I agree that species membership as such is irrelevant, although it is in practice an extremely powerful summary piece of information about a creature's capabilities, psychology, relationship with moral agents, ability to contribute to society, responsiveness in productivity to expected future conditions, etc.

Animal happiness is good, and animal pain is bad. However, the word anti-speciesism, and some of your discussion, suggests treating experience as binary and ignoring quantitative differences, e.g. here:

Such a view leaves room for probabilistic discounting in cases where we are empirically uncertain whether beings are capable of suffering, but we should be on the lookout for biases in our assessments.

This leaves out the idea of the quantity of experience. In human split-brain patients the hemispheres can experience and act quite independently without common knowledge or communication. Unless you think that the quantity of happiness or suffering doubles when the corpus callosum is cut, then happiness and pain can occur in substructures of brains, not just whole brains. And if intensive communication and coordination were enough to diminish moral value why does this not apply to social groups like firms, herds, flocks, hives and the like?

Animals vary enormously in the number of neurons and substructures, including ones engaged in reinforcement learning responsive to pleasure and pain. For example, a fly's brain contains 100,000 neurons, where a human's contains about a million times as many. Here are brain masses for some animals:

• Adult elephants at around 5000 g
• Adult humans 1300-1400g
• Chimpanzees are about 420 g, about a 3:1 ratio with humans, with the ratio for cortex neurons around 3:1 to 4:1
• Cows are 425-458g, about a 3:1 ratio; if their cortex neuron counts resemble horses that would be closer to 8:1
• Pigs are at 180 g, a ratio of 7.5:1
• Domestic cats stand at 25-30 g, ~50:1 with the cortex ratio somewhat bigger
• Pekin Duck at 6.3 g, 214:1 ratio
• Owl brains are 2.2 g, around 600:1, and European quail at 0.9 g, about 1500:1
• Goldfish have 0.097 g, just under a 14,000:1 ratio

Particularly for birds, fish, and insects one sees extremely large ratios. If, as is quite plausible in light of the decentralized operations of brains (stunningly demonstrated in split-brain patients, but also a routine feature of information processing in nervous systems), smaller subsystems can experience pleasure and pain, then animals with large nervous systems may be orders of magnitude more important than one would otherwise think. Importantly, this is not a consideration lowering the expected experience of animals with small nervous systems, but increasing the expected experience of animals with large nervous systems, so it does not need to be held with very high confidence to much affect behavior: "what if small neural systems suffer and delight?" is analogous to "what if snails sufffer and delight?").

Would you say that making such adjustments is speciesist? For example wikipedia gives the world chicken population as 24 billion, mostly kept in horrible conditions, and 1.3 billion cows. If one ignores nervous system scale the welfare of the chickens dominates in importance, but if one thinks that quantity of experience scales then the aggregate welfare of the cows looms larger. Is it speciesist to prioritize cows over chickens or fish on this basis?