What in the world is uca?
Uca is actually the name of the genus of fiddler crabs.
What are fiddler crabs?
For starters, this image above is a picture of a male fiddler crab. Notice the huge claw that the crab in the picture is waving? Only the males have them. Remember the nursery rhyme, whereby one line goes: " the cat and the fiddle"? It was given this name because the movement of the small claw from the ground to its mouth, across the big claw, resembles the action of a person moving a bow (small claw) across a fiddle (big claw).
This picture shows a male uca annulipes, a species of fiddler crabs. It can be easily identified by the black and white markings on its back. Other species of fiddler do not have such markings on its back.
I know that this is a question that will definitely be asked, but since the story of its origin is too tedious to be told, let us just leave it as that, shall we? Perhaps some other fine day, where the sky is clear and the flowers blossom in the refreshing breeze, I will relate the tale.