Distribution, Growth and Mortality of Juvenile Clams in the San Juan Islands, WA

Abstract

An understanding of the ecology and life history of juvenile organisms is crucial for understanding these aspects of the adult organism. Intertidal bivalves rely on the successful settlement of larvae and the survival and growth of juveniles. Determining the distribution of juvenile bivalves is important for understanding the distribution and predicting the presence of adult bivalves, for both ecological and commercial reasons. Measuring growth rates and mortality of juvenile bivalves can shed light on what is necessary to carry a juvenile bivalve through adulthood. I surveyed several sites in the San Juan Islands, WA, recording sediment type, tidal height, beach slopes and wave exposures to determine the abundance of juveniles at these sites and what factors can be used as indicators of juvenile presence. I found that surface granules, surface mud, beach slope and subsurface shell hash were correlated with patterns observed in clam assemblages. It also appears that a particular site is a better predictor of clam assemblages and physical features than tidal height. I also tested the response of several species of intertidal clams—Tresus capax, Saxidomus giganteus, Macoma spp., Protothaca staminea, Venerupis philippinarium and Mya arenaria—to increased sediment temperatures during low-tide of different lengths. I found that growth increased in ideal conditions from species that live lowest in the intertidal to those that live in the high intertidal. Mortality in elevated temperatures increased with decreasing intertidal elevation. Effects of predation were also tested—Hemigrapsus nudus and Cancer productus were found to prey on juvenile Macoma spp. H. nudus’s preference between Macoma spp. and Ulva depended on previous experience handling clams.