The shifting balance of littoral predator-prey interaction in regimes of hydrodynamic stress

Above lowshore levels of wave-beaten rocky shores, desiccation from tidal e xposure and hydrodynamics stresses from wave action are thought to create r efuges from predation, allowing concentrations of sedentary prey such as mu ssel beds. Underwater time-lapse photography on rocky shores in Southern Ca lifornia revealed that dense aggregations of spiny lobsters prey on mussels during nocturnal high tides. In contradiction of the refuge hypothesis, th e densest aggregations occurred on midshore levels of the most wave-exposed site, a semi-protected site showed intermediate densities, and a protected site showed only sparse numbers of lobsters. On wave-beaten shores, the lo bsters' high mobility and rapid prey handling allowed them to exploit inter tidal prey in the brief period at extreme high tide, when both desiccation and hydrodynamic stresses were at a minimum. The spatial differences in lob ster densities were, however, positively related to the recruitment rates o f juvenile mussels, the preferred prey. A field experiment demonstrated tha t predation by lobsters within a mussel bed affects the age/size structure of the bed without changing primary percent coverage. Therefore, concentrat ions of adult prey on some wave-swept sites appear to result from elevated rates of prey recruitment that surpass rates of predation, rather than abso lute refuges from predation.