While little is known about sex pheromones in fish, their alarm pheromones are among the best-studied in vertebrates. Their discovery is associated with the name of Nobel laureate Karl von Frisch. In 1938, he trained a school of wild minnows (Phoxinus phoxinus) to approach the riverbank when a bell rang. He would then feed them worms. On one occasion, he caught a fish and performed a minor surgical procedure—he injured the sympathetic nerve, which controls the chromatophore muscles. Chromatophores are skin cells near the tail that can change their color. After releasing the operated fish, von Frisch noticed that the other fish in the school swam away and did not return for hours. He assumed that they were frightened by the altered appearance or that he had somehow managed to inform their companions of the improper act committed by this seemingly benevolent man. To rule out the possibility of exchanging information with the other fish, von Frisch killed the animals after the operation and then threw them into the water. The reaction was the same. He then cut them into pieces so that they no longer resembled a fish. When he threw the pieces into the water, the school scattered again. It was obvious that the panic reaction had nothing to do with the altered appearance of the fish. He suspected that the injury had led to the release of a substance that frightened the other fish. To prove his hypothesis, von Frisch prepared an aqueous extract from crushed fish, which he filtered to remove all traces of animal tissue. When the extract was added to the water, the fish again swam away. There was no doubt that the surgical procedure was accompanied by the release of an alarming substance that triggered a fear response. According to modern terminology, this is an alarm pheromone.