If I am not wrong, some biological classifications are tags because they encompass species with easily noticeable characteristics and that have (together) considerable representativeness in e621. If this is the case, the phylum mollusca (mollusks) meets the requirements.

Mollusks have soft, unsegmented body, usually not vermiform, with lateral symmetry and sometimes protected by a calcareous shell.
Easily noticeable characteristics » OK

Mollusk subclassifications (gastropods, cephalopods and bivalves) totalize more than 2500 posts.
Considerable representativeness in e621 » OK

Based in these informations, I belive the mollusk tag should be created (technically the tags mollusk and mollusc already exist, but even together they have less than 30 posts).

Logical implications:

cephalopod → mollusk
gastropod → mollusk
clam → mollusk