My New Kind of Rainbow Fish

[BillT]

I made a previous post about this zebrafish mutant (emerald green or now shortened to emerald) I found a while ago:
http://www.mugwump-fish-world.com//index.php?topic=433.msg3839#msg3839
(is this how you're supposed to make internal links here?)

I took some nice pictures a while ago and have been looking at them closely as well as looking into possible explanations for their looks.

Here is my rainbow fish picture:


Here is a cropped version (close-up!!!):


This shows a series of color bands (blue to red) from the top down to the side. This kind of pattern is produced by iridophore pigmentation cells. They have STRUCTURAL pigment, rather than the normal pigment based colors. Blue light has the shorter wavelengths and red has the longer wavelengths.

Pigment colors are chemicals that can be extracted and isolated and will still have the same color as in the animal because it is based on properties of particular molecules.

Structural colors can not be extracted in this manner because they are not based on a chemical. They are based on small reflecting or refracting surfaces separated at the scale of the particular wavelengths. This sets up interference patterns among the light reflected to your eyes or camera, which can result in different colors appearing depending on the angles of illumination and observation WRT the surface of the animal. This interference effect acts something like a color filter (showing only certain wavelengths of light) that changes with your point of view. This stuff was first hypothesized in the time of Newton (late 1600's, early 1700's). Oil slicks do the same thing optically.
https://en.wikipedia.org/wiki/Structural_coloration

This picture shows a rainbow effect because of the smooth curvature of the fish's surface causes a smooth change in the angle of illumination (the angle of observation of the different parts of the body would also have some effect but is not a very big difference across the rainbow).

This explanation assumes that the guanine platelets, that are the reflecting surfaces in iridophores (shiny pigment cells), are mostly parallel to the surface (parallel to the tangent to the curve of the body surface at a particular place) of the fish.

I sent the fish to Dave Parichy's lab in Seattle a few years ago.
They think its a new mutant (to researchers), a single (or two closely linked) gene(s).
It has melanosomes (dark brown or black pigment cells) but they lack pigment and are therefore clear. However, melanosomes affect the patterning of other pigment cells like iridophores

I recently contacted Dave because I wanted to better understand the cytology of iridophores (what the cells look like and where they are) in zebrafish. He gave me some papers from a Japanese lab. They describe two kinds of iridophores s-type (small guanine crystals) and L type (large guanine crystals). The L type crystals are always close to parallel to the body surface, making a good reflective surface. The S type crystals are sometimes parallel to the surface and sometimes not. When they are not, the iridophores have a white color. If they are parallel to the surface they appear shiny like the L type cells.
Normally the L type iridophores are under the dark stripes in the zebrafish and the S type iridophores are in the non-dark stripes. My fish have a leopard background, so it should be spots and non-spots. the S type iridophores normally haven a very dense layer of cells with lots of reflecting guanine platelets while the L type cells have only a few large platelets (making dots (or a cytologist would say its punctate)). This can be seen in many pictures.

This explanation makes a lot of sense to me and I can see how this works now in lots of other pictures.

Payoff for reading this crap:
I would expect that if you have fish with iridescent pigments (basically shiny or metallic looking) you can probably maximize the iridescent effect by using a single strong light rather than several different lights illuminating the fish from lots of different angles. In this way, the interference pattern of the light coming off the fish's iridophores will not be diluted by other light patterns.

The angle of the light illuminating the fish can greatly change the color of what you see. My pictures were taken with the flash at about 60 degrees up. This gave the blue green color of the fish I got in my first pictures. The rainbow fish was flashed from directly above. Dave sent me a picture he took that is red. This predicts his light was probably off to the sides, thereby accentuating the longer wavelengths.
This is definitely something that could be manipulated to get fish to show different colors. I have a bunch of things to try out (experiments) based on this ideas (hypotheses).
I'll take pictures from different angles (should shift around the rainbow effect) and use some color filters (ordered but not yet here) to illuminate only with particular wavelengths (which should produce a single monchromatic (single color) peak of light intensity at some point.

[Mugwump]

Pretty little thing....reminds me of the 'pearl' danios...