New insight into colour changing Neons

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The Neon tetra (Paracheirodon innesi) swimming in your tank and the Venetian blind hanging from your window may have a lot more in common than you think.

The new findings are to be published by a team of Japanese scientists in a forthcoming issue of the Journal of the Royal Society Interface.

Shinya Yoshioka and coauthors made this conclusion while studying the mechanism behind the coloration of the neon stripe in the Neon tetra, which can change from yellow to violet depending on the time of the day and the mood of the fish. 

It has been known for some time that the colour effects of the Neon tetra's stripe are due to light-reflecting cells known as iridiophores. Each iridiophore is known to contain two, occasionally one or three, stacks of thin platelets of guanine crystals. 

The exact mechanism by which these platelets in the iridiophores work in changing colour was not known until now, although several models had been proposed (but not verified). 

One such model, known as the Venetian blind model, proposed that the angle of the platelets could be physiologically controlled in a similar manner to the way a Venetian blind works; colour change would be achieved in this model by changing the angle of the platelets.

The authors constructed a microscope apparatus that could measure the angle and the colour of the guanine platelets in the iridiophores simultaneously, and used this to test if the Venetian blind model applied to the iridiohores of the Neon tetra. 

The apparatus directed a strong beam of light to the iridiophores and measured the angle at which this light is reflected. At the same time, a spectrometer attached to this apparatus measured the wavelength (i.e. the colour) of the reflected beam.

Previous experiments had shown that increasing the amount of potassium in the solution in which the iridiophores were bathed could induce a colour change, so the authors added potassium to the solution in which a piece of skin from the Neon-tetra stripe had been placed under the microscope. 

They found that when the platelets appeared blue, they were oriented about 12° to the floor of the iridiophores, and when they appeared yellow, this angle changed to about 16°C.  The results of this study thus validated the Venetian blind model.

The exact physiological mechanism by which the angle of the platelets is controlled remains unknown, however.  The observation that potassium is involved in colour change and given potassium's role in neuronal responses suggests that this form of control is likely to be neuronal.

For more information, see the paper: Yoshioka, S, B Matsuhana, S Tanaka, Y Inouye, N Oshima and S. Kinoshita (2010) Mechanism of variable structural colour in the neon tetra: quantitative evaluation of the Venetian blind model. Journal of the Royal Society Interface doi: 10.1098/​rsif.2010.0253