Evolutionary Biological Data Science Laboratory

Abstract

Fiddler crabs (Ocypodidae, Uca) are a well known group of small, intertidal Brachyuran crabs, characterized by strong sexual dimorphism and male asymmetry. Male fiddler crabs exhibit one of the most extreme levels of body asymmetry of any organism, having a large major claw containing a third to half of the animal’s body mass and a small minor claw. The morphology of major and minor claws varies tremendously across species. These studies are concerned with understanding the phylogenetic history of the genus, describing patterns of claw shape within and across species, and exploring the relationship of claw morphology with behavior and ecology.

The systematic and phylogenetic history of the genus is explored in some detail before a morphological phylogenetic analysis was performed on 88 of the 97 recognized species. These results were compared to a molecular study of 16S ribosomal DNA for 28 species. The results resolve most of the subgeneric taxonomic conflicts and allow one to perform interspecific analyses in a comparative methodological framework.

Geometric morphometrics was used to study claw shape across the genus. Within species, major claws show allometric growth in both shape and size; minor claw growth is isometric. Both major and minor claws are isometric for size and allometric for shape across species; accounting for phylogenetic dependence has little effect on the analyses. There is evidence for evolutionary allometry explaining some of the diversity of claw forms seen within the genus.

Four behaviors are associated with claw use: visual signaling, acoustic signaling, combat, and feeding. The first three are examined with respect to major claw morphology; the later with minor claw morphology. Only combat can explain a significant amount of variation in major claw morphology. Differences in habitat choice are able to explain some variation in minor claw morphology. Species in sandy habitats have minor claws with wider gapes and longer chela than those in muddy habitats.