Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Calappoidea is a superfamily of crabs comprising the two families Calappidae and Matutidae. The earliest fossils attributable to the Calappoidea date from the Aptian.Crabs often show marked sexual dimorphism. Males often have larger claws, a tendency which is particularly pronounced in the fiddler crabs of the genus Uca (Ocypodidae). In fiddler crabs, males have one claw which is greatly enlarged and which is used for communication, particularly for attracting a mate. Another conspicuous difference is the form of the pleon (abdomen); in most male crabs, this is narrow and triangular in form, while females have a broader, rounded abdomen. This is due to the fact that female crabs brood fertilised eggs on their pleopods.