[Federico Sibona]

Per cominciare:

Gnathonemus petersii:
The best we can do is look at the physical dimensions of a fish's brain. We cannot just look for the biggest brain, because naturally bigger animals have larger brains, yet nobody would suggest a cow was smarter than a dog. Two parametres that can help us assess a fish's intelligence are the relative brain weight to body weight and the complexity of the brain. I have found no information comparing the brains of fish in terms of complexity, or surface area (foldedness). This leaves us with the brain to body weight relationship. The winner in this category is the Elephantnose fish (Gnathonemus petersii) which has a brain that amounts to 3.1% of its total body weight. To put this into perspective most fish have brains that weigh less than 1% of their total body weight and even the human brain only averages out at 2.3% of body weight. Whether this makes it the world's most intelligent fish however I cannot say.
The Elephantnose fish is a tropical, freshwater fish from Central Africa that grows to a maximum length of 35 cm (14 ins). It is a species that is commercially available to aquarists and it is known for its playful behaviour. It also seems to have a strange karma because not only is it incorrectly named, it is its chin not its nose, that is enlarged, but it is also spelled incorrectly (in terms of its scientific name) in at least 3 major works on fish that I have looked at, including a university level textbook.
http://scholar.google.it/scholar?hl=...ed=0CA8Q0AIwAA
http://www.springerlink.com/content/m01184l2x6271462/

In generale:
http://www.earthlife.net/fish/learning.html