Changes in the visual environment affect colour signal brightness and shoaling behaviour in a freshwater fish

Aquatic organisms are exposed to highly variable light environments, which can affect the efficacy of colour patterns that are used for communication or camouflage. Specifically, dissolved organic matter that is common in turbid freshwater habitats tends to absorb short wavelength light causing a shift towards environments that are rich in long wavelengths (orange/red). We investigated how changes in the intensity and wavelength of light affect colour pattern expression and shoaling behaviour in a colourful freshwater fish, the western rainbowfish, Melanotaenia australis. We used light filters to simulate environments rich in organic matter (yellow filters: long wavelength dominated, reduced luminance), habitats with full-spectrum lighting but with reduced luminance (neutral-density filters) and a control in which no changes in the light environment occurred (no filters). We measured changes in the area and brightness of colour patterns using digital photography and spectrometry and we evaluated the effect of lighting on fish social (shoaling) behaviour. Rainbowfish in the dissolved organic matter treatment showed an increase in the area and brightness of their colour patterns and individuals shoaled further apart than those in the control group. The increased brightness of red colours in environments rich in organic matter could act to enhance colour pattern conspicuousness, allowing individuals to maintain communication in altered visual environments. However, an understanding of the species’ visual system is required to determine levels of contrast of the colour patterns with respect to variable background environments.