I don't know about crabs but there's something called "high pressure lobster" where the animal is killed very swiftly through very high pressure soon on the boa

I don't know about crabs but there's something called "high pressure lobster" where the animal is killed very swiftly through very high pressure soon on the boat. While I am not sure about this is being more humane than throwing it in boiling water (cf. https://news.ycombinator.com/item?id=29301843 https://news.ycombinator.com/item?id=29301843) there's also the question of keeping lobsters alive for a long time all the way from catch through the market while keeping their claws tied etc.