swimming crab

from The Century Dictionary.

• noun A shuffie-crab or shuttle-crab; a paddle-crab; any crab one or more pairs of whose legs are expanded and fin-like or fitted for swimming, as in the family Portunidæ. See cut under paddle-crab.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Any of many marine crabs, of the family Portunidae, some of whose legs that are flattened into paddles

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun marine crab with some legs flattened and fringed for swimming