crab has 14 distinct synonyms spread across 6 of its 11 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
decapod having eyes on short stalks and a broad flattened carapace with a small abdomen folded under the thorax and pincers
the fourth sign of the zodiac; the sun is in this sign from about June 21 to July 22
the edible flesh of any of various crabs
a louse that infests the pubic region of the human body
a stroke of the oar that either misses the water or digs too deeply
“he caught a crab and lost the race”
Synonyms, antonyms and the broader and narrower relations for crab come from Princeton University’s WordNet. Full sources and licences.