decapod vs hermit crab

decapod and hermit crab both are nouns.

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

As nouns, hermit crab is a hyponym of decapod; that is, hermit 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
hermit crab: small soft-bodied marine crustaceans living in cast-off shells of gastropods

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

decapod (noun)	hermit crab (noun)
crustaceans characteristically having five pairs of locomotor appendages each joined to a segment of the thorax	small soft-bodied marine crustaceans living in cast-off shells of gastropods
cephalopods having eight short tentacles plus two long ones