Researchers in ltaly carried out an odd recent study involving eels. To determine the drug's effect on eels, they exposed them to low-level doses of cocaine. They found this caused a degree of hyperactivity in the eels, as you'd probably expect, but also led to thickening of their skin and the linings of their intestines. It also affected their hormone production.
This study did actually have a serious point. Eels can be exposed to cocaine as a result of polluted river water, and it's estimated that 15g of cocaine flow through ltaly's Sarno River every day. The study suggests cocaine could impact the eels' ability to protect themselves from disease, and produce alarm pheromones, and could affect their life cycles.