They are unclear, but not arbitrary. I enjoy shrimp. I don't think a shrimp is capable of experiencing suffering. I feel no guilt about the treatment of shrimp raised for me to enjoy. It is completely obvious to me that higher mammals such as dogs and pigs experience suffering. I enjoy pork. I don't feel bad about a pig being humanely killed so that I may enjoy pork. I do feel bad about a pig being made to suffer for his entire life as some do on factory farms.

Just because it is difficult and subjective to know what suffers doesn't mean we shouldn't think about it.