Until now, the way fish detect changes in day length has remained an unsolved mystery.
Now, thanks to the analysis of the Atlantic salmonthat discover something new and not only sheds light on the mystery of ongoing migration, but also helps improve understanding of how climate change affects migratory fish populations.
One of the greatest ichthyological mysteries revolves around how migratory fish know when it is time to move from their winter habitats to summer habitats.
The ability to know when seasons change is critical for a wide range of essential events in their lives, including feeding, spawning and migration.
Many animals are sensitive to changes in day length between seasons.
Their physiological response to the day night cycle, a process that occurs in plants, animals and even humans, is called photoperiodism.
While researchers have a good understanding of how this process occurs in birds and mammals, how fish detect changes in day length has remained a mystery until now.
A team of researchers led by Steven McCormick, a biologist at the University of Massachusetts Amherst, revealed how fish know when to migrate.
The research not only sheds light on the mystery of ongoing migration, but also helps improve understanding of the impact of climate change on migratory fish populations.
“Many animals, especially in temperate regions, need to sense the changing seasons,” McCormick said.
“They need to prepare for winter or know when spring arrives, which is critical for everything from mating to finding food. One reliable way to do this is to detect seasonal changes in daylight.”
When the salmon’s pituitary glands are exposed to daylight, they illuminate through thyrotropin, red and green.
Researchers have long known that in birds and mammals, increasing daylight triggers a cascade of hormonal activity.
First, the pituitary gland produces TSH.
Which changes the production of the enzyme deiodinase, converting the thyroid hormone thyroxine, T4, into its more active form, triiodothyronine, T3. From there, a variety of seasonal responses occur, from migration to growth and reproduction.
McCormick and his colleagues at the University of Tokyo, Tohoku University and the University of Gothenburg are the first to confirm that this basic pathway exists in fish and they also discovered that the way fish sense day length changes is very different from birds and mammals.
In their study, the team focused on Atlantic salmon.
“I have a great interest in this species,” McCormick said, emphasizing its status as at risk of extinction. What is true for Atlantic salmon likely applies to many other, which spend most of their lives in the ocean but return to freshwater, rivers and streams, to reproduce.
Daylight changes influence the timing of salmon migration.
Salmon typically live in freshwater streams and lakes for one to three years, and when they reach the desired size, they migrate to the ocean in the spring.
During this change, they shift from dark to bright silver coloring, develop high tolerance to saltwater, and begin altering their behavior.
“All of this is triggered by changes in day length,” explained McCormick, “and now we know how and why it happens.”
It turns out salmon can perceive daylight beyond just their eyes.
About 7 to 9 percent of sunlight penetrates the head and the gland, which contains light receptors.
They removed their pituitary glands, exposing them to different wavelengths and observing how thyrotropin levels changed in response to longer days.
This ability of the pituitary gland to directly sense day length has never been observed in any vertebrate before.
For many years, migratory animals evolved to start their migration so they arrive at their destinations in time to feed on preferred food sources or when conditions are suitable for mating or raising offspring.
However, climate change has completely disrupted this process, making it common for migratory species to reach feeding areas in summer after their food source is already gone.
“Now that we understand how photoperiodism works in fish, we better understand the pace of their evolution.
This can help determine the best ways to support fish survival in a warming world,” concluded McCormick.