Biomechanics of pleon attachment in the European shore crab Carcinus maenas (Linnaeus, 1758) (Brachyura: Portunoidea: Carcinidae)

Brachyuran crabs have a distinct outer morphology with a highly reduced pleon, which is folded underneath a usually broad cephalothorax. The pleon is secured in this position with a specialised holding mechanism, which consists of a sternal knob, or press-button, and, in most eubrachyurans, a corresponding pit or socket on the pleon. The effectiveness of this mechanism has not been described so far. In this study a method to investigate the force needed to move the pleon from its folded position was measured in male and female crabs under different experimental conditions is presented. In male specimens a median pull-off force of 167.20 mN was measured in air. The measured force decreased slightly underwater. When the press-buttons were removed artificially, no pleon attachment was feasible underwater. The variation observed in the pull-off forces measured is most likely a result of different levels of wear or damage of the holding structures. In mature females, which lack these structures, no pleon attachment was possible underwater. In air, residual moisture and the morphology of the pleon resulted in attachment via capillary interactions. The method presented in this study can be used in further investigations to evaluate the effectiveness of pleon-holding structures in various brachyuran crabs.