Consider the oyster - Why even vegans should feel comfortable eating oysters by the boatload...
Geege Schuman stashed this in Nutrition
Biologically, oysters are not in the plant kingdom, but when it comes to ethical eating, they are almost indistinguishable from plants. Oyster farms account for 95 percent of all oyster consumption and have a minimal negative impact on their ecosystems; there are even nonprofit projects devoted to cultivating oysters as a way to improve water quality. Since so many oysters are farmed, there's little danger of overfishing. No forests are cleared for oysters, no fertilizer is needed, and no grain goes to waste to feed them—they have a diet of plankton, which is about as close to the bottom of the food chain as you can get. Oyster cultivation also avoids many of the negative side effects of plant agriculture: There are no bees needed to pollinate oysters, no pesticides required to kill off other insects, and for the most part, oyster farms operate without the collateral damage of accidentally killing other animals during harvesting. (Relatedly, although it's possible to collect wild oysters sustainably, the same cannot be said for other bivalves like clams and mussels. These are often dredged from the seabed, disrupting an entire ecosystem. For that reason, it's best to avoid them.)
Moreover, since oysters don't have a central nervous system, they're unlikely to experience pain in a way resembling ours—unlike a pig or a herring or even a lobster. They can't move, so they don't respond to injury like those animals do, either. Even monkish ethicist Peter Singer sanctioned oyster eating in Animal Liberation —the best-argued case for a vegan diet I've read—before reversing his opinion for later editions of the book. To justify the flip-flop, he wrote that "one cannot with any confidence say that these creatures do feel pain, so one can equally have little confidence in saying that they do not feel pain." This is unconvincing: We also can't state with complete confidence that plants do, or do not, feel pain—yet so far Singer hasn't made a stand against alfalfa abuse.
Too bad oysters aren't more inexpensive to produce, or they could be a World Hunger solution.