shell collecting vs malacology

shell collecting and malacology both are nouns.

Word	Noun	Adjective	Verb	Adverb
shell collecting	Yes	No	No	No
malacology	Yes	No	No	No

As nouns, malacology is a hypernym of shell collecting; that is, malacology is a word with a broader meaning than shell collecting:

shell collecting: the collection and study of mollusc shells
malacology: the branch of zoology that studies the structure and behavior of mollusks

Other hypernyms of shell collecting include aggregation, assembling, collecting, collection.

shell collecting (noun)	malacology (noun)
the collection and study of mollusc shells	the branch of zoology that studies the structure and behavior of mollusks