The Pygmy Fiddler Crab (Uca pygmaea) is an obscure, very small fiddler found on the Pacific coast of the Americas, from southern Costa Rica through northwestern Colombia. It’s name refers to it’s tiny size. Almost nothing is known about this species; there appear to be no observational studies of it in nature beyond its original collection along the muddy bank of a stream in Costa Rica.
The large claw of this species is interesting in that the hand is very thick and the fingers are relatively short and stubby. Many species have juveniles with claws that are thick and stubby; as they get older the proportions shift into those that we tend to associate with most fiddler crab claws today. This change in shape with size is known as allometry. It has been proposed that the Pygmy fiddlers’ claws stop developing at a more “juvenile” shape, a pattern known by the technical term paedomorphosis. It is certainly not a requirement that a species this size have a claw of this shape; while thicker, stubbier claws are not uncommon among some of the other very small species (e.g., U. saltitanta and U. inaequalis), one of the very smallest, Uca batuenta, has a perfectly “normally” proportioned claw.