spider crab

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of various crabs of the family Majidae, having long thin legs and a relatively small triangular body and often camouflaging the body with seaweed or sponges.

from The Century Dictionary.

• noun A spider-like crab, or sea-spider, with long slender legs and comparatively small triangular body.

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

• noun Any of various species of crab of the family Majidae with long legs.

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

• noun any of numerous crabs with very long legs and small triangular bodies