Autotomy-induced effects on the locomotor performance of the ghost crab Ocypode quadrata

The voluntary amputation of an appendage, or autotomy, is an effective defensive mechanism that allows an animal to escape aggressive interactions. However, animals may suffer long-term costs that can reduce their overall fitness. Atlantic ghost crabs (Ocypode quadrata) are one of the fastest terrestrial invertebrates, and regularly lose one or more limbs in response to an antagonist encounter. When running laterally at fast speeds, they adopt a quadrupedal gait using their first and second pairs of legs while raising their fourth, and sometimes the third, pair of legs off the ground. This suggests that some limbs may be more important for achieving maximal locomotor performance than others. The goal of this study was to determine whether the loss of certain limbs would affect running performance more than others, and what compensatory strategies were used. Crabs were assigned to four different paired limb removal treatments or the control group and run on an enclosed trackway in their natural habitat. Ghost crabs were found to adjust stride kinematics in response to limb loss. Loss of the second or third limb pairs caused a reduction in running speed by about 25%, suggesting that the remaining intact limbs were unable to compensate for the loss of either limb, either due to a lack of propulsive forces produced by these limbs or issues stemming from re-coupling limb arrangements. Loss of any of the other limbs had no detectable effect on running speed. We conclude that compensatory ability varies depending on the limb that is lost.