Animal vision and colour change for camouflage

Abstract

Camouflage is a well-studied form of antipredator defence. A key issue is how animals ensure camouflage effectiveness when the visual environments many camouflage strategies rely on vary. Phenotypic plasticity allows animals to adjust coloration to best match such environmental variation. It is assumed that vision is used in identifying this variation and guides changes in colour. However, questions still exist regarding the opportunities and limitations afforded from vision-guided changes for camouflage. Carcinus maenas, the green shore crab, already a widely used species to investigate a variety of questions regarding camouflage, was used to test the assumption that vision is directly responsible for guiding (and limiting) colour change for camouflage. In the first chapter, tests of spectral sensitivity and colour discrimination were performed, which were then compared to colour change responses. Following this, crabs’ spatial resolution was tested and compared to pattern change responses on uniform and patterned backgrounds. Finally, crabs’ brightness change responses to varying illumination and substrate brightness conditions were recorded to examine directional light’s role in substrate perception for plasticity. My results indicate that C. maenas colour change for camouflage is determined and limited by their vision. First, spectral and colour discrimination results indicate C. maenas cannot perceive differences in colour. This aligns with colour change results, with crabs only showing significant achromatic change despite apparently possessing the chromatophores needed for chromatic change. Following this, crab’s changed patterning by increasing pattern contrast proportionate to background pattern size, without changing pattern shape or size. This change in patterning is indicative of a shift from uniform background matching to disruptive markings. Finally, C. maenas colour change corresponds to the relative reflectance of substrates, accounting for illumination. This indicates some level of assessment of directional light, likely dependent on the differential stimulation of an eye perceiving light from multiple directions at once. These results indicate that while species’ vision can limit colour change for camouflage, effective improvements in camouflage are still capable within these limits.