Comparative analysis of the antennae of three amphipod species with different lifestyles

Crustaceans detect chemical stimuli in the environment with aesthetasc sensilla, which are located on their 1^st antennae. With the transition to other environments, chemoreception faces physical challenges. To provide a deeper understanding of the relation between the morphology of olfactory organs and different lifestyles, we studied the peripheral olfactory system of three amphipod species, the marine Gammarus salinus, the blind subterranean freshwater species Niphargus puteanus, and the terrestrial Cryptorchestia garbinii. We compared the 1^st and 2^nd antennae of these species with respect to length and presence of aesthetascs and other sensilla. The females of N. puteanus reveal the longest 1^st antennae in relation to body size. G. salinus shows the largest aesthetascs and the same relative length of the 1^st antennae as male N. puteanus. C. garbinii has very short 1^st antennae and reduced (putative) aesthetascs. Our findings show that the compensation of vision loss by olfaction cannot be generally assumed in animals from dark environments. Furthermore, the behaviour of C. garbinii indicates a chemosensory ability, despite the reduction of the 1^st antennae. A comparison with other terrestrial crustaceans suggests that the loss of the olfactory sense on the 1^st antennae in C. garbinii might be compensated with chemoreception by the 2^nd antennae.