Unraveling the nature of individual recognition by odor in hermit crabs.

Individual recognition is a key element in the social life of many invertebrates. However, most studies conducted so far arrived at documenting that several species are capable of a “binary” discrimination among conspecifics, but not of “true individual recognition”. Our objective here was to learn more about the mechanisms that underlie individual recognition by odor in hermit crabs using Pagurus longicarpus Say 1817 as model species. We found that: (1) P. longicarpus is unable to chemically recognize familiar and high quality empty shells; (2) there is no odor specific of a rank; (3) individual crabs discriminate their own odor from the odor of other individuals; (4) they can chemically distinguish between larger crabs inhabiting higher quality shells and smaller crabs inhabiting lower quality shells, provided that these crabs are familiar to them; (5) they associate the odor of an individual crab with the quality of the shell it inhabits; and (6) this association quickly changes when social partners switch to shells of different quality. These results suggest that the nature of chemical recognition in P. longicarpus is more refined than a simple binary system and hints at the potential of this species for relatively high-order knowledge about conspecific individuals.