vegan

The original article. Bivalves' nervous systems can be summarized as follows:

The central nervous system (CNS) of bivalves is bilaterally symmetrical, of ganglion type. The nervous system consists of aggregations of nerve cells arranged into a chain of paired, sequentially connected ganglia. The paired ganglia are connected via commissures to each other and via connectives to neighboring ganglia. There are three pairs of ganglia in the nervous system of bivalves of the subclass Autobranchia: cerebro-pleural (cerebral), pedal, and visceral (viscero-parietal). The pedal and visceral ganglia communicate with the cerebro-pleural ganglia via the cerebro-pedal and cerebro(pleuro)-visceral connectives, which makes such a nervous system tetraneurous. The major difference between bivalves and other classes of mollusks is the reduction of the head region and, as a result, the absence of some structures: bivalves lack buccal ganglia like those in gastropods, while the cerebral ganglia merge with the pleural ones at the later stages of embryogenesis. The simplification of the nervous system in bivalves is suggested to be a consequence of a slow-moving lifestyle due to the filter-feeding on substrate.

It's first and foremost incorrect to call yourself vegan if you eat oysters; the commonly accepted definition by the Vegan Society is just objectively contravened here. But semantics aside, as noted in the article, the question becomes "is there something wrong with it?" I definitely think there is. Bivalves are still shown to proactively avoid noxious stimuli in the way a more developed nervous system might, and while the existing research is too sparse to definitively call it "pain", this feels like yet another step in a long, storied history where humans decided animals didn't feel pain until researchers stepped in and found out yes, they definitely do (see, e.g., fish). It's easy not to eat them, and it's pretty ridiculous to treat the waiting period for more robust scientific literature as a "grace period" instead of something that should be treated with caution. Getting it out of the way, because it's often presented in bad faith, the whole "plant pain" argument is absurd on its face, both because a basic understanding of entropy still means veganism would be the way to go even taking that asinine premise at face value (vastly more plants per calorie for meat than simply via directly eating plants), and more importantly, plants lack a nervous system at all. They don't feel pain, and the argument exists 5% to be sincerely believed by nutjobs and 95% to soothe cognitive dissonance felt by people who pay to have animals feel pain.