Mirror carp possess an extraordinary feature that distinguishes them from their common carp relatives—their scales. Unlike the even and regular scale pattern of common carp, mirror carp boast irregular and patchy scales that bear a striking resemblance to mirrors. These distinctive scales make each mirror carp a unique and easily identifiable individual. It is worth noting that in the United Kingdom, mirror carp weighing over 40 lb (18 kg) often acquire colorful nicknames, adding to their charm and allure.
Additionally, leather carp share a resemblance to mirror carp in terms of scale appearance. However, leather carp are permitted to have a few scales along the dorsal line or the wrist of the tail, differentiating them from the mirror carp variety.
The genetic makeup of mirror carp unravels the mystery behind their awe-inspiring scales. The most significant distinction between mirror and common carp lies in the presence of large mirror-like scales in the former. This unique scale phenotype is the result of a genetic mutation present at either the S or N scale trait loci. The genotype responsible for producing the mirror scale phenotype is "ssnn," where the lowercase letters represent recessive alleles. In contrast, wild-type carp may have either the SSnn or Ssnn genotype, denoting the presence of dominant alleles.
Researchers have identified the "S" locus as containing the gene encoding fibroblast growth factor receptor Fgfr1A1. During the evolutionary process of carp, this gene duplicated, resulting in the absence of typically lethal phenotypes when only one locus is mutated. However, the "N" locus, which also contributes to the scale development of mirror carp, has yet to be identified. It is hypothesized that the "N" locus plays a role in embryonic mesenchyme development.
Contrary to popular belief, a leather carp is not merely a mirror carp without scales. True leather carp are homozygous recessive at the "S" locus (ssnn genotype), but they are heterozygous for a dominant mutant allele at the "N" locus (ssNn genotype). Moreover, leather carp exhibit reduced red blood cell counts and slower growth rates compared to scaled carp. The differences in pharyngeal teeth among mirror carp from Hungarian and Asian stocks, as well as nude carp, have also been observed.
Mirror carp introduced to Madagascar, where they became an invasive species, exhibited a remarkable transformation. Although they maintained their mirror phenotype, the feral Malagasy carp developed increased scale coverage, closely resembling that of wild-type carp. Researchers suggest that this phenotypic reversion was due to compensation by quantitative trait loci, driven by a selective disadvantage for partial scaling in the wild. This selective pressure might be related to an impairment in parasite resistance, providing an intriguing insight into the adaptive nature of mirror carp.
Anglers worldwide are drawn to the challenge of capturing large leather carp, driven by their rarity and the thrill of the chase. One notable example is Heather the Leather, a colossal leather carp weighing an impressive 52 lb (24 kg). Such exceptional catches not only exemplify the allure of leather carp but also testify to the dedication and skill of passionate anglers.