PREDATION ON DECAPOD CRUSTACEANS THROUGH THEIR ONTOGENY: APPLYING REVERSED HANDEDNESS METRICS TO THE FOSSIL RECORD

The role of durophagous crabs as major predators is well established. These crabs, however, are also preyed upon, and this interaction is potentially observable in the fossil record using claw reversals as a proxy of predation on fossil crabs. Most durophagous crabs have dimorphic claws with a given juvenile right-major/left-minor frequency. Reversed handedness (and, thus, deviations from this frequency) primarily results from claw autotomy as an escape response to predation. Because dactyls (movable fingers) are readily preserved in the fossil record, the incidence of reversals is detectable in death and fossil assemblages.

Interpreting changes in reversal frequency with size can be difficult, however, because reversals persist through successive molts. For example, in mud crabs (Brachyura: Panopeidae) from oyster-bed and seagrass death assemblages from six Florida localities, reversal frequencies differed significantly among size classes for most assemblages. Reversal frequencies increased, and for major dactyls (averaged across assemblages) were 9% (n=253), 16% (n=275), and 24% (n=145) for small, medium, and large size classes, respectively. Minor dactyls also had higher reversal frequencies in larger size classes. We found, however, in modeling predicted changes in reversal frequencies that an increase through ontogeny can be produced by increasing, leaving constant, and even decreasing predation intensities.

We also detected significant differences in reversal frequency between major and minor dactyls within size classes. Because reversals of minor dactyls occur immediately following autotomy, while major dactyl development may take several molts to complete, we initially inferred that these differences related to differences in development rate between major and minor dactyls. We found, however, that the numbers of forms transitional between major and minor claws are not high enough to account for handedness differences between major and minor dactyls. In addition, transitional forms were equally divided between left and right dactyls. In spite of these complications, ontogenetic changes in frequencies of left and right major, left and right minor, and transitional dactyls may provide important data on the incidence and ontogenetic timing of nonlethal injury and mortality of crabs.