By: Matt Lyon
Below we explore some advanced goldfish topics. The public’s understanding on these topics has not advanced significantly since the early 1970’s. This is an invitation for advanced goldfish breeders to contribute to the goldfish world’s collective knowledge base on these topics. If you are interested in sharing additional information beyond this article, or contributing to our efforts for better understanding, please contact The Goldfish Council. Further, if we have made any errors, please bring them to our attention and we will update this document as appropriate.
The purpose of this article is to define and clarify the terms mock metallic, blue belly (or bluebelly), and pseudomatt (or pseudomatte) for goldfish breeders. We hope to harmonize the available information as far as presently possible. We will also discuss some related terms and contrast the matte gene against the netlike transparent mutations. Because this topic relates to the presence and absence of iridescent pigment in goldfish, we will begin with a discussion on guanine.
Guanine is the colorless, iridescent pigment present on birds, some herps and on the surface layers of most fish. It is the substance responsible for giving fish their metallic appearance. Guanine is contained in microscopic cells known as iridocytes. In goldfish, the normal or wild state is for guanine to be present on the rear surfaces of the scales, inner margin of the skin, rear surfaces of the gill covers (opercula), on the fins and in the irises of the eyes. All of this guanine gives goldfish an opaque, metallic appearance by blocking the visibility of any deep-seated pigments, deeper tissue layers, internal organs, etc. The presence of guanine behind colored pigments gives those pigments a uniform and opaque quality, while the absence of guanine can give colors the appearance of more depth. In contrast to guanine, which is opaque, the color pigments of goldfish have varying levels of transparency or translucency.
Without guanine on the scales, it is possible to see through the scales to pigments and guanine in the skin. Without guanine on either the scales or skin, it is possible to see through the skin and into the deeper tissues and even internal organs. Without guanine on the opercula, we can see through to the reddish hue of the gills. Without guanine in the eyes, a goldfish’s eyes appear as solid black (this is sometimes termed “button eyes” or “shoe button”). The absence of guanine is usually described as “transparent” or “transparency”. In goldfish, there are at least 2 known transparency mutations that create the effects mentioned above in distinctive ways.
The most common and well-known transparency mutation is known as the “matt” or “matte” gene. This is the gene that causes the transparent scales on calico and matte goldfish. In calico goldfish, there are usually several randomly-placed metallic scales present, giving the fish a spangled appearance. Calico goldfish can also be referred to as “nacreous” (though there are numerous color combinations in nacreous fish in addition to calico). The term nacreous stems from the pearlescent sheen caused by the presence of a guanine layer in the skin. This guanine layer would not be visible on metallic fish, because the metallic scales would obscure our view beneath. Scale transparency, combined with the presence of this pearlescent layer underlying color pigments produces a palette of colors not seen in metallic fish: lavender, salmon, coral, pink, pastel yellow, etc. The matte gene has been long established as incomplete dominant. Due to this, the effects of the matte gene are visible in both heterozygous (nacreous) fish and homozygous (matte) fish. Matte goldfish have no metallic scales while also having absent or reduced guanine in the skin. Matte goldfish are also known as “pinkie” or “pinkie matte”. The matte mutation has been observed in goldfish for well over a hundred years, possibly as early as 1688. It should be noted that the matte gene also has an effect on the quantity and placement of pigment-containing cells (chromatophores), producing the mottling effect of many nacreous color patterns. Pinkie mattes are often nearly colorless (their pink appearance is caused by the blood cells in capillaries). Stated another way, when comparing nacreous and matte fish, there seems to be a relationship between the presence, quantity, and pattern of both guanine and color pigments. Matsui represented the matte mutation using (T) for matte and (t) for metallic. The genotypes for the matte mutation are:
TT = matte
Tt = nacreous or calico fish (heterozygous for the matte mutation)
tt = normal metallic scaled fish
As an aside, we have recently become aware of another transparency mutation, described by Dr. Toki-o Yamamoto in 1977, and termed “nacreous-like”, but will not explore that topic here.
 Goldfish Breeding & Genetics, Smartt & Bundell, 1996, Page 117  The Goldfish, Hervey & Hems, 1968, Page 87  Inheritance of Nacreous-Like Scaleness in the Ginbuna, Carrasius auratus langsdorfii; Yamamoto, Toki-O, 1977
Another less-familiar transparency mutation has resulted in the descriptive terms “mock metallic”, “netlike transparent”, “blue belly”, and “pseudomatt” (or “pseudomatte”). Even though the earliest of these phenomena was first described nearly 100 years ago, our general understanding of them is far from complete. There has been debate on whether these terms all describe the same mutation or possibly two or three different (but similar) mutations. On more than one occasion, it has been suggested that these mutations are actually the same. In Goldfish Varieties & Genetics, Smartt concludes that they are the same mutation. But recent personal correspondence suggests it is more complex than that. One of the few things we can conclude for certain is that these mutations seem to have arisen independently and geographically separate, as will be explained further.
For the terms “mock metallic”, “netlike transparent”, and “blue belly”, these fish are described as having reduced guanine (in both the skin and scales), often with a netlike appearance to the scales. Though these fish may have both transparent and metallic scales, the placement of the metallic scales isn’t random as in the matte mutation. The metallic scales tend to be present in wide swaths below and, to a lesser extent, above the lateral line. And the lower rear abdomen is often void of metallic scales. According to both Matsui and Smartt, when metallic scales are present, the rear surface of each scale has a central guanine deposit and transparent edges. The size of the guanine deposit relative to scale size can vary widely. Also, when viewing each metallic scale individually, the guanine sometimes has tiny transparent flecks or streaks, giving the fish a sparkly or “crystal” appearance. However, the scalation of these fish can also vary from fully transparent to fully metallic, due to both genetic and environmental factors . Young fish often express less guanine than older fish, and the guanine expression often increases throughout a fish’s life. In some cases this results in an adult fish visibly indistinguishable from a normal metallic.
In contrast to the matte mutation, it should be noted that these mutations only affect the production of guanine, not the presence or pattern of color pigments. Therefore, fish with these mutations display the same general colors and patterns as metallic fish. These fish come in both solid colors (black, blue, chocolate, etc) and patterns (sarasa, etc). But they do not come in calico or finely speckled patterns unless combined with the matte mutation (see the term pseudomatte below). Also, colored fish with this mutation tend to have a broader color palette and more vivid colors than metallic fish. Reduced guanine at the scale level and absence of guanine at the skin level allows light to penetrate deeper into the layers of the fish’s skin. When that light travels back through translucent tissues and transparent scales, refraction occurs, resulting in the unique hues we perceive in these fish. Further, the color of the scales may blend with the color of the skin beneath. This effect combined with refraction creates fish with deeper red, tangerine orange, lemon yellow, etc.
Fish with these mutations are sometimes seen in the commercial trade, marketed under various trade names (examples include “crystal” ranchu, “Sunburst” comets, etc.)
Goldfish Varieties & Genetics, Smartt, 2001, Page 167
Netlike transparent – In Japan, Dr. Yoshiichi Matsui observed and described this mutation as early as 1927. It was described as a recessive mutation. He used both the terms “netlike transparent” and “reticular transparent” to describe it. According to Matsui, it emerged in a population of metallic demelanizing Wakin. Matsui provided one of the most complete discussions available on this topic. He also printed a diagram showing the theoretical array of genotypes and phenotypes possible when combining the matte and netlike transparent mutations. To designate the genotype, he used (N) for normal metallic and (n) for the netlike transparent. Netlike transparent is also useful as a descriptive term, as it describes the overall appearance resulting from the deposition of guanine in the center of each scale (with each scale also having a transparent margin). Viewed as a whole, a fish’s scales (or portions of the scales) will have a netlike appearance, when homozygous for this trait.
Mock Metallic – In the UK, this recessive mutation was described and given the name “mock metallic” by Daphne Morris. Mock metallics were first observed in her shubunkin spawns. Matsui referenced Miss Morris’ work as early as 1972. These fish did not demelanize and had the olive bronze coloration found in wild type metallic fish arising from calico goldfish spawns. They were apparently named “mock metallic” because in some ways they resembled metallic fish, but produce different breeding results.
Dave Mandley describes the mock metallic mutation as the “bluebelly fish of the nacreous group”.
Mock metallics display a netlike transparent guanine pattern. In general, mock metallics are associated with higher guanine expression than fish that would be labelled as bluebelly.
Bluebelly – In the United States, a similar mutation was described by Al Thomma and named the “blue belly” gene. It is a recessive guanine-reducing mutation, and was first observed in demelanizing feral goldfish populations in Ohio, USA. It is so-named because of a dark bluish color present in the lower abdomen of these fish (particularly that of young fish, whose tissue is thinner and less opaque, and which tend to have less guanine overall). According to Smartt, the blue coloration is deep-seated melanin pigment in the abdomen. But according to Dave Mandley, the blue coloration is caused by large, dark-colored scales in the peritoneum. Other goldfish breeders have also supposed that the blue coloration is due to the visibility of the internal organs (the lack of guanine in the scales and skin allowing a translucent view into the peritoneum). Fish described as bluebelly are generally associated with lower guanine levels than mock metallics, though varying guanine expression is also observed.
Pseudomatte – This is a term coined by Daphne Morris in the UK. Presumably when breeding mock metallics with calico fish in her shubunkins, she produced an interesting fish that she named the “pseudomatt”. Smartt wrote that the pseudomatte is heterozygous for the matte mutation (aka nacreous), and homozygous for the mock metallic mutation. But Matsui contradicts this by stating that the pseudomatte genotype is TTnn (homozygous matte and homozygous netlike transparent). It generally has the color pattern of a calico goldfish, while having a complete lack of guanine (owing to a double dose of transparency), thus making its transparency similar to that of a matte fish. However, it produces breeding results more similar to a nacreous goldfish. Pseudomattes generally have black button eyes, transparent gill plates, no metallic scales, and calico color patterns. Like non-nacreous fish with a netlike mutation, pseudomattes have particularly vivid colors. A pseudomatte cannot necessarily be positively identified by sight, because they may be so similar to regular calico or nacreous fish (of which there is a wide variety of color expression).
 Goldfish Varieties & Genetics, Smartt & Bundell, 2001, pages 168 & 174
In theory, to determine if fish are pseudomatte, their genetic makeup can be determined by breeding them and observing the progeny. The breeder would need to raise the fry for several months in order to observe their scale types. In theory, if the statistically-expected distribution of scale types is produced, then the breeder can conclude the genetic makeup of the parents. On the Bristol Aquarists goldfish color types chart, it states “pseudo-matt x pseudo-matt is said to produce mock metallic, calico, and pseudo-matt”. Generally speaking, we would expect pseudomatte spawns to produce scale type distributions similar to that of nacreous fish, but with the addition of mock metallics.
Transparency mutations have resulted in some of the most beautiful colors and attractive patterns in ornamental goldfish. Calico, Sakura, shubunkin, milk cow, ink wash, midnight, and sky blue are all poetic-sounding terms that stretch to capture in words the visual effects of the matte mutation. The netlike transparent mutations have resulted in equally descriptive names and trade names, such as Apricot Blush, Sunburst, and crystal. The netlike transparent mutations have seen far less “mass acceptance” than the matte mutation, but both still hold endless future potential for goldfish breeders to discover and experiment.
The Goldfish; Hervey & Hems, 1968
Goldfish Varieties & Genetics; Smartt, 2001
Goldfish Breeding & Genetics; Smartt & Bundell, 1996
Goldfish Guide; Matsui, Yoshiichi, 1972
Inheritance of Nacreous-Like Scaleness in the Ginbuna, Carrasius auratus langsdorfii; Yamamoto, Toki-O, 1977
Tippitt, R.R. & Bennett, M.C. (1965) Rediscovery of the net-like transparent goldfishes. The Aquarium, December, pp. 10–11
Preliminary Note on the Inheritance of Scale Transparency in Gold-fish of Japan; Matsui, Yoshiichi, 1932
https://goldfishkeepers.com/GoldfishCullingScaleTypes.php (pages 1 through 4)