I'm not linking to them, but there are pages on the web claiming oysters are vegan, or at least only technically non-vegan. I disagree with this rather strongly, but the argument is that since an oyster has no brain, it's unable to suffer and is therefore ethically at the same level as a plant.
You probably know I'm panpsychist, but I can still leave that aside and make an argument against eating oysters. Oyster nervous systems consist of a number of nerve centres connected by nerve fibres. They are able to sense chemicals in the water, concentration of salt and somehow the position of Cynthia. An oyster on land will know when it would be high tide if the sea was covering its location, even if that location is thousands of nautical miles from the nearest body of salt water. I have no idea how it does this. They have tentacles and something called a pallial organ, both of which are sense organs, and they have an adductor muscle which pulls the shell shut when a possible threat is perceived. Effectively, they have a sense of taste. If certain predators are nearby, they pick up their taste in the water and slam shut, and they also respond to bitter substances in the water and changes in salinity. What they don't do, of course, is move away from threats or towards light, darkness or food.
There are similar molluscs which do move around, notably scallops. Scallops swim through the water by opening and closing their shells, and they swim away from potential predators. If you see a shellfish belting hell for leather through the water pursued by something which wants to eat it, you would probably easily be able to imagine that it is conscious and fleeing for its life in terror.
Now consider the oyster again. It sits on a rock and slams shut when it perceives a threat. It doesn't swim through the water because it's stuck to a rock, but it does respond in the only way it can. It will contract its adductor muscle and close its shell.
Back to the scallop. It's not stuck to a rock, but it too will slam shut when it perceives a threat, then it will open again, shut again and continue, thereby propelling itself through the water. It moves like the things we more often tend to think of as animals move, so we're more likely to be able to relate to it as being afraid or sentient.
This happens with humans too. Many humans would run away if their lives were threatened. But what about someone with locked in syndrome? They're able to hear, feel, touch, taste and so on. The only thing they can't do is move voluntarily. Suppose you're lying in bed and you hear the doctors near you, or more likely your relatives, discussing whether to turn off your life support. You can't run away because you have locked in syndrome, but presumably the heart monitor would show an increase in pulse rate and your blood pressure would rise, and there would probably be more adrenalin in your bloodstream, as a result of your sensation of hearing people discussing ending your life.
I would contend for the sake of this bit of this entry that oysters are an example of an entire species with locked in "syndrome", i.e. in their healthy state they are unable to move their entire bodies, but their reactions to sensations are otherwise similar to other bivalves to which we would have no problem attributing consciousness.
However, there is a further problem. Venus fly traps, like oysters, snap shut when they are stimulated in a particular way, and they're plants. Does this mean they are also sentient? I would actually say yes. So the question then arises of why I eat plants if they might be able to suffer. The answer is that plants are on a lower tropic level, i.e. they consume fewer resources during their lives. Hence their consumption would harm or kill fewer organisms than the consumption of animals regardless of whether they are sentient or not. It takes more land to raise a cow than the equivalent calories and nutrients usable from wheat plants because the latter don't move around as much.
Oysters are filter feeders. They eat other animals and plants which do move around. Therefore, eating oysters is likely to harm more animals than eating the same nutrition in plant form of any kind with the possible exception of insectivorous plants.
That is why consciousness is not the only reason why veganism is a good idea, and that's why oysters are not vegan.