Sensitivity differences in fish offer near-infrared vision as an adaptable evolutionary trait.

Near-infrared (NIR) light constitutes an integrated part of solar radiation. The principal ability to sense NIR under laboratory conditions has previously been demonstrated in fish. The availability of NIR in aquatic habitats, and thus its potential use as a cue for distinct behaviors such as orientation and detection of prey, however, depends on physical and environmental parameters. In clear water, blue and green light represents the dominating part of the illumination. In turbid waters, in contrast, the relative content of red and NIR radiation is enhanced, due to increased scattering and absorption of short and middle range wavelengths by suspended particles and dissolved colored materials. We have studied NIR detection thresholds using a phototactic swimming assay in five fish species, which are exposed to different NIR conditions in their natural habitats. Nile and Mozambique tilapia, which inhabit waters with increased turbidity, displayed the highest spectral sensitivity, with thresholds at wavelengths above 930 nm. Zebrafish, guppy and green swordtail, which prefer clearer waters, revealed significantly lower thresholds of spectral sensitivity with 825-845 nm for green swordtail and 845-910 nm for zebrafish and guppy. The present study revealed a clear correlation between NIR sensation thresholds and availability of NIR in the natural habitats, suggesting that NIR vision, as an integral part of the whole spectrum of visual abilities, can serve as an evolutionarily adaptable trait in fish

Mean angle (°) of the preferred fish head alignment with regard to direction of the NIR-light source (± mean angular deviation).

<p>Direction to NIR-light source = 0°.</p

Allocation preference of fish in the spectral range between 825 and 890 nm (Spectrum S1).

<p>(A). Sector diagrams show mean allocation time [%] with regard to mean head position of fish in 24 sectors of the swimming vessel, representing 15° each. Each left and right sector diagram shows mean data for 15 fish of each species. (B). Bar graphs show mean allocation time of individuals of each fish species [s] ± standard deviation (n = 30) for NIR (black) and control halves (white) of the swimming vessel, respectively.</p

Spectral characteristics of applied light.

<p>Spectral characteristics of applied light.</p

Mean swimming time (%) of fish during experimental testing period.

<p>Mean swimming time (%) of fish during experimental testing period.</p

Comparison of allocation preference of tested species in different NIR spectra.

<p>(A–E) Lines represent mean allocation time [s] ± standard deviation in NIR (line with circles) and control (line with squares) halves of the swimming vessel for each tested spectrum and species.</p

Allocation preference of fish in the spectral range between 910 and 1020 nm (Spectrum S3).

<p>(A). Sector diagrams show mean allocation time [%] with regard to mean head position of fish in 24 sectors of the swimming vessel, representing 15° each. Each left and right sector diagram shows mean data for 15 fish of each species. (B). Bar graphs show mean allocation time of individuals of each fish species [s] ± standard deviation (n = 30) for NIR (black) and control halves (white) of the swimming vessel, respectively.</p

Length of the mean directional vector <i>R</i>.

<p>Length of the mean directional vector <i>R</i>.</p

Spectra used.

<p>The range of each spectrum was measured by spectroradiometer ILT950 at a distance of 10 cm between swimming vessel and light source. S1: 825–890 nm; S2: 845–950 nm; S3: 910–1020 nm; S4: 930–1020 nm. For wavelengths above 1020 nm only noise of the detecting device was observed.</p

Allocation preference of fish in the spectral range between 845 and 950 nm (Spectrum S2).

<p>(A). Sector diagrams show mean allocation time [%] with regard to mean head position of fish in 24 sectors of the swimming vessel, representing 15° each. Each left and right sector diagram shows mean data for 15 fish of each species. (B). Bar graphs show mean allocation time of individuals of each fish species [s] ± standard deviation (n = 30) for NIR (black) and control halves (white) of the swimming vessel, respectively.</p