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Lobster Predation on Mussels: Shore‐Level Differences in Prey Vulnerability and Predator Preference

Robles, C., Sweetnam, D., Eminike, J.
Ecology 1990 v.71 no.4 pp. 1564-1577
Mytilus californianus, Panulirus interruptus, aquariums, coasts, field experimentation, intraspecific variation, laboratory experimentation, littoral zone, lobsters, mussels, population structure, predation, refuge habitats, shell (molluscs), surveys, tides, California
On a Rocky islet off the coast of Southern California, intertidal beds of the mussel Mytilus californianus are subject to predation by spiny lobsters, Panulirus interruptus, during evening high tides. Mussel shell thickness and degree of inflation (width—to—height ratio) increased with length, but the increases were significantly less for mussels growing on relatively low shore levels. Vertical differences in mussel density, size frequency distribution, and shell morphology were described with transect surveys at low tide. Lobster distribution and abundance were recorded by divers during nocturnal high tides. Laboratory experiments and field observations were done to determine (1) the maximum vulnerable, "critical" lengths above which the mussels cannot be crushed, (2) preferred sizes below the critical lengths, and (3) whether critical lengths or predator preferences varied with the shore—level differences in mussel morphology. Critical lengths, preferences, and other measures of predator performance were determined from predator electivity experiments in laboratory aquaria. The lobsters displayed size preferences that fell far below the critical lengths. Both critical and preferred sizes increased with lobster carapace length, and, consequently, the largest lobsters were able to kill the largest mussels in the bed. Prey handling times ranged from 5 s to 1.6 h, and increased with prey/predator size ratios. For a given carapace length, the critical length was greater for lower shore mussels. For a given mussel length, lower shore mussels required shorter handling times and were preferred over upper shore mussels. Intraspecific variation in prey defenses can affect predator performance. In the intertidal mussel bed, such variation occurs over small spatial scales. In contrast to studies of intertidal prey refugia, few of the prey were invulnerable. Therefore, predator preferences and the relative availability of preferred types may determine aspects of prey population structure.