Undergraduate Thesis Or Project


Use of tethered prey for estimating the impact of the invasive European green crab Public Deposited

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  • Various mollusks, including small bivalves and gastropod snails, are a common food source for intertidal crabs. Prey opening techniques used on hard-shell prey are dependent on claw size and morphology. For example, large, strong claws can crush a snail outright while smaller, weaker claws leave characteristic peels, pulls and upper whorl peels. It is therefore often possible to identify the predator responsible for a specific breakage pattern. A feeding study was conducted to "fingerprint" the shell opening techniques of the European green crab and five common native Pacific Northwest crab species on three size classes of the intertidal snail Littorina sitkana. A multiple linear regression analysis utilizing a statistical program resulted in an odds ratio that identified the crab species most likely to perform a given technique. For example, the green crab was 19 times more likely to utilize the pulling technique than a Dungeness crab, while the Dungeness crab primarily relied on crushing. The green crab was the only crab species to utilize an upper whorl peel technique. Shell breakage patterns found on snails tethered to predation lines at various sampling sites in Coos Bay and Yaquina Bay, Oregon, were then used to identify the size and species of foraging crab predators at those sites. A predation line is a tool designed to quantify the foraging impact upon small gastropod snails by various crab species in the intertidal zone. A given number of Littorina sitkana snails are attached to monofilament line with marine epoxy. These lines are then tied to metal rebar rods and left in the intertidal for one full tidal cycle. The fate of the snails on the lines is then scored as: live, attempted peel, peeled, pulled or crushed. The final results yield the overall crab foraging rate and indicate the most likely crab species responsible for the predation. This information can be very useful when comparing predation rates between sampling sites that host the invasive European green crab, and those sites that have not yet been invaded.
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