Spiny lobsters and sea urchins: Analysis of a predator-prey interaction

Abstract

Spiny lobsters, Panulirus interruptus (Randall), are important predators of two species of sea urchins, Strongylocentrotus franciscanus (A. Agassiz) and S. purpuratus (Stimpson), which are major herbivores in southern California kelp forest communities. Aquarium experiments indicate that spiny lobsters strongly prefer S. purpuratus over S. franciscanus of comparable test diameter, probably accounting for the higher number of S. purpuratus mortalities observed in the field. Juvenile S. franciscanus (< 60 mm test diameter, TD) are preferred over larger conspecifics, but the smallest S. franciscanus (< 30 mm TD) are protected by association with the adult urchin spine canopy. Mid-sized S. franciscanus too large to fit under the spine canopy are subject to intense predation. All sizes of P. interruptus tested showed a notable reluctance to attack large S. franciscanus (> 90 mm TD) when offered a range of sizes, although the larger lobsters are capable of handling the largest urchins. Relative movement, spine length, test thickness, and gonad weights of sea urchins were examined to evaluate the factors affecting the lobsters' choice of prey, and behavioral responses to lobsters were assessed to determine urchin availability to these predators. The handling problems associated with the long spines of larger S. franciscanus appear to be the major considerations in prey choice despite the much greater potential energy gain from these urchins. Thus predation by spiny lobsters appears to be sufficient to explain the bimodal size-frequency distribution typical of S. franciscanus populations in areas where these predators are common. Heavy fishing pressure on spiny lobsters during this century probably resulted in the release of sea urchin populations and contributed to episodes of destructive urchin grazing observed in the 1950's and subsequent years.