Predator Foraging Characteristics and Prey Population Structure on a Sheltered Shore

I investigated predation on mussels occurring at upper levels of a sheltered rocky shore on Santa Catalina Island, USA. Despite continuous recruitment, the mussels Mytilus californianus, M. edulis, and Septifer bifurcatus seldom survived to lengths >1.5 cm., and they remained inconspicuous beneath a canopy of perennial algae. Surveys using scuba at high tide and gut—content analyses showed that intertidal mussels were preyed upon by whelks (Ceratostoma nuttali and Maxwellia gemma), diurnal fishes (Halichoeres semisinctus and Semicossyphus pulcher), and nocturnal spiny lobsters (Panulirus interruptus). Whelks and fishes foraged primarily on mid to low shore levels, the lower half of the mussels' vertical range. However, the lobsters foraged over the entire intertidal zone and were the only predator to specialize on mussels. Exclusion of lobsters from upper shore levels caused significant increases in the densities and sizes of the mussels and herbivorous molluscs. Increases in the percent cover of mussels coincided with decreases in the percent cover understory algae. Previous studies open temperate shores indicate that mussel beds form in upper shore refugia where brief submergence intervals preclude predators that require long foraging times and are intolerant of exposure. On some sheltered temperate shores, predators that move quickly with the tides may influence upper shore zonation. Pronounced variation in prey population structure can result from shifts in the relative effectiveness of different foraging characteristics within diverse predator faunas.