DEVELOPING A PROXY FOR PREDATION ON FOSSIL CRABS: GROWTH AND REVERSAL OF HANDEDNESS OF THE MUD CRAB EURYPANOPEUS DEPRESSUS UNDER LABORATORY CONDITIONS

Because fossil crab claws are used both for defense and feeding, they may be the best tools for testing the relative roles of top-down and bottom-up evolutionary forces. However, their use as a proxy for predation intensity assumes that reversed-handedness frequencies (switching of the major claw from right to left) accurately reflect injury rates.

Eurypanopeus depressus is the numerically dominant crab of most intertidal and shallow subtidal oyster reefs in the Gulf of Mexico and western Atlantic. While studies of commercially important species of crabs have documented handedness reversals following removal of the major claw, the effects of claw autotomy on E. depressus has not been reported. In addition, previous work on E. depressus dactyls from death assemblages demonstrates that frequencies of reversed handedness rise through successively larger size classes. Without knowledge of molting frequency, however, these patterns are difficult to interpret.

In order to better understand growth and patterns of claw regeneration of Eurypanopeus, approximately 60 juvenile crabs collected from oyster bars in Lemon Bay, Florida were isolated in individual glass jars inside two ten gallon tanks with differing temperatures for seven months. Half the crabs from each tank were forced to autotomize their major claw. Results indicate that crabs with carapace widths (CW) of 0.5 cm molt after 10 days on average, while crabs with 1.0 cm or greater CW have an average molt frequency of 25 days. The number of times a crab has molted can be predicted from isolated fingers, the most commonly preserved crab part, not only using linear regression models, but also because successive molts fall into discrete size classes. We also found that handedness reversals occurred only as a result of major claw autotomy. The first regenerative molt following autotomy results in the development of minor claws in place of major claws. Original minor claws increase in size, but change little morphologically with the exception of a newly formed, pointed, proximal tooth on the dactylus and a slight increase in dactylus curvature. Transformations of minor claws into major claws take approximately three molts after autotomy of major claw. This expimental work supports the utility of reversed handedness as a proxy for injury in crab death and fossil assemblages.