Brightly colored males of freshwater fish are forced to find trade-offs between conspicuousness for sexual mates and cripticity for potential predators. Divergence in spectral sensitivity of prey and predators is one of the possible ways to solve this problem in its evolutionary development.

The maximums of spectral sensitivity in threespined stickleback, Gasterosteus aculeatus (605 nm: Rowe et al., 2004), as prey, and perch, Perca fluviatilis (635 nm: Protasov, 1968), as natural predators, are strongly distanced. It means that for P. fluviatilis, as specialized observers in the longwave part of the spectrum, bright red breast and blue iris of G. aculeatus (Rowe et al., 2004) must look darker (that is cryptic) than for the owners of these colors. Similarly, the maximums of spectral sensitivity in pumpkinseed sunfish, Lepomis gibbosus (612 nm: Tamura & Niwa, 1967), as prey, and largemouth bass, Micropterus salmoides (673 nm: Kawamura & Kishimoto, 2002), as natural predators, are distanced even more. So, for M. salmoides, with fine color vision in the far red part of the spectrum, conspicuous red iris and opercular flaps, all-important for breeding sunfish (Stacey & Chiszar, 1978), must be dimmer than for the sunfish exploited these signals.

Another way is the divergence of spectral sensitivity in the shortwave part of the spectrum.

Vision in the ultraviolet (UV) part of the spectrum (with the maximum of spectral sensitivity near 365 nm) affects female mate choice in threespined sticklebacks, G. aculeatus (Rick et. al., 2004; Boulcott et al., 2005; Rick et al., 2006). According to Leech & Johnsen (2009), however, ability to see in the UV region (with the maximum near 385 nm) occurs only in planktivorous individuals of Perca flavescence (or P. fluviatilis) disappearing in adult predatory fish.

Basic References

Boulcott P.D., Walton K., Braithwaite V.A. 2005. The role of ultraviolet wavelengths in the mate-choice decisions of female three-spined sticklebacks. Journal of Experimental Biology 208, 1453-1458

Kawamura G., Kishimoto T. 2002. Color vision, accomodation and visual acuity in the largemouth bass. Fisheries Science 68, 1041-1046

Leech D.M., Johnsen S. 2009. Light, Biological Receptors. Encyclopedia of Inland Waters. (Gene E. Likens, Editor), Oxford, Elsevier. Volume 2, 671-681

Protasov V.R., 1968. Vision and near orientation in fish. Israel program for scientific translations, Jerusalem

Rick I.P., Modarressie R., Bakker T.C.M. 2004. Male three-spined sticklebacks reflect in ultraviolet light. Behaviour 141, 1531-1541

Rick I.P., Modarressie R., Bakker T.C.M. 2006. UV wavelengths affect female mate choice in three-spined sticklebacks. Animal Behaviour 71, 307–313

Rowe M.P., Baube C.L., Loew E.R., Phillips J.B. 2004. Optimal mechanisms fo finding and selecting mates: how threespine sticklebac ... Read more »

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 erythropteru ... Read more »