Not all crabs are created equal: diverse evolutionary paths of female preferences for courtship structures in fiddler crabs (genus Uca)

Abstract

Males of fiddler crabs (genus: Uca) construct courtship structures using mud or sand to attract mate-searching females for underground mating. A sensory trap hypothesis proposes that the courtship structures have evolved by trapping the sensory bias for any objects which the crab approach to evade predators. We tested if the sensory bias commonly exists in three species of fiddler crabs and the outgroup Ocypode stimpsoni, by placing crabs individually at the center of an arena, from which they could choose between burrows with and without structures. Contrary to previous results in other species, male U. lactea showed no orientation response toward holes with or without semidomes. Female U. lactea predominantly visited holes with semidomes during reproductive periods but did not show the response during non-reproductive periods. In contrast, female U. arcuata and male U. terpsichores and even the outgroup female O. stimpsoni showed a strong orientation response to an artificial structure. By inferring the evolutionary history of sensory biases and courtship structures, we conclude that the preference for the courtship structures evolved before the genus Uca branched out from the common ancestor of genus Uca and Ocypode and yet that the sensory bias has been modified or lost in U. lactea. Significance statement We tested the sensory trap hypothesis for preference for courtship structures in three species of fiddler crabs in different locations and the outgroup O. stimpsoni. Contrary to previous results in other species, one species which build courtship structures (U. lactea) showed sex-and context-dependent orientation responses in the non-courtship context. On the other hand, the other species tested had a strong orientation response to structures. This is the first comparative study supporting that the preference for the courtship structures in fiddler crabs could have different evolutionary paths.