When flounders are small larvae, they swim around with one eye on each side of their body, just like ordinary fish. But then their appearance undergoes a dramatic change. Their bodies start to tilt and one eye starts to migrate to the other side. Finally, their bodies become horizontal, with a white blind side and a pigmented ocular side. The flounder has thus gained its characteristic appearance.
This process is called metamorphosis, which means a major change of appearance, and can be compared to the process whereby a tadpole turns into a proper frog.
But what actually happens? How does a flounder manage to move its eye and have different colours on each side? Scientists have now found the answer.
Turn towards the vitamin
‘We have discovered that light controls the migration of the eye and pigmentation in Japanese flounders. And the same mechanism is probably at work in all flatfish species,’ says Kristin Hamre, senior scientist at NIFES in Bergen, Norway.
An international research team comprising mostly Chinese scientists, but also members from Europe and the USA, has discovered that the skin of the Japanese flounder contains the protein rhodopsin. When this protein registers light, it releases vitamin A. As flounders tilt more and more, the skin on the side facing up registers most light.
‘More vitamin A is released when the intensity of the light that hits the surface of the fish increases, so that a gradual increase, a so-called gradient, arises of vitamin A from the underside to the upper side. The eye on the underside gradually migrates to the upper side, where there is a lot of vitamin A,’ says Hamre.
Light also affects the colour of the flounder. The ocular side, where vitamin A is released in the skin, becomes pigmented, while the blind side remains white.
The entire process takes around one month in Japanese flounders, but what triggers the process is still a mystery.
‘We still don’t know why the fish start to tilt,’ says Hamre.
Problems with halibut larvae
The answer to how the flounder gains its characteristic appearance has just been published in the renowned journal Nature Genetics. At NIFES, the hunt for the answer started several years ago.
‘We worked a lot with halibut larvae, but they often had developmental defects, their eyes didn’t migrate and they had the wrong pigmentation,’ says Hamre.
The scientists at NIFES therefore tried to find out why this was happening to the halibut larvae.
‘We searched the literature on other types of flatfish extensively and came up with a number of hypotheses, including that pigmentation and eye migration were dependent on a vitamin A gradient. We didn’t know though how it was formed and we didn’t have the funding to pursue it,’ says Hamre.
Valuable lack of funding
The lack of funding led Hamre and her fellow scientists to publish their hypotheses in a review article in 2007.
‘Then I suddenly got an email from Changwei Shao, a scientist from Yellow Sea Fisheries Institute in Qingdao in China,’ says Hamre.
Shao had read the article, and though the hypothesis for the flounder metamorphosis was excellent. The Chinese scientists had just sequenced the genome of the Japanese flounder, i.e. they had mapped all of the species’ genes. The hypothesis provided the Chinese scientists with a pointer to what to look for in the vast genetic material. By applying the NIFES scientists’ hypothesis to an overview of the genes that are particular to flatfish, they eventually found a connection between light, vitamin A and the metamorphosis.
Once they had studied the genome and confirmed their findings in various trials, the scientists were finally certain: Light controls eye migration and pigmentation in the flounder. Hamre and the scientists at NIFES’ hypothesis was correct.