ELLIOTT, JK*; CHAMBERS, R; MCKINLEY, M; YOUNG, E; University of Puget Sound, Tacoma: The influence of predation on the distribution and abundance of an introduced mussel

Introduced species are considered to be one of the largest threats to biodiversity in marine ecosystems because they can displace and/or hybridize with native species, and significantly alter food webs. The European blue mussel (Mytilus galloprovincialis = Mg) has been introduced into the Pacific Northwest in ballast water from ships and by shellfish culture operations. Populations of Mg have become established in a variety of areas throughout Puget Sound, and Mg is hybridizing with the native mussel M. trossulus (Mt). Hybrids (Mgt) are typically much larger than Mt or Mg. Mussels are found predominantly on urban structures (floating docks, pilings, and concrete bulkheads) in South Puget Sound with Mg and Mgt being most abundant on floating docks. The large Mg and Mgt mussels also occur at low tide levels on concrete walls and pilings. A variety of factors (e.g., predation, growth rates, desiccation, larval recruitment) may be causing these observed distribution patterns. In this study we examined the effects of predation by crabs and seastars. Red rock crabs, Cancer productus, preferred to consume small (20-40 mm) and medium (40-60 mm) size mussels over large (>60 mm) mussels. The seastar Pisaster ochraceus consumed mostly large and medium size mussels, but if these larger mussels were covered with epibionts (e.g., sea anemones) then they were least preferred. Mussel attachment strength, which is proportional to resistance to crab predation, increased with mussel size and decreased with tide height on concrete walls and pilings. Thus, large Mg and Mgt mussels survive best in areas with higher crab predation (e.g., low rock walls and the lowest sections of pilings). Large mussels covered with epibionts survive best in areas with high seastar predation (e.g., floating docks).