Crab should win.
The crab is both a diverse and highly adaptive species. They come in a multitude of shapes, sizes and colours, so adaptive are crabs that they have come to inhabit all of the oceans of the world.
Crabs are also of an ancient genus, the fossile record shows they were around in highly evolved, land dwelling states as far back as the Cretaceous Period, some 65 million to 130 million years ago. One of the oldest and most recently discovered examples being estimated to be at least 94 million years old. The following link is an interesting read.
http://record.wustl.edu/news/page/normal/1739.html
I will for now hold my argument in the support of the crab. Would anyone else like to contribute?
EDIT: Compare this with my earlier picture.
