Sexual Dichromatism of “Rusty” Spots in a Population of Faxonius rusticus (Girard)

Sexual dichromatism is a common phenomenon among animal taxa and consists of differing coloration between sexes in a species. Specimens of the crayfish Faxonius rusticus (Girard) typically have a distinct “rusty” red spot on each side of the cephalothorax. These spots can display variation in both redness and size among specimens; however, little work appears to have been done examining variation in this trait. As a means of investigating possible sexual differences in the color and relative spot size (RSS) of the “rusty” spot in F. rusticus, this work utilized digital image analysis techniques to quantify the color and size of spots in a population of this species. Analyses revealed no difference in RSS between sexes; however, spots of females were significantly redder than those of males. Sexual dichromatism in the spot color of F. rusticus in this population could be relevant to mate discrimination during mating; however, this or other possible functions of this sexual color difference will need to be examined by behavioral studies.