Dorsal fins in freshwater fish are located on the upper most illuminated part of the fish’s body. Therefore, clearly visible colored patterns of dorsal fins can play an important role in fish behaviour.

Color patterns of dorsal fins can be divided into several groups.

Uniform Color Patterns

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 signals to contact with schooling and sexual mates.

Sail Fins

Studies of variable platyfish, Xiphophrus variatus, and other poeciliids point out to female biases for males with larger dorsal fin size and lateral projection area (LPA). Using dummy females varying in dorsal fin size, body size, and dorsal fin to body size ratio, MacLaren & Fontaine (2013) have found that males prefer larger bodied females when fin size and total LPA are constant, but not for larger fins when body size are constant (see data by MacLaren et al., 2004, for sailfin molly, Poecilia latipinna) . Unlike the permissive preferences of females, males are able to discriminate between female body size (as an visual indicator of fecundity) and fin size.

Dorsal Fins with Small Spots

This group of colored patterns includes relatively small spots located on the inter-ray tissue of dorsal fins.

Color patterns of this type occur, for example, in graylings such as the European grayling, Thymallus thymallus, and other species with their huge dorsal fins. Dorsal fins of these fish are covered with the chromatic inter-ray spots that form irregular or regular (row) patterns (e.g., Knizhin et al., 2006). In general, rainbow colored dorsal fins of graylings are an element of cryptic rheophilous, or stream coloration. At the same time dorsal fins are used both in agonistic (fin lateral displaying) and spawning (fin clasping) behaviour (e.g., Kratt & Smith, 1980; Darchambeau & Ponchin, 1997). Dorsal fins in graylings have commonly colored edge rims, adipose fins may be red similar to other salmonids like brown trout, Salmo trutta, both in parr and adults.

In males of cichlid fish, Astatotilapia burtoni, dorsal fins are covered with numerous spots (yellow, orange or reddish) located on each inter-ray space and formed some longitudinal regular rows, togerther with the edge row-like rims (Henning & Meyer, 2012).

Dorsal Fins with Large Spots

In addition to spotted dorsal fins, anal fins in males of A. burtoni are covered with 4-8 large spots that occupy several inter-ray spaces and mimic eggs ((e.g., Henning & Meyer, 2012; Theis et al., 2012). As shown in aggression trials (Theis et al., 2012), males of A. burtoni with fewer egg-spots receive more attacks of rivals, suggesting that egg-spots are an important signal (that refers to 1500 species of cichlids with egg-spots in males).

Cichlid fish called kribensis, Pelvicachromis pulcher, have some large egg-sports in the rear part of the dorsal fin and the upper part of the caudal fin.

Single Patch

In freshwater fish, large spots or patches that occupy all or an appreciable part of dorsal fins are very visible and play an important role mainly in schooling, alarm and aggressive behaviour (Guthrie, 1986). For example, dorsal fin yellow in the base, black and white on the tip is actively used in characin fish, Pristella riddlei (Keenleyside, 1955), in schooling and alam behaviour. According to manipulative experiments conducted by Petfield (1983), large black and white dorsal fin spot in perch, P. fluviatils, is an important stimulus for conspecific aggregation. Many of single patch patterns of dorsal fins in freshwater fish are reviewed by Guthrie (1986), but most of them are waiting for detailed description, systematization and validation of the signaling role.

Edge Rims

Colored rims are developed in many freshwater fish commonly on the dorsal, caudal and anal fins. For example, white, yellow and blue rims occur in breeding black males of gobiid fish. Brightly colored blue and red fin rims are developed in breeding males in many species of darters.

In gobies, colored rims display the readiness of males for spawning and called gamosematic signals. These temporary edge rims disappear when males begin to care for the nests with laying eggs.

Figure 1. Colored rims in redspot darter, Etheostoma artesiae (powered by Joseph Tomelleri)

Club Like Fins

Fleshy proliferations are well developed on the fins of breeding males in egg-clustering darters of the subgenera Boleosoma and Catonotus of the genus Etheostoma. Shape and pigmentation, commonly white and yellow, of these knobs suggest that they function as egg-mimics (e.g., Bart & Page, 1991; Page et al., 2000). Size of egg-mimics can achieve 1,8 – 2.2 mm (Page et al., 2000). Breeding males of darters typically develop fleshy proliferations on spines, rays of anal, pelvic and pectoral fins. Only in Boleosoma and Catonotus, which uniquely share the egg-clustering behavior, breeding males have fleshy masses on the tips of the dorsal fin elements.

Basic References

Bart H.L.Jr., Page  L.M. 1991. Morphology and adaptive significance of fin knobs in egg-clustering darters. Copeia 1991, 80-86

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

Guthrie D.M. 1986. Role of vision in fish behaviour. In: The behavior of teleost fish (Pitcher T.J. Editor). Johns Hopkins University Press, Baltimore, Maryland, 75-113

Henning F., Meyer A. 2012. Eggspot number and sexual selection in the cichlid fish Astatotilapia burtoni. PLoS ONE 7(8): e43695

Keenleyside M.H.A. 1955. Some aspects of the schooling behaviour of fish. Behaviour 8, 183-249

Knizhin I.B., Bogdanov B.E., Vasil’eva E.A. 2006. Biological and morphological characteristic of the Arctic grayling Thymallus arcticus (Thymallidae) from alpine lakes of the basin of the upper reaches of the Angara river. Journal of Ichthyology 46, 709-721

Kratt L.F., Smith R.J.F. 1980. An analysis of the spawning behaviour of the Arctic grayling Thymallus arcticus (Pallas) with observations on mating success. Journal of Fish Biology 17, 661-666

MacLaren R.D., Rowland J.W., Morgan N. 2004. Female preferences for sailfin and body size in the sailfin molly, Poecilia latipinna. Ethology 110, 363-379

MacLaren R.D., Fontaine A. 2013. Incongruence between the sexes in preferences for body and dorsal fin size in Xiphophorus variatus. Behavioural Processes 92, 99-106

Page L.M., Knouft J.H., Schaefer S.A. 2000. Variation in egg-mimic size in the guardian darter, Etheostoma oophylax (Percidae). Copeia 2000, 782-785

Patfield I.M. 1983. Visual aspects of intraspecific recognition in the perch (Perca fluviatilis L.). University of Manchester

Theis A., Salzburger W., Egger B. 2012. The function of anal fin egg-spots in the cichlid fish Astatotilapia burtoni. PLoS ONE 7(1): e29878