bivalve vs piddock

bivalve and piddock both are nouns.

Word	Noun	Adjective	Verb	Adverb
bivalve	Yes	Yes	No	No
piddock	Yes	No	No	No

As nouns, piddock is a hyponym of bivalve; that is, piddock is a word with a more specific, narrower meaning than bivalve:

bivalve: marine or freshwater mollusks having a soft body with platelike gills enclosed within two shells hinged together
piddock: marine bivalve that bores into rock or clay or wood by means of saw-like shells

Other hyponyms of bivalve include clam, cockle, oyster, ark shell, blood clam, mussel, escallop, scallop, scollop.

bivalve (noun)	piddock (noun)
marine or freshwater mollusks having a soft body with platelike gills enclosed within two shells hinged together	marine bivalve that bores into rock or clay or wood by means of saw-like shells

bivalve (adjective)	piddock (adjective)
used of mollusks having two shells (as clams etc.)