Countershading is an examplary pattern of animal coloration in which an animal’s pigmentation is darker on the upper side and lighter on the under side of the body. When light falls on any volume uniformly colored object from above, it makes the upper side appear lighter and the under side darker with the gradual transition between them. Thanks to the counetrshading with the dark upper side and the light under side, the same object appears flat and thus invisible on the surrounding background.

This basic pattern is found in many species of mammals, reptiles, birds, fish and other animals.

Color countershading is determined by color vision of fish that is characterized by the different bell like curves of spectral sensitivity. Spectral sensitivity is an ability of the eye to perceive monochromatic light of equal power with the different wavelengths. Eyes of saltwater fish, and human, are most sensitive to light with the wavelengths of 550-560 nm (the green-yellow part of the spectrum). Because fresh waters are optically turbid, eyes of freshwater fish are more sensitive to light with the wavelengths of 600-680 nm (the orange-red part of the spectrum). Due to bell like dependence of spectral sensitivity equipower monochromatic light of different wavelengths are not equally bright to the eye. Green light is most bright for human and saltwter fish, red light is most brigth for freshwater fish contrary to our perception.

In the terms of cryptic countershading, coloration of freshwater fish may be conditionally divided into achromatic, or bright countershading patterns and chromatic, or color countershading patterns. Greyish back, silver (mirrored) sides and whitish belly are typical for fish with the achromatic countershading. Green, yellow (goldish or golden), orange and red colors are main components of the color countershading patterns. Darker colors lie on the top, brighter colors lie on the bottom with the corresponding gradual transitions. The first sign of the presence of color countershading in an appearance of freshwater fish is red color of pectoral, pelvic, anal and caudal fins lied in the less illuminated parts or in the shadow of the fish’s body.

Pelagic Coloration

In general, coloration of freshwater pelagic fish is ranged from blank achromatic countershading patterns to color countershading patterns, depending on the optical properties of the water.

For idealized mixed countershading patterns, greenish back, silver sides, whitish belly as well as yellowish, orange, reddish or red pectoral, pelvic, anal and caudal fins are typical. This type of coloration is found in European asp, Aspius aspius, Aral redlip, A.a. taeniatus, in many Amur fish (see Nikolski, 1956) like Amur redfin, Pseudaspius leptocephalus, skygazer, Erythroculter erythropterus, Mongolian redfin, E. mongolicus, yellowcheek, Elopichthys bambusa, three-lip, Opsariichthys uncirostris amurensis, and in many other freshwater pelagic fish in other regions. Note, yelllow golden color of gill covers and yellow color of the lower lip in the yellowcheeks is in the full conformity with the theory of color countershading in fresh waters.

In general, the colored lower fins are practically absent in pelagic fish with the wedge like shadowless body (Aleyev, 1975) and occur in pelagic fish with the roundish shadow made body.

Littoral Coloration

Mixed color countershading patterns with greenish back, silver sides, whitish belly and the colored lower fins are ultimately developed in frshwater littoral fish. Such generally known fish as roach, Rutilus rutilus, rudd, Scardinius erythrophthalmus, ide, Leuciscus idus, chub, L. cephalus, and many other are classic examples. In contrast to pelagic fish in which coloration of the lower fins may be missing, in littoral fish this feature is constant and exptremely distinct.

Natural littoral zones are commonly represented by open deep areas and overgrown shallow areas, with the significantly different optical properties of the water in these areas. In overgrown shallow areas, floating aquatic plants (like water lily and other) and submerged aqauatic plants work as optical filters and reflectors. Moreover, the water in these areas contains more organic substances than the water in open areas. Thanks to the coaction of these two factors, the water as the optical medium is enriched in overgrown areas by rays in the green-yellow part of the spectrum. In these optical conditions, roach, rudd, ide and other fish have more or less distinct golden sides instead of silver ones and yellowish or even orangish belly instead of whitish one.

Steady olive golden sides is found in freshwater fish, like tench, Tinca tinca, that constantly live in the overgrown areas.

According to calculations (Johnsen, 2003; Johnsen & Sosik, 2003), in the described optical conditions colored diffusely reflective surfaces are more cryptic than mirrored directionally reflective ones.

Generally, in freshwater littoral fish with the high flattened body, like bream, Abramis brama, coloration of the lower fins is absent or less developed. In addition, deep dwelling bream, A. brama, have golden sides versus silver ones and yellowish or orangish belly versus whitish one.

In addition, there are wild gene based golden color morphs of cyprinid fish (such as carp, rudd, ide, tench and more: for example, see Kvasnička et al., 1998).

Phytophilous Coloration

In freashwater fish with the so called phytophilous coloration, mixed color countershading patterns with greenish back, silver greenish or golden greenish sides, whitish belly and yellowish, orange, reddish or red fins are basic. At the same time, coloration of sides in these fish is supplemented with the achromatic or chromatic spots, irregularities and transverse bands that form the so called disruptive patterns cryptic on the background of underwater plants and woods.

Phytophilous colorations with the various disruptive patterns are found in many freshwater fish worldwide. Examples are generally known pike Esocidae, except Amur pike, Esox reicherti, with the rheophilous cryptic patterns (Nikolski, 1956), freshwater perch Percidae, such as Perca fluviatilis and P. flavescens, piranha Characidae, sunfish Centrarchidae like largemouth bass, Micropterus salmoides, bluegill, Lepomis macrochirus, and many other fish. Furthermore, bellies in these fish are frequently yellowish, orangish and reddish outside the breeding season.

Rheophilous Coloration

Mixed color countershading patterns with greenish back, silver or golden sides, whitish or yellowish belly and yellowish, orangish or reddish fins are basic in fish with the so called rheophilous, or brook coloration. In these fish, basic countershading coloration is supplemented with the numerous achromatic (black) and chromatic (orange, red) dots, simple and ringed, that cover usually all the upper part of the body, the head and the upper fins. Such well known freshwater fish as brown trout, Salmo trutta, European grayling, Thymallus thymallus, taimen, Hucho taimen, and many other are typical examples. With color countershading and dots, the rheophilous fish are cryptic in murmuring streams and on the background of mottled stony bottom with flecks of sunlight from the surface waves. In addition to dots as elements of the rheophillous coloration, young individuals of these fish, called parr, have on their sides per an ordered lateral row of large spots that are basic elements of the so called lithophilous coloration.

Graylings have the less dotty body than other rheophilous freshwater fish, but they have huge rainbow-patchy dorsal fins with an evident cryptic function. According to Darchambeau & Ponchin (1997), brightly colored dorsal fins play an important role in the spawning behaviour of these fish.

In sum, an idealized coloration of fish consists of cryptic patterns of two types, achromatic and chromatic countershading, and disruptive patterns of three types: phytophilous, rheophilous, lithophilous. The so called psammophilous (sandy) pattern and some other patterns are not considered here. By definition (Endler, 2006), disruptive patterns are conspicuous, because they must dismember an appearance of an animal into the disconnected parts. Taken together, however, cryptic and disruptive patterns create an overall camouflage coloration in which, in addition, some elements may be used as signals in intraspecific and interspecific communication.

Basic References

Aleyev Y.G. 1976. Nekton. Naukova Dumka Publishers, Kyiv

Darchambeau F.,  Ponchin P. 1997. Field observations of the spawning behaviour of European grayling. Journal of Fish Biology 51, 1066-1068

Endler J.A. 2006. Disruptive and cryptic coloration. Proceedings of the Royal Society, Biological Scienses 273, 2425-2426

Kvasnička P., Flajšhans M., Ráb P., Linhart O. 1998. Inheritance studies of blue and golden varieties of tench (Pisces: Tinca tinca L.). Journal of Heredity 89, 553-556

Johnsen S. 2003. Lifting the cloak of invisibility: the effects of changing optical conditions on pelagic crypsis. Integrative an Comparative Biology 43, 580-590

Johnsen S., Sosik H.M. 2003. Cryptic coloration and mirrored sides as camouflage strategies in near-surface pelagic habitats: Implications for foraging and predator avoidance. Limnology and Oceanography 48, 1277-1288

Nikolski G.V. 1956. Fishes of the Amur basin. USSR Academy of Science Publishing, Moscow