Summary: Kreysing et al. (2012)

Vertebrate retinae are diverse, but, in general, are specialised either to maximise the amount of light hitting the retina (increased number of rods) or to enhance visual acuity (increased number of cones). Kreysing et al. (2012) noted a different functional type of retinae in elephantnose fish (Gnathonemus petersii) that is optimised for neither purpose. They found that cone photoreceptors group together in "reflecting, photonic crystal-lined cups" and these act as macroreceptors. In contrast, they found that rod photoreceptors occur behind these reflector cups, not in them. Interestingly, this arrangement matches photoreceptor sensitivity and allows for detecting mixed-colour stimuli and the grouping of these photoreceptor cells results in insensitivity to spatial noise. These allow for a more reliable flight reaction, given the fish's habitat (dim and turbid).