1. A collector and student of mollusc shells: Joe was a conchologist who was a specialist in the branch of zoology that deals with the study of mollusks and shells.

Conchologists (practitioners of conchology) may study animal shells to gain an understanding of the diverse and complex taxonomy of mollusks, or simply appreciate them for their aesthetic value.

Conchologists deal mainly with gastropods (snails), bivalves, Polyplacophora (chitons) and Scaphopoda (tusk shells).
2. A carrier-shell mollusk: A conchologist is the name of a particular type of mollusk that has a habit of attaching other shells, stones, etc., to its own shell.