The moral difference is the degree to which the creature can suffer and the capacity for it to flourish.

Suffering is a subjective experience, impossible to see any but your own. I experience so it's likely humans do, other animals are similar and different to various degrees. Do you doubt the size of the animal's brain or the configuration of their nervous systems has something to do with their capacity to experience suffering?

Flourishing considers the best potential life vs the worse potential life. Is it full-filling to be a cow just eating grass with the herd? Not in comparison to a human's potential experiences. Could a pet cow have a fulfilling life with a loving owner as a pet dog? Not in all the same ways as it's different, but perhaps it could have an equally high peak. As much as I know about a tuna fish, they won't form as a meaningful bond with their owner - their peak life is not as high as a dog.