AMHERST — Most birds and mammals rely on their eyes to sense daylight, but a new study finds fish also directly “see” daylight with the pituitary gland in their brains.
A team of researchers led by UMass adjunct professor Stephen McCormick discovered the unique attribute while researching how Atlantic salmon use the length of day, or photoperiod, to know when to migrate.
“We had known it from 30 years of research that this whole process is controlled by photoperiod,” McCormick said, “But we didn’t know how the animals were actually sensing photoperiod until now.”
The pituitary’s ability to sense light also explains how salmon undergo a smoltification, a transition from freshwater to saltwater. University of British Columbia professor Colin Brauner said timing is key for smoltification. Fish have a narrow two-week window to adapt to their saltwater environment or they will die.
“This paper is really exciting in that it tells us what the mechanism is that tells the fish it’s time to go to sea water, and it initiates all the changes that have to occur,” Brauner said.
By understanding the pituitary’s role in smolitification, aquaculture farmers can use photoperiod to grow bigger fish. Similarly, light pollution from cities could negatively alter this mechanism. As climate change uncouples seasonal changes from the length of the day, fish may change their response to photoperiod in order to survive.
“Fish live in every aquatic environment on the planet, and we’re only just starting to scratch the surface in how they do that,” Brauner said. “This is a great study toward that goal.”
The hormones in your head
As a key producer of hormones, the pituitary gland is also critical in seasonal changes in birds and mammals. As daylight increases in late winter, the pituitary gland releases a hormone called thyroid stimulating hormone. This hormone tells an enzyme in the brain to increase production of triiodothyronine. Once the brain senses higher levels of triioothyronine, the animal undergoes its other seasonal changes.
“It’s not just migration, but it also will affect reproduction. It’ll affect feeding,” McCormick said. “The next step is to see whether or not this is universal among fish.”
If this finding does apply to all fish species, Brauner said, it means that over half of vertebrates use this mechanism. He agrees that the physiological pathway is likely the same in most other fish species. However, more research is required to confirm the biologist’s hypothesis.
‘Seeing’ without eyes
Despite previous research that found salmon had non-visual opsins, or light-sensing proteins, on their pituitary gland, McCormick conceded that he and his colleagues at University of Tokyo, Toho University, and the University of Gothenburg were surprised by the finding.
“That was probably the most novel thing that we that we found,” McCormick said. “We’ve not seen that in mammals or birds where this sort of same mechanism is in place.”
According to the paper, researchers estimate 7% to 9% of ambient of light gets to the fish’s pituitary gland. Brauner hypothesizes this is due to the thinness of juvenile fish. In his own research, Brauner found that delaying smoltification in farm-raised salmon resulted in bigger salmon, but lower smoltification success.
“The bigger the fish are, the less of kind of smolt signal they get when you give them an artificial kind of spring photoperiod,” Brauner said. “So there’ll be a lot of interest in this to more applied aspects, like aquaculture.”
Bad for nature, good for aquaculture
Vertebrates use photoperiod to sense seasonal change rather than temperature because the latter is unreliable, McCormick said. One warm week in March or an early winter thaw could send a false signal to fish. Seasonal temperature changes much later than photoperiod, so fish may miss their food source or their smolting period if they wait for warmer weather.
“Photoperiod is the part that gets them ready, and temperature is the last bit of the signal that cues them to migrate,” he said.”
Yet photoperiod may not be as dependable if it uncouples with seasonal change. The Earth’s climate is moving toward hotter summers and warmer winters. Climate Central found winter cold streaks have shrunk an average of six days in 240 cities since 1970. Winter, the nonprofit science policy group found, is also the fastest warming season, blending winter and spring. Climate change has shifted the seasons, and put animals’ photoperiod mechanism out of sync.
“Now animals may be mismatched to what the what their evolutionary past has shaped,” McCormick said. “This can be a big problem, and it’s been shown (that) birds that rely on photoperiod for their migration timing are doing more poorly because they’re not timed properly anymore.”
Fish are slowly catching up with these changes. In fact, McCormick said there is some evidence of changes in photoperiod response in the last 50 years.
“As with all science, it sort of opens the door for 10 more experiments that we need to do,” Brauner said.