decapod vs crayfish

decapod and crayfish both are nouns.

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

As nouns, crayfish is a hyponym of decapod; that is, crayfish 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
crayfish: small freshwater decapod crustacean that resembles a lobster

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

decapod (noun)	crayfish (noun)
crustaceans characteristically having five pairs of locomotor appendages each joined to a segment of the thorax	large edible marine crustacean having a spiny carapace but lacking the large pincers of true lobsters
cephalopods having eight short tentacles plus two long ones	small freshwater decapod crustacean that resembles a lobster
	tiny lobster-like crustaceans usually boiled briefly
	warm-water lobsters without claws; those from Australia and South Africa usually marketed as frozen tails; caught also in Florida and California