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 ... Read more »

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). It is important that blue stripes are located in these fish on the upper most illuminated part of their body while the red area is located on the under shadowed part of the body.

Note also that coloration of neon tetra does not differ between sexes or among growth stages.

In order to understand principles of the foregoing coloration design, we must address to the spectral sensitivity of eyes in human and fish. Because, generally, the curves of spectral sensitivity have the bell like form, different equipower monochromatic colors are not equally bright.

For example, for eyes of human (as an agreed standard observer), the maximum of spectral sensitivity is near 555 nm. It means that for our eyes blue-green colors (with wavelengths, say, 510-520 nm according to the reflection spectrum of blue stripes in tetra: Lythgoe & Shand, 1983) are brighter than red colors (620-640 nm to the red area). On the contrary, the curve of spectral sensitivity in tetra is shifted, similarly to other freshwater fish, to the red part of the spectrum, with the longwave maximum at 607 nm (Lythgoe & Shand, 1983). Thus, for eyes of tetra red colors must look brighter than blue-green colors as opposed to our visual perception.

In sum, blue-green stripes that are bright for us must be dim for neon tetra. Hereof location of blue colors on the upper most illuminated part of the fish’s body (above the line of body convexity) and red colors on the under shadowed part of the body are in conformity with the theory of color countershading in fresh waters.

Importantly, blue-green and near red (near to orange) colors in the reflection spectra of tetra’s body (Lythgoe & Shand, 1983) form an exactly matched pair of complementary colors. Developing these colors, tetras are able to find the trade-off between conspicuousness (maximising it) for conspecifics (Endler, 1992) and crypticity (minimising conspicuousness) for potential predators.

Main natural predators for neon tetras are butterfly peacock, Cichla ocellaris, red-finned pike cichlid, Crenicichla johanna, and leaf fish, Monocirrhus polyacanthus, which eat tetras without any signs of avoidance (Ikeda & Kohshima, 2009). Nothing is known about spectral sensitivity of these predatory fish. However, in waters saturated with the litter organics, where C. ocellaris, C. johanna and M. polyacanthus live, the curves of their spectral sensitivity must have red displacement, by definition. On the other hand, C. ocellaris and C. johanna are ecologically close to largemouth bass, Micropterus salmoides, with the maximum of spectral sensitivity at 673 nm (Kawamura & Kishimoto, 2002). Comparison of these cichlids with cichlids from optically clean African lakes is incorrect. In Florida’s eutrophic optically turbid channels, introduced C. ocellaris and native M. salmoides are forced to co-exist, hunt practically the same prey (Fill et al., 2004), and there are no doubts that their eyes have similar spectral sensitivities.

For information, eyes of piranchas, Serrasalmus sp., and other predatory characins have well developed long wavelength sensitive visual pigments in the range of 600-630 nm (Kusmic & Gualtieri, 2000).

Similar patterns, with the upper location of blue colors and the under location of red colors, occur in many other freshwater fish. For example, breeding males of threespined stickleback, Gasterosteus aculeatus, have blue eyes and red breast. The maximum of spectral sensitivity in stickleback is near 605 nm (Rowe et al., 2004). The same value in pike, Esox lucius, their natural predators, is 630 nm (Protasov, 1968). In the similar way, bluegill, Lepomis macrochirus, have blue gill covers and orange belly. The maximum of spectral sensitivity in sunfish is near 612 nm (Tamura & Niwa, 1967). The same value in largemouth bass, their natural predators, is 673 nm, as metioned above. So blue colors are brighter, that is signaling, for their owners and much more darker, that is cryptic, for co-existent predatory fish.

Pecos ... Read more »