Seascapes of Fear in Prince William Sound, Alaska: Decorator Crabs and the Giant Pacific Octopus

Abstract

Decorator crabs (Brachyura: Majoidea) attach objects from their surrounding environment to their bodies as external camouflage. It is thought that these crabs rely on decoration, combined with minimal movement, as an important anti-predator strategy. Oregonia gracilis is a particularly abundant species of decorator crab in the intertidal zones of Prince William Sound, Alaska, where the juvenile giant Pacific octopus (Enteroctopus dofleini) resides in relatively high densities. In Prince William Sound, the giant Pacific octopus is an important consumer of small invertebrates and especially crabs, but O. gracilis is mysteriously underrepresented in octopus diets. Here I investigate the effects of two fear signals on the anti-predator behaviors of O. gracilis: (1) the presence of the giant Pacific octopus and (2) the odor of injured conspecifics. I also investigate the aspects of giant Pacific octopus hunting behavior that may be effectively defended against by O. gracilis decoration. I found that the odor of injured conspecific had a strong effect on O. gracilis behavior, reducing both movement and aggression, but the presence of an octopus had no effect on these behaviors. Surprisingly, decoration rate was reduced in the presence of octopuses as compared to in an octopus-free environment or when the crabs encountered the odor of an injured conspecific. I hypothesize that this surprisingly low decoration rate in the presence of octopuses resulted from habituation to octopus odor in the crab captive tank, which circulated through the same water as the captive octopuses due to space limitations. This habituation could have reduced the sense of fear for crabs in the octopus-odor treatment, and thus reduced the urgency to redecorate. Observations of octopus hunting behavior suggested that visual camouflage in O. gracilis could be an effective deterrent of giant Pacific octopus predation.