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.

Image

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. The research, published recently in the journal Proceedings of the Royal Society B, not only sheds light on the enduring migration mystery, but also will help in planning for the increasingly disruptive role climate change is already having on migrating fish populations.

“Many animals, especially in the temperate zones, need to be able the sense the changing of the seasons,” says McCormick. “They need to prepare for winter or know when the spring is arriving, and this is crucial for everything from mating to finding food. One reliable way to do this is to be able to judge the season by the amount of daylight.”

Researchers have known for years that, in birds and mammals, increased daylight triggers a whole chain of hormone activity. First, the pituitary gland produces a hormone, called thyroid stimulating hormone (TSH), that flows to the brain, which in turn alters the production of a deiodinase enzyme that converts the thyroid hormone thyroxine (T4) to its more active form, known as T3. It’s this increasing level of brain T3 that seems to stimulate all sorts of seasonal responses, from migration to growth and reproduction.

Image

Not only are McCormick and his colleagues, who hail from the University of Tokyo, Toho University, and the University of Gothenburg, the first to confirm that this basic pathway is the same in fish, but they also discovered that the way fish sense changes in daylength is very different from how birds and mammals do it.

For their research, the team focused on Atlantic salmon. “I have a lifelong interest in the species,” says McCormick, who notes that they’re endangered and that what holds true for them should also hold true for many other anadromous, migratory fish. 

Salmon normally live in freshwater streams and lakes for one to three years and, upon reaching a critical size, will migrate in the spring to the ocean. As part of this transformation, they change from being dark sided to a bright silver color, acquire a high tolerance for saltwater and begin to change their behavior by schooling and swimming downstream.

“All of this is stimulated by changes in the daylight,” says McCormick, “and now we know how and why.”

Image

It turns out that salmon can perceive daylight with more than just their eyes—between 7-9% 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.

This capacity of the pituitary to directly perceive changes in daylength has not been previously seen in any vertebrate.

All of this has implications for the survival of migratory fish. Over eons, migratory animals have evolved to begin their migrations so that they arrive at their destinations in time to feast on their favorite food source, or when conditions are right for mating or raising their young. Climate change has uncoupled seasonal changes from the length of the day, however, and it is now not uncommon for migratory species to arrive in their summer feeding grounds after their food sources has itself migrated somewhere else.

“Now that we know how photoperiodism works in fish,” McCormick says, “we can get a much better sense of the pace of their evolution, and this can help us determine the best methods for helping fish to survive a warming world.”