus were not significantly different in June, Mytilus sp. became heavier than the same size S. virgatus for all sizes in August. Indices of valve thickness(valve thickness-shell length ratio)athe point in mussel shells where dogwhelks preferred to make a hole before feeding were not significantly diffberent between the two mussel species, indicating that the drilling times of both prey types may be almost the same. These resultsuggest that Mytilus sp. and S. virgatus have almost the same prey value in June, but Mytilus sp. is a more valuable prey in August. Although the energy content of one prey and drilling effort required were used to estimate the prey value, this study suggests that other factors(e. g., morphological differences of prey, nutritional quality of prey)might be also important for seasonal prey selection. Dogwhelks selected significantly larger mussels in summer than in winter both in the laboratory and field. Hence, dogwhelks consume optimal species(more valuable species)and prey size(bigger sizes)in warm periods, buthe foraging behavior is restricted by environmental conditions in cold periods.