pea crab vs oyster crab

pea crab and oyster crab both are nouns.

Word	Noun	Adjective	Verb	Adverb
pea crab	Yes	No	No	No
oyster crab	Yes	No	No	No

As nouns, oyster crab is a hyponym of pea crab; that is, oyster crab is a word with a more specific, narrower meaning than pea crab:

pea crab: tiny soft-bodied crab living commensally in the mantles of certain bivalve mollusks
oyster crab: tiny soft-bodied crab living within the mantle cavity of oysters

Other hyponyms of pea crab include Pinnotheres ostreum.

pea crab (noun)	oyster crab (noun)
tiny soft-bodied crab living commensally in the mantles of certain bivalve mollusks	tiny soft-bodied crab living within the mantle cavity of oysters