Grazing pressure on juvenile Nereocystis luetkeana sporophytes: potential top-down effects of the majid crab Pugettia producta

Abstract

The range and distribution of Nereocystis luetkeana kelp forests in the San Juan Islands have changed in the past 100 years. Urchin removal experiments designed to test the paradigm of top-down control of kelp forests by urchins in the San Juan Channel showed no significant differences in macroalgal density or community composition after urchins were removed, suggesting that grazing by other invertebrates may control kelp populations. Few studies have been completed on the effects of grazing by crabs like Pugettia producta on kelp forest communities. A caging experiment was conducted over 15 days in the field to compare the grazing impacts of P. producta to those of mesograzers in the kelp forest canopy on juvenile Nereocystis sporophytes. Six types of blocks were suspended off the Friday Harbor Labs breakwater in triplicate. Kelps were attached to blocks with 1) two different closed cages with 13 mm and 50 mm mesh, 2) two different open cages with 13 mm and 50 mm mesh, 3) a 13 mm mesh cage with an individual P. producta, or 4) no cage and exposed to grazing. At the end of the experiment, the ampithoid amphipod Peramphithoe was the most commonly seen mesograzer associated with the blocks. Significant differences in kelp mass and area change were observed between control blocks and crab-only blocks, implying that individual P. producta have a much greater impact on juvenile sporophyte survival than canopy mesograzers associated with the blocks.