The role of kelp crabs as consumers in bull kelp forests—evidence from laboratory feeding trials and field enclosures

The Northern kelp crab (Pugettia producta) and the graceful kelp crab (Pugettia gracilis) are common primary consumers in bull kelp beds near the San Juan Islands (Salish Sea, NE Pacific). In this system, urchins (often considered the most voracious herbivores exerting top-down control on kelp beds) tend to remain sedentary because of the high availability of detrital macroalgae, but the extent to which kelp crabs consume kelp (and other food options) is largely unknown. I conducted four types of laboratory feeding experiments to evaluate kelp crab feeding patterns: (1) feeding electivity between bull kelp (Nereocystis luetkeana) and seven species of co-occurring local macroalgae; (2) feeding electivity on aged vs. fresh bull kelp; (3) feeding preference between N. luetkeana and small snails (Lacuna sp.); and (4) scaling of feeding rate with body size in P. producta and P. gracilis. In choice experiments, P. producta consumed greater mass of N. luetkeana than of other macroalgal species offered and elected to eat fresh bull kelp over aged. However, P. producta also consumed snails (Lacuna sp.), indicating more generalized feeding than previously suspected. Feeding rates for P. producta exceeded the expected 3∕4 scaling rule of metabolic rates, indicating that larger P. producta may have a disproportionately large impact on bull kelp. A subtidal field experiment, designed to assess the influence of consumers on juvenile bull kelp net tissue gain, found that only fully enclosed (protected) bull kelp increased in wet mass and blade length. Herbivory by kelp crabs, among other consumers, is likely to play a previously unrecognized role in mediating the growth and survival of this annual kelp species within the Salish Sea.