What shellfish doesn't feel the pain?

Answer 1

The answer is currently unknown.

The shrimp nervous system is decentralized, having a brain at the front and a series of ganglia. They are decapod crustaceans, and hence are quite closely related to lobsters and crabs.

In 2007, Professor Elwood of Queen's University showed that grass prawns would react to acetic acid and sodium hydroxide by grooming the afflicted area. Grooming behaviour decreased when local anaesthetics were applied to the afflicted area. However, in another study where sodium hydroxide, hydrochloric acid, and benzocaine were used, there was no behavioural changes. In another study, stress from eyestalk ablation in the Pacific white shrimp was reduced with an anaesthetic.

In 2009 and 2013, Elwood et al had demonstrated that crabs (who are considered Decapods, along with lobsters and shrimp) can learn to associate electric shocks with an unrelated cue. In the 2009 study, hermit crabs that were shocked when entering one shell often forfeited the shell. In the 2013 trial, crabs were shocked when entering a dark shelter (in which they preferred), and many of the crabs escaped and did not return after being shocked. This demonstrates that they are able to learn to avoid electric shocks. However, trials on lobsters have not been conducted.

All being said, this is still not 100% proof that they feel pain, at least like we do. It is still better to give them the benefit of the doubt and destroy their nervous system quickly.

Answer 2

The one possible one is a starfish as it has no brain therefore it should not feel any pain as the brain is what processes pain from the neurons and nerve endings. Basically any shellfish with a brain should process pain therefore can feel pain.