Nice read! I liked this part in particular: "In conclusion, the difficulty of drawing a line between the species that can be used without any concern and the pain-sensitive species in the same group, does not by itself justify drawing an arbitrary line."

I've been working on/off on the topic of pain in crustaceans for about a year now, and I still don't know what to believe. In my opinion, the article by Magee and Elwood currently provides some of the best support in favor of pain, (along with this http://science.sciencemag.org/content/344/6189/1293). And yet, it's not terribly convincing. If you have a look at Fig. 1 in Magee and Elwood's article, you'll see that on the tenth trial, about a third of all crabs still entered a shock chamber. Better than would be expected by change, but not much. If the learning ability of crabs is really this bad, one can't help but wonder how much they benefit from it in nature. If the answer is: "they don't", then what benefit does pain provide that nociceptive reflexes don't?

The fact that anxiety-like behavior can be induced with electric shock and then removed pharmacologically in the same way in humans and crayfish (see the linked article) makes me think that decapods do feel pain. But I won't feel safe in drawing that conclusion until I've seen evidence of more complex harm-induced learning.