KEYSTONEin: a glycoprotein cue drives predation on mussels and structures rocky intertidal communities

Foundation species provide critical resources to ecological community members and are major determinants of biodiversity. One such species, the California mussel Mytilus californianus, is a superior competitor and dominates space on rocky, wave-swept shores (northeast Pacific Ocean). Live mussels secrete a 29.6 kDa glycoprotein (named 'KEYSTONEin'), with expression localized in epidermis, extrapallial fluid, and periostracum (organic shell coating). Hence, KEY-STONEin is available for contact recognition as predators crawl on rocky surfaces. Purple sea stars Pisaster ochraceus, whelks (Nucella emarginata and Acanthinucella spirata), and lined-shore crabs Pachygrapsus crassipes are common intertidal carnivores. Spanning 3 phyla and differing substantially in sensory mechanisms and hunting techniques, all are major mussel consumers. Here, we constructed faux prey to mimic both physical and chemical properties of mussels. Purified KEYSTONEin was presented at a typical mussel concentration. All 4 predatory species attacked, and ate, KEYSTONEin-infused faux prey as opposed to the organic enrichment or seawater controls. Sea stars, whelks (both species), and crabs also did not distinguish between KEY-STONEin-laced mimics and their live, intact counterparts. Mechanisms of chemical recognition thus have converged across phylogenetically diverse taxa to promote the exploitation of a valuable, shared, prey resource. By providing critical sensory information, KEYSTONEin drives mussel predation and initiates trophic cascades that shape community structure and function. KEY-STONEin is necessary and sufficient as a predatory cue of considerable ecological consequence