A comparison of the strengths of gastropod shells with forces generated by potential crab predators

Carcinus manenas, Liocarcinus puber and Cancer pagurs are thought to be three likely crab predators of the gastropod Calliostoma Zizyphinum. In order to compare the strenghts of predators and their prey, the whole shell and aperture lip strengh of white and pink Calliostoma morphotypes and the maximum forces exerted by the chelipeds of three crab species were measured. Although white shells were thicker than pink shells, Calliostoma colour morphotyes did not differ significantly in either the force required to break the shell lip or the whole shell. Both Liocarcinus puber and Carcinus maenas have dimorphic chelipeds and their “crusher” chelipeds deliver almost double the forces generated by the‘cutter’chelipeds. In constrast, Cancer pagurus has monomorphic chelipeds both delivering similar forces. When compared with Calliostoma shell strenght, the forces generated by the‘crusher’chelipeds of most L. puber tested were, in general, sufficient to break the shell lip of Calliostoma shells, whereas forces generated by the‘cutter’chelipeds of only the larger individuals were sufficient to break the shell lip. In C. manenas, forces generated by both the‘cutter’and‘crusher’chelipeds often exceeded the minimum recorded force required to break the shell lip and the‘crusher’cheliped reached the minimum force required to break whole Calliostoma shells. Both chelipeds of all C. pagurus tested generated forces in excess of the minimum required to break the shell lip, and the proportion of individuals capable of generating the minimum force required to break the whole shell increased with the size of the size of the crab. Carcinus maenas and Cancer pagurus were capable of breaking both the shell lips and the whole shells of a wider range of shell sizes than L. puber.

No evidence of morphine analgesia to noxious shock in the shore crab, Carcinus maenas

A number of criteria have been suggested for testing if pain occurs in animals, and these include an analgesic effect of opiates (Bateson, 1991). Morphine reduces responses to noxious stimuli in crustaceans but also reduces responsiveness in a non-pain context. Here we use a paradigm in which shore crabs receive a shock in a preferred dark shelter but not if they remain in an unpreferred light area. Analgesia should thus enhance movement to the preferred dark area because they should not experience 'pain'. However, morphine inhibits rather than enhances this movement even when no shock is given. Morphine produces a general effect of non-responsiveness rather than a specific analgesic effect and this could also explain previous studies claiming analgesia. However, we question the utility of this criterion of pain and suggest instead that behavioural criteria be employed. (C) 2011 Published by Elsevier B.V.