Foraging Ecology of Intertidal Crabs (Hemigrapsus oregonensis) at Anthropogenic Sulfide Seeps in Commencement Bay, Washington

Stella Mosher, University of Puget Sound


Organic pollution from wood waste produced by the lumber industry between 1869 and 1977 resulted in 30 or more feet of debris on the bottom of Commencement Bay in Washington State. Woodchip decomposition by sulfate-reducing bacteria inhabiting this zone has created a hydrogen sulfide (H2S) rich environment, which contains many similarities to the anoxic and H2S-rich environments found at hydrothermal vents and cold methane seeps. The H2S-rich intertidal zone in Tacoma supports extensive mats of free-living chemoautotrophic (sulfide-oxidizing) bacteria that form on hard substrates such as rocks and the carapaces of intertidal crabs (Hemigrapsus oregonensis) that live under the rocks. This study examined how, or if, bacteria contribute to crab diet. In the field, the diversity and abundance of food sources available to crabs were measured. In lab, experimental feeding trials were used to observe feeding behavior. Carbon nitrogen and sulfur stable isotope analyses of crab and food source tissues, as well as algae, were used to build trophic food webs and answer whether of not the crabs were feeling on the bacteria. Fluorescence In Situ Hybridization (FISH) and DAPI staining were also used to determine the presence or absence of bacteria in crab gut contents. Results suggested that Hemigrapsus oregonensis prefers to feed on green algae and does not feed on sulfur-rich bacteria given the choice, but that it is being incorporated into their diet in small amounts. Crabs appear to eat the bacteria if they are not provided with another food source.