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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.

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Category: Coloration | Views: 1453 | Added by: nickyurchenko | Date: 2013-05-06

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. For example, the maximum of spectral sensitivity in largemouth bass, Micropterus salmoides, is near 673 nm (Kawamura & Kishimoto, 2002). For eyes of freshwater fish red and orange colors are brighter than all other equipower monochromatic colors, in particular green and blue. This feature occurs in nuptial coloration of fish with location of bright red colors on the under shadowed parts of the body, in conformity with the theory of color countershading in fresh waters.

However, there are as minimum two outstanding cases of nuptial coloration in freshwater fish with location af red or orange colors on the upper most illuminated parts of the fish’s body.

Fig.1illustrates, perhaps, one of the outstanding examples of contrast melanin and carotenoid based color patterns in freshwater fish. Bilateral longitudinal red (orange) and black (dark) stripes are basic elements of nuptial coloration in males of the anadromous Far-Eastern dace, Tribolodon brandtii (Cyprinidae), and related daces. The maximum of spectral sensitivity in T. hokonensis, adapted to the optically turbid freshwater, is shifted to the red part of the spectrum and is near 612 nm (Kawamura & Kishimoto, 2002). So, red stripes are brightest to the eyes of tribolodons, herewith dorsal stripes are located on the upper most illuminated parts of the body.

Figure 1. Anadromous Far-Eastern dace, Tribolodon brandtii (Cyprinidae), with nuptial coloration

Another outstanding case of bright nuptial coloration on the upper most illuminated parts of the body or even throughout the body occurs in anadromous Pacific salmons.

Sockeye salmon, Oncorhynchus nerka, display the most extreme nuptial colors among other species of this genus, with 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 show preferences to abstract (three ribbed) female models of red color over models of other colors in prespawning period and choose exceptionally red models during spawning. The curve of spectral sensitivity in young O. nerka is adapted to fresh water with one of the maximums at 635 nm (Flamarique & Hawryshyn, 1996), eyes of the adult fish entered fresh water must be readapted to the corresponding optical conditions.

The appearance of bright dorsal colors , both visible for conspecifics and predators, in anadromous fish can be explained by the less predation pressure in fresh waters. As mentioned above, red or orange colors are determined by intensive fattening of daces and salmons on the crustacean (krill) carotenoid rich diets  in the sea.

Basic References

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

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

Novales Flamarique I., Hawryshyn C.W. 1996. Retinal development and visual sensitivity of young Pacific sockeye salmon (Oncorhynchus nerka). Journal of Experimental Biology 199, 869-882

Category: Coloration | Views: 1150 | Added by: nickyurchenko | Date: 2013-04-29



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