decapod vs lobster

decapod and lobster both are nouns.

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

As nouns, lobster is a hyponym of decapod; that is, lobster 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
lobster: any of several edible marine crustaceans of the families Homaridae and Nephropsidae and Palinuridae

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

decapod (noun)	lobster (noun)
crustaceans characteristically having five pairs of locomotor appendages each joined to a segment of the thorax	any of several edible marine crustaceans of the families Homaridae and Nephropsidae and Palinuridae
cephalopods having eight short tentacles plus two long ones	flesh of a lobster