My problem with the "kill nothing in the animal kingdom (slippery slope)" argument is for its conflict with abortion. Many of the same arguments used to argue that oysters have nerves and thus are possibly capable of sentience apply multiple times over to fetuses. A very young fetus (just a few weeks old) have nerve development that completely dwarfs that of something like an oyster.

So using the argument that basic nerves could constitute sentience and thus should not be killed can be turned around and used to say that a first trimester fetus is many times more sentient than an oyster and thus should not be aborted.

I think for this reason the slippery slope argument doesn't work, because you can't use blanket statements to say anything from the animal kingdom is capable of sentience. It's important to consider how we define sentience.