More or less uniformly colored yellow, orange and red dorsal fins without additional marks occur in many freshwater fish, first of all in cyprinid fish such as roach, Rutilus rutilus, rudd, Scardinius erythrophthalmus, and more. In roach, for example, the rear part of dorsal fin is constantly red, this fin can be folded and rise. Theoretically, roach can fold its red dorsal fin with the approaching of natural predators (like perch, Perca fluviatilis, or pike, Esox lucius), becoming cryptic, and rise it using as red signal to contact with schooling and sexual mates.

In this experiment, the hypothesis that perch can eliminate roach with the bright red dorsal fins was verified.

An experiment was carried out in an enclosure fenced off the shore by three nets 2x2x2 m of 40 cm height. The maximum depth in the enclosure with pure sandy bottom was 30 cm. From the side of open water, dense thickets of pondweed (Potamogeton) were located. 25-30 roach (4-5 cm standard length) and 3 perch (10 cm each) were released into the enclosure. After adaptation (1 day and 1 night), roach formed an actively moving school and fed from the bottom. 3 perch were in pondweeds and sometimes, through 5-15 min, attacked the roach perhaps unsuccessfully.

Then three artificially red-marked roach were added to the natural roach. Experimental fish were caught separately by net and marked with the assistance of needle with the bright red thread (1 cm) tied to the base of the dorsal fin. These fish quickly joined an available school of natural roach performing synchronized actions. However, after two hours of observations these red-marked roach were eaten by perch.

Thereafter, an experiment was closed because of ethical demands and an evidence of its results.

Previous experiments with artificial fish models have shown that roach can use their bright red dorsal fins in schooling and other forms of social behaviour. However, this color signal does not evolve into the trait because of the lack of clear sexual dimorphism in roach (selection of males) beeing at the same time under the strong predation pressure.

For more information, please read Roach prefer red-marked school mates

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 ... Read more »