One of the enduring ichthyological mysteries is how migratory fish know when it is time to move from their winter to summer habitats. The ability to tell when the seasons are changing is crucial for a wide range of major life events, including feeding and spawning, as well as migration.

Many animals are sensitive to photoperiods, or the changing length of the days across seasons, but while scientists have a good understanding of how photoperiodism works in birds and mammals, how exactly fish recognize changes in day length has remained a mystery—until now.

A team of researchers led by Stephen McCormick, a biologist at the University of Massachusetts Amherst, has just uncovered how fish know when to migrate—with the surprising discovery that their pituitary gland, located at the base of their brains, is studded with photoreceptors that can "see" the changing daylight.

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It turns out that salmon can perceive daylight with more than just their eyes—between 7% and 9% of the sunlight penetrates the fish's head and reaches the pituitary gland itself, which is studded with photoreceptors. McCormick and his team discovered this by removing the pituitary glands, exposing them to various lengths of daylight, and observing as the glands' TSH levels changed in response to long days.

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