Abstract

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 longwavelengths (orange/red).We investigatedhowchanges 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.