decapod vs crab

decapod and crab both are nouns.

Word	Noun	Adjective	Verb	Adverb
decapod	Yes	No	No	No
crab	Yes	No	Yes	No

As nouns, crab is a hyponym of decapod; that is, crab is a word with a more specific, narrower meaning than decapod:

decapod: crustaceans characteristically having five pairs of locomotor appendages each joined to a segment of the thorax
crab: decapod having eyes on short stalks and a broad flattened carapace with a small abdomen folded under the thorax and pincers

Other hyponyms of decapod include lobster, crawdad, crawdaddy, crawfish, crayfish, hermit crab, shrimp, prawn, tropical prawn.

decapod (noun)	crab (noun)
crustaceans characteristically having five pairs of locomotor appendages each joined to a segment of the thorax	a stroke of the oar that either misses the water or digs too deeply
cephalopods having eight short tentacles plus two long ones	decapod having eyes on short stalks and a broad flattened carapace with a small abdomen folded under the thorax and pincers
	a louse that infests the pubic region of the human body
	the edible flesh of any of various crabs
	a quarrelsome grouch

decapod (verb)	crab (verb)
	complain
	fish for crab
	scurry sideways like a crab
	direct (an aircraft) into a crosswind