Serious answer: why does it matter?

Are you an organization promoting veganism? Maybe direct your limited awareness raising funds at less ambiguous examples.

Are you looking to score points against a self-professed vegan acquaintance? Maybe don't offer guidance on what other people eat unless they ask for input.

Are you a conflicted vegan? If you're feeling inner turmoil about consumption of scallops/oysters then maybe abstain to be safe.

Are you (as I suspect) merely concerned about definitions? Define "vegan" sufficiently and the answer should follow from that. Different self-professed vegans will use different definitions, some seek to avoid "animal products", others "animal suffering". In the latter case, an organism without a nervous system cannot "suffer" as the term is commonly defined, in the former "animal" is typically used synonymously with the kingdom Animalia.

This is indeed an interesting question, and debated by vegans. The exact definition of what makes something permissible for vegans is just not there. With "no central nervous system" a creature should not be able to feel suffering, but are we sure. There are some videos that show these shells escaping a preditor, that would show they are aware to some extend.

For me it is simple, I eat everything Fungi and to the left of it in this image: https://evogeneao.s3.amazonaws.com/images/tree_of_life/tree-...

It's... an argument. One me and most vegans I know avoid by not being particularly interested in neither scallops nor oysters.

I'm very "sentience" oriented, and have a (relatively, for a veg person) high bar for one, so personally I consider them fair, um, game. But one probably shouldn't put "vegan" on the label.

A lettuce is a living being