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To study the evolution of smell-driven behaviours in a laboratory setting, the researchers chose two closely related fish of the Mexican tetra species -- the blind cave-dwelling subtype, and its sighted surface river-dwelling counterpart.

When floods carried some Mexican tetra river fish, Astyanax mexicanus, into about 30 different caves, other fish of the same species remained at the surface, providing a natural study in contrasting evolutionary paths.

This form of pork-barrel politics is one reason so much of Japan's coastline is blighted by tetra pods, its rivers walled with grey concrete.

But hunting in groups can enable predators to hunt prey that would otherwise be too fast, such as the tiny tetras.

Small fish, called tetras, are the target of the attack; they fly into the air and land stunned and motionless on the water.