Coral trout communicate with moray eels (through body language such as headstands and head shakes) to convince the eels to partner with them and to point their eel comrades in the right direction. (This is not uncommon. Groupers also collaborate with moray eels: see a NatGeo video here. In the video, a grouper solicits an eel's help by shimmying, then indicates to the eel the location of a prey species' hiding place).

A recent study in a controlled aquarium setting tested whether coral trout could distinguish between good and bad hunting partners: indeed, the trout were 3 times more likely to partner with an eel model that had previously helped with hunting! This study joins a growing body of evidence that there is more going on in a fish brain than we often give them credit for.

This study, and the clear evidence of cooperation and communication between different species of fish, reminds me of a heartwarming tale titled The Eel and the Bartender. It is a true recounting by zoo veterinarian Beth Chittick Nolan (the story is within the collection called The Rhino With Glue-on Shoes, available on Amazon).

There was once an eel who lived at a bar. But the eels grew, as eels do, and the bartender decided to donate the eel to the New England Aquarium, where it could properly be taken care of. 

But the eel did not take to its fancy new aquarium home: day after day, it hid in a crevice and refused to eat. The aquarists and vets tried everything, but the eel showed no signs of illness or parasites. The eel was perfectly healthy- it simply wouldn't come out, and refused any food offered to it, no matter how tasty! Finally, in desperation, the zoo vet called the bartender and asked him to come in. He agreed, sad to hear about his eel's decline.

The bartender arrived at the aquarium, walking past tank after tank of calm exotic fish going about their routines. But the second the bartender came into view of the eel's tank, something surprising happened: the eel-- who for weeks had cowered listlessly in a crevice- shot out of his hiding place and shimmied energetically along the sides of the tank. It was a reunion of old friends; the bartender laughing and smiling, the eel undulating and swimming the circumference of its tank. The bartender offered the eel food, and the eel gladly accepted. The eel wasn't sick; the eel hadn't forgotten how to eat; he simply missed his oldest friend.

So the next time someone tells you fish can't think, or fish can't feel, tell 'em the story of the eel and the bartender.