Northern kelp crab (Pugettia producta) feeding preferences - kelp crabs eat kelp, and lots else?

Bull kelp (Nereocystis luetkeana) is an important source of both habitat and primary production in the Salish Sea and appears to be in decline in certain areas. Northern Kelp Crabs (Pugettia producta) are large crustacean consumers that preferentially consume N. luetkeana over other local seaweed species in laboratory feeding trials and exert some level of top-down control on kelp populations in the field. We have observed differences in kelp crab density around the Salish Sea and noted P. producta living on invasive wireweed (Sargassum muticum) as well as on kelp and in other habitats. We used a combination of laboratory feeding assays, field surveys, and crab foregut dissection to help quantify P. producta feeding preferences and determine if kelp crab gut contents “match” the substrate from which they were collected. Whole tissue laboratory feeding trials indicated the kelp crabs prefer to eat bull kelp; however, when offered artificial food (“algal jello”), crabs can and do eat Sargassum. The gut contents of crabs collected near San Juan Island contained sometimes very high levels of Sargassum, while crabs collected further south (Squaxin Island, Joemma State Park) contained less Sargassum even though it was still present in the environment. Crab foreguts were sometime filled with diatoms, ulvoid algae, and even eelgrass blades, indicating that they may be opportunistic feeders in the field. A better understanding of the ecological relevance of Northern Kelp Crab feeding preferences has important implications for future bull kelp management, conservation, and restoration efforts