amphipod vs beach flea

amphipod and beach flea both are nouns.

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

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

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

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

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