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About blue-red coloration of neon tetra in comparison with similar blue-red patterns

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 pupfish, Cyprinodon pecosensis, with the upper location of blue color and the under location of orange color is another example.

Generally, with decreasing of the brightness of some colors the distance of recognition of these colors under water also decreases (Emmerson & Ross, 1986). This rule is valid for red colors located in the shadowed part of the fish’s body. According to Emmerson & Ross (1986), in green water blue colors are recognized at shorter distances than yellow colors, especially fluorescent.

Basic References

Emmerson P.G., Ross H.E. 1986. The effect of brightness on colour recognition under water. Ergonomics 29, 1647-1658

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

Hill J.E., Nico L.G., Cichra C.E., Gilbert C.R. 2004. Prey vulnerability to peacock cichlids and largemouth bass based on predator gape and prey body depth. Proceedings of the Annual Conference of the Southeastern Association of Fish and Wildlife Agencies 58, 47-56

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

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

Kusmic C., Gualtieri P. 2000. Morphology and spectral sensitivities of retinal and extraretinal photoreceptors in freshwater teleosts. Micron 31, 183-200

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

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

Rowe M.P., Baube C.L., Loew E.R., Phillips J.B. 2004. Optimal mechanisms fo finding and selecting mates: how threespine stickleback (Gaserosteus aculeatus) should incode male throat colors. Journal of Comparative Physiology A190, 241-256

Tamura T., Niwa H. 1967. Spectral sensitivity and color vision of fish as indicated by S-potential. Comparative Biochemistry & Physiology 22, 745-754

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