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Color signs on the body in freshwater fish

Because fresh waters are optically turbid in comparison with pure sea waters, curves of photopic spectral sensitivity in freshwater fish are strongly displaced to the red part of the spectrum. It means that for eyes of freshwater fish orange (585-620 nm) and red (longer than 620 nm), depending on the value of displacement, colors are brighter than all other equipower monochromatic colors. Red colors are located commonly on the under shadowed parts of the fish’s body (like the lower fins), in conformity with the theory of color countershading in fresh waters.

On the contrary, in many freshwater fish red and other colors are located on the upper most illuminated parts of the body. Colors with this location become conspicuous both for social mates and predators. There are many strategies in fish to find trade-offs between these extreme positions that, however, will not be considered here.

Colored Body

No doubts, the most known example of coloration of this type in freshwater fish is nuptial coloration of anadromous sockeye salmon, Oncorhynchus nerka, that they acquire in the home rivers in the reproductive period. Breeding sockeye salmons have the olive-green heads and conspicuous carotenoid based red bodies in both sexes but brighter in males (Foot et al., 2004). According to the same authors, in field experiments males of O. nerka show preferences to abstract female models of red color over models of other colors in pre-spawning period, but choose only red models during spawning.

According to spectroscopic measurements (Foot et al.,, 2004), green color of the head and red color of the body form an exactly matched pair of complementary colors that is extremely conspicuous (Endler, 1992).

Another example is black nuptial coloration in males of Ponto-Caspian gobiid fish which readily migrate into the freshwater rivers of this basin and breed here (beeing also invaders). Round goby, Neogobius melanostomus, monkey goby, N. fluviatilis, ratan goby, N. ratan, black goby, Gobius niger, and racer goby, Mesogobius gymnotrachelus, are among them. In general, black coloration of males repel rivals and is more attractive for females than mottled one (see data by Yavno, 2010, for round goby, N. melanostomus).

Black and white mollies, Poecilia latipinna, prefer to aggregate with conspecifics of the same body coloration (McRobert & Bradner, 1998; Bradner & McRobert, 2001).

Colored Stripes

Bilateral longitudinal red (orange) stripes are basic elements of nuptial coloration in males of the anadromous Far-Eastern dace, Tribolodon brandtii (Cyprinidae), and related daces. On the flanks of tribolodons, one of red stripes is located ventrally, another is located dorsally beeing very visible.

Neon tetra, Paracheirodon innesi, cardinal tetra, Cheirodon axelrodi, and some other characins, which inhabit optically turbid blackwater streams and flood lagoons of the Amazon basin, have extremely bright coloration with lateral blue or blue-green stripes and red rear abdominal area (e.g., Lythgoe & Shand, 1983; Ikeda & Kohshima, 2009). Precisely, brilliantly colored lateral stripes are located above the line of the body covexity that is on the most illuminated part of the body.

For comparison, the same patterns with longitudinal colored stripes occur in many marine fish. In Elacatinus gobies, for example, colored high contrast stripes, located on the sides centrally or dorsally, are signals of cleaning service on coral reefs (e.g., Lettieri et al., 2009; Cheney et al., 2009). Longitudinal patterns with some stripes are more attractive for clients than cross banded patterns.

Mixed Patterns

Females of fighting fish, Betta splendens, display readiness to associate with conspecific females depending on coloration of their body that varies dramatically in this species (Blakeslee et al., 2009). Uniformly colored white fish and fish with brown longitudinal stripes prefer to associate with conspecifics of matched body coloration.

Rothenthal & Ryan (2005) have studied schooling preferences in wild zebrafish, Danio rerio, two artificially selected strains (gold and leopard) with reduced stripe expression, stripeless D. albolineatus and striped D. nigrofasciatus. Each species and strain of Danio prefer the phenotype that most resembles its own, with preference for striped stimuli by fish with dark-stripe phenotypes and preference for no stripes by fish lacking stripes (for more details, see Snekser et al., 2010).

Basic References

Blakeslee C., McRobert S.P., Brown A.C., Clotfelter E.D. 2009. The effect of body coloration and group size on social partner preferences in female fighting fish (Betta splendens). Behavioural Processes 80, 157-161

Bradner J., McRobert S.P. 2001. The effect of shoal size on patterns of body colour segregation in mollies. Journal of Fish Biology 59, 960-967

Cheney K.L., Grutter A.S., Blomberg S.P., Marshall N.J. 2009. Blue and yellow signal cleaning behavior in coral reef fishes. Current Biology 19, 1283-1287

Endler J.A. 1992. Signals, signal conditions, and the direction of evolution. American Naturalist 139, S125-S153

Foote C.J., Brown G.S., Hawryshyn C.W. 2004. Female colour and male choice in sockeye salmon: implications for the phenotypic convergence of andromous and nonanadromous morphs.  Animal Behaviour 67, 69-83

Ikeda T., Kohshima S. 2009. Why is the neon tetra so bright? Coloration for mirror-image projection to confuse predators? "Mirror-image decoy” hypothesis. Environmental Biology of Fishes 86, 427-441

Lettieri L., Cheney K.L., Mazel C.H., Boothe D., Marshall N.J., Streelman J.T. 2009. Cleaner gobies evolve advertising stripes of higher contrast. Journal of Experimental Biology 212, 2194-2203

Lythgoe J.N., Shand J. 1983. Diel colour changes in the neon tetra Paracheirodon innesi. Environmental Biology of Fishes 8, 249-254

McRobert S.P., Bradner J. 1998. The influence of body coloration on shoaling preferences in fish. Animal behaviour 56, 611-615

Rothenthal G.G., Ryan M.J. 2005. Assortative preferences for stripes in danios. Animal Behaviour 70, 1063-1066

Snekser J.L., Ruhl N., Bauer K., McRobert S.P. 2010. The influence of sex and phenotype on shoaling decisions in zebrafish. International Journal of Comparative Psychology 23, 70-81

Yavno S. 2010. Behavioural responses of the Round Goby, Neogobius melanostomus, to signals from conspecifics. University of Windsor

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