“Hey there, wanna check out my burrow?” Fiddler crabs (genus: Uca) have sexual dimorphism, meaning that males and females have different characteristics aside from their sexual organs. This male thick-legged fiddler crab (Uca crassipes) has a claw that is much larger than the female (on the right) and can be used for communication, courtship, and combat. The male fiddler crab will use his claw in waving displays that signals to the female they are ready to mate. Females will choose males based on their claw size and the quality of waving. More vigorous waving display indicates a healthier male and a larger claw indicates a wider burrow which will provide better temperatures for egg incubation.

[Photo and caption contributed by Karli Chudeau]

Reference
Salmon, M. & Atsaides, S.P. (1968). Visual and Acoustical Signaling during Courtship by Fiddler Crabs (Genus Uca), American Zoologist 8(3), 623–639.