Crayfish “are more comfortable in darker water. After giving [them] a series of mild electric shocks … [they] hardly entered the lit paths of the aquarium at all. … The creatures responded positively to chlordiazepoxide (CDZ), a drug used to treat anxiety in humans. Treated crustaceans lost their nerves” (http://www.theguardian.com/science/2014/jun/13/crayfish-feel-stress-claim-scientists)
Stunning and chilling.