amphipod vs sand flea

amphipod and sand flea both are nouns.

Word	Noun	Adjective	Verb	Adverb
amphipod	Yes	No	No	No
sand flea	Yes	No	No	No

As nouns, sand flea is a hyponym of amphipod; that is, sand flea is a word with a more specific, narrower meaning than amphipod:

amphipod: a kind of malacostracan crustacean
sand flea: small amphipod crustaceans that hop like fleas; common on ocean beaches

Other hyponyms of amphipod include beach flea, sand hopper, sandhopper, skeleton shrimp, whale louse.

amphipod (noun)	sand flea (noun)
a kind of malacostracan crustacean	small amphipod crustaceans that hop like fleas; common on ocean beaches