How Salmon Sense Seasons

Japanese researchers have identified an area in the salmon brain that functions as an internal clock to sense seasonal changes in day length.

AsianScientist (Jul. 3, 2013) – An area of the fish brain called the saccus vasculosus (SV) has baffled scientists since it was described by anatomists in the 17th century – no one could figure out what it did.

Now, Japanese researchers reporting in Nature Communications say that it functions as an internal clock that senses seasonal changes in day length, and plays an important role in regulating seasonal reproduction in fish.

Many animal species breed only during certain times of the year. In mammals and birds, seasonal changes in day length are sensed by photoreceptors in the eyes or other parts of the brain, and transmitted to the pars tuberalis, an area of the pituitary gland that is the regulatory hub for seasonal reproduction. Appropriate conditions activate the production of reproductive hormones that favor fertility.

Fish do not have a pars tuberalis; yet many fish species living in temperate climates are also seasonal breeders. In this study, Prof. Takashi Yoshimura and his colleagues at Nagoya University in Japan, wanted to understand how masu salmon, which breed when days are short, sense seasonal changes.

Looking in different parts of the fish brain, the researchers tracked expression patterns of key genes known to be involved in the regulation of seasonal reproduction. They zeroed in on the SV, where these genes showed the strongest responses to changes in light exposure.

They found that the SV expresses both photoreceptors as well as genes that regulate hormone production, indicating a complete signaling pathway from light input to hormone output. In fact, when the researchers grew the SV in organ culture outside of the fish, its ability to respond to changes in light exposure remained intact.

Finally, the researchers wanted to see if fish without an SV would lose their ability to sense seasonal changes. To remove the SV while keeping the fish alive, fish were anesthetized and their right eyes removed, allowing the researchers access to the SV through the area behind the eye; the only ill-effect the resulting one-eyed fish suffer is that, understandably, they are less able to compete with intact fish for food, said Prof. Yoshimura. “Sham”-operated fish, with right eyes removed but with intact SV, were used as controls.

The researchers observed the fish for five more weeks after the surgery. In the intact and sham-operated fish, short day length induced the production of reproductive hormones and the growth of reproductive organs; however no such changes were seen in fish without an SV. This proved the SV is required to detect seasonal changes.

Masu salmon, known as yamame in Japan, is farmed and consumed on a large scale there, and Prof. Yoshimura hopes that the group’s research could have agricultural applications in the future. He also hopes to expand his group’s studies of seasonality in animals to include other factors besides light.

“It is well established that day length is the dominant seasonal cue for many organisms. But, temperature changes also affect the seasonality in some species. I would like to understand how animals sense changes in temperature to adapt to seasonal changes in environment,” he said.

The article can be found at: Nakane et al. (2013) The saccusvasculosus of fish is a sensor of seasonal changes in day length.

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Source: Nature Communications. Photo: mrjorgen/Flickr/CC.
Disclaimer: This article does not necessarily reflect the views of AsianScientist or its staff.

Shuzhen received a PhD degree from the Johns Hopkins Bloomberg School of Public Health, USA, where she studied the immune response of mosquito vectors to dengue virus.

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