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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.