THE PRECAUTIONARY STANCE: THE BENEFIT OF THE DOUBT
It is never possible to state with 100% certainty what another animal is experiencing. Some scientists have commented that there is as much evidence for crustaceans feeling pain as there is for mice, and that therefore it is "illogical" not to treat them as sentient. (1). Where there is still doubt in the face of substantive positive scientific evidence one has to pitch the potential risks to the animal concerned of continuing to treat them as non-sentient; against the inconvenience to humans of treating them as pain-feeling creatures.
The costs to the millions of crabs and lobsters who could suffer and die in pain if we don't give them the benefit of the doubt is clearly very high. However, for humans, the steps taken to mitigate their suffering are relatively simple and the inconvenience hardly equal to the pain of a sentient creature being boiled or shrinkwrapped alive. Therefore, as long as the evidence is compelling enough to make a rational case, the moral imperative is clear: follow other countries, take the "precautionary stance" and give decapod crustaceans the benefit of the doubt where doubt still exists. Take steps to ensure they are kept and slaughtered humanely without pain or suffering.
But don't just take our word for it:
"It is now widely accepted that all vertebrates (mammals, birds, reptiles, amphibians and fish) are sentient - that they have the capacity to feel pain, to experience distress and suffering, to experience both positive and negative feelings. Scientific evidence is that this also possibly applies to some invertebrates, such as cephalopods and crustaceans. UFAW believes that where there is any doubt the animal should be given the benefit of the doubt and treated as if it were sentient - and that, from an ethical point of view, it is essential that we take these feelings into account in all our dealings with them."
1. Hubrect, R (2013) quoted in Cressey, D., "Experiments Reveal that Crabs and Lobsters Feel Pain", accessed 5-1-17