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My fancy goldfish, Rashi, was purchased as a black Moor. However, over the years she has been slowly turning orange. In an effort to understand this transformation, I have done a bit of reading about colors in fish and why and how they change, and I would like to share some of my findings with other curious fish owners.
The colors of fish are due to the presence of pigment cells called chromatophores. Chromatophores come in two varieties: those that absorb light and those that reflect light. Some light-absorbing chromatophores that occur in fish are melanophores, erythophores, xanthophores, and cyanophores. Leucophores and iridophores are examples of light-reflecting chromatophores. Inside chromatophores are organelles called chromatosomes. The type of chromatosome found in a chromatophore determines the color of the pigment cell. For example, melanophores, which are black, contain melanosomes, ie. melanin (black) is the pigment in the chormatosomes of melanophores.
Scientists have identified two types of color changes in fish: physiological and morphological. Physiological color changes are due to the spreading out or aggregation of chromatosomes. When the chromatosomes are spread throughout the cell, the color is more pronounced to the naked eye. However, when the chromatosomes aggregate in the center of the cell, the color is muted or not visable. Morphological color changes, on the other hand, are due to a change in the number of chromatophores. So a fish that loses a number of melanophores will appear lighter, and a fish that gains melanophores will appear darker. Physiological color changes can become morphological color changes over time. For example, a fish that is kept in a tank with a dark background and dark rocks will become darker, initially because of movement of melanosomes in the already exisiting melanophores. However, if enough time goes buy, the fish will start to produce more melanophores and then the color change is considered morphological.
The common phenomenon of black goldfish turning orange, or young goldfish losing black markings as they grow is an example of a morphological color change. As the fish mature, they lose melanophores in a process called apoptosis. Apoptosis is directed cell death, or cellular suicide, and is an important phenomenon in many aspects of development. However, the exact molecular cues that tell a cell it's time to die are still very mysterious. In addition to apoptosis, goldfish that lose black coloration are also supressing the birth of new melanophores. The loss of melanophores reveals the other pigment cells present in the skin. The type and extent of color change a young fish will go through depends on their individual genetic makeup, and there is a lot of variation between individuals! Some fish even go through a second color change from the destruction of xanthophores (red pigment cells).
My Moor/telescope goldfish, Rashi, has been undergoing a very slow morphological color change over the last three years. When she first came into my life, in the spring of 2003, she was all black with a hint of orange on her belly. This picture is from July 2003:
Slowly the orange coloration spread as melanophores died and were not replaced. Here is Rashi in February 2006:
The rate of color change in Rashi is definitely not constant. Rather, she appears to go through bursts of change. Her coloration will remain stable for a few months or more, and then suddenly I will notice new orange patches on her. Here she is about a month after the above picture, in March 2006. Already the orange has spread further up her body.
Her coloration stayed like that pictured above until early May 2006, when I noticed new orange spots on her head.
Rashi has an on-going issue with mysterious spots on her tail fins. In hopes of diagnosing her, I took a small sample from the tip of her fin and looked at it under a microscope. While I was unable to determine what the spots are, I did get some interesting pictures of chromatophores. Here you can see the black melanophores and orange erythrophores.
To the naked eye, the fin appeared black. It is interesting to note how the melanophores in the above pictures appear to have more processes radiating from their centers than the erythrophores. This is indeed what we would expect to see in a black fin. As the fin changes from black to orange, however, the melanophores will retract their processes before they are destroyed. There have not been any reports of other light-absorbing pigment cells changing in this manner.
At GAB, we have a couple of reports of Moors turning orange as they grow, as well as orange Ryunkins and telescopes turning white, and calicos of various breeds loosing their black markings. Some of these goldfish have changed color within a few months while others, like Rashi, take a few years. Do you have a coloring-changing goldfish? Please share his or her story with us!
Table shows the progression of Rashi's color change
Last updated: June 28th, 2006
References:
Goodrich, HB. The Development of Hereditary Color Patterns in Fish. The American Naturalist, Vol. 69, No. 722, pp. 267-277. (May-June 1935).
Goodrich, HB and Hansen, IB. The Postembryonic Development of Mendelian Characters in the Goldfish, Carassius auratus. The Journal of Experimental Zoology, Vol. 59, No. 2, pp. 337-358. (April 1931)
Kajishima, Takao. Genetic and Developmental Analysis of Some New Color Mutants in the Goldfish, Carassius auratus. Genetics, 86: 161-174. (May 1977).
Sugimoto, Masazumi. Morphological Color Changes in Fish: Regulation of Pigment Cell Density and Morphology. Microscopy Research and Technique, 58:496-503 (2002).
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