How do fish identify with other fish so as to join into a school? They can't know what they look like.
-- Bob Kammerer, the net
Grade point average? Name tags? Uniforms? Well, sort of. More than half of fish species have the inborn urge to school for safety, and they gang up kind of the same way we and some other animals do. Appearance and behavior, based on a schooling instinct. At least that's science's best guess at the moment. Fish schools include individuals that are the same age and size and that swim at the same rate, with the instinct to maintain a certain distance from the fish in front and to the sides. That's what makes the school an effective defense mechanism against predators. Tiny, hairy neuromasts on a fish's head and along its side sense water currents and help all fish in a school react almost like one big organism. But most schooling behavior relies on vision, so even if a herring doesn't know what he looks like, he knows a good, reliable neighbor when he swims next to one.