Until relatively recently, scientists believed the elephantnose fish to be completely blind. An interdisciplinary collaboration comprising scientists from 16 institutes across the UK, Germany and Russia have now discovered that the unusual configuration of photoreceptors in the retina of an elephantnose fish makes it colour-blind and insensitive to spatial clarity, which aids navigation through dark and turbid waters (Science 336, 1700–1703; 2012).

The retina of a vertebrate contains two types of photoreceptor cells — rods and cones — for operation at different light levels. Cone receptors are less light-sensitive than rods and therefore provide higher acuity, which makes them useful for daytime or bright-light conditions. Rod receptors, on the other hand, lack acuity but are capable of sensing even a few photons, which allows them to dominate at night or in low-light conditions such as the deep sea. Andreas Reichenbach and co-workers have now shown that the grouped retinas of the elephantnose fish suggest a third specialization of the retina, in which both types of photoreceptor are active simultaneously.

Credit: © 2012 AAAS

The retina of the elephantnose fish is lined with cup-shaped photonic crystals made from four thin layers of guanine-crystal lamellae. These photonic crystals form a parabolic mirror, with grouped cone receptors embedded at the base and rods located directly underneath. Simulated results from the finite-difference time-domain implementation of Maxwell's equations show that the reflecting walls of the cup cause the cones to receive around five times the intensity of the incident light, while only a small fraction of light reaches the rods. This is at the expense of reduced spatial resolution; because each cone sees the same part of the image, a group of cones essentially act as one. The reflection is wavelength-dependent and most sensitive in the red, which is fittingly the colour of the fish's native waters in West and Central Africa.

The structure of the grouped cones means that only very large objects can be detected; if a large predator is surrounded by bubbles and floating debris, the elephantnose fish will see only the predator. The researchers confirmed this inherent spatial low-pass filtering by conducting behavioural tests. When pitted against a goldfish, which are known for their high-acuity vision, the elephantnose fish outperformed its competitor by displaying more reliable flight reactions when subject to digital simulations of a predator surrounded by small particles.

The researchers anticipate that future sensing technology could make use of this configuration to detect objects in noisy environments.