Transition from sea to land: olfactory function and constraints in the terrestrial hermit crabCoenobita clypeatus

Abstract

The ability to identify chemical cues in the environment is essential to most animals. Apart from marine larval stages, anomuran land hermit crabs (Coenobita) have evolved different degrees of terrestriality, and thus represent an excellent opportunity to investigate adaptations of the olfactory system needed for a successful transition from aquatic to terrestrial life. Although superb processing capacities of the central olfactory system have been indicated inCoenobitaand their olfactory system evidently is functional on land, virtually nothing was known about what type of odourants are detected. Here, we used electroantennogram (EAG) recordings inCoenobita clypeatusand established the olfactory response spectrum. Interestingly, different chemical groups elicited EAG responses of opposite polarity, which also appeared forCoenobita compressusand the closely related marine hermit crabPagurus bernhardus.Furthermore, in a two-choice bioassay withC. clypeatus,we found that water vapour was critical for natural and synthetic odourants to induce attraction or repulsion. Strikingly, also the physiological response was found much greater at higher humidity inC. clypeatus, whereas no such effect appeared in the terrestrial vinegar flyDrosophila melanogaster. In conclusion, our results reveal that theCoenobitaolfactory system is restricted to a limited number of water-soluble odourants, and that high humidity is most critical for its function.