A chemical defense in Crustacea?

Abstract

The free-living fourth crab stage of the inquiline pea crab Pinnotheres ostreum (Say) lacks typical antipredator defenses frequently found in planktonic and pelagic crustaceans, such as spines, cryptic coloration and/or diel vertical migration. However, in laboratory feeding trials, four species of estuarine fishes consumed many more first stage juvenile blue crabs Callinectes sapidus Rathbun than P. ostreum even though the prey were similar in size. Observations of predator-prey interactions in the laboratory revealed that, once captured, pea crabs were frequently rejected by mummichogs Fundulus heteroditus (L.). Experiments in which mummichogs were offered six prey items (live, heat-killed and cold-killed crabs of each species above) revealed that live pea crabs were rejected >90% of the time, killed crabs were rejected approximately one-third of the time, and blue crabs, live or dead, were always consumed. These data indicate that the unpalatability of pea crabs is not a consequence of akenisis and suggest an active component to the defense. We argue, on the basis of morphological and behavioral evidence, that mechanical irritation is not the source of pea crab unpalatability. The accumulated evidence strongly suggests that this stage of P. ostreum employs a noxious antipredator substance. The origin and composition of this substance and the prevalence of this defensive system (in other stages and species) remain to be investigated.