Elasmobranchs have highly developed sensory systems that help them find food – a sense of smell that can detect prey at long distances, and the ability to read the minute electrical fields emitted by fish. But what about their vision? Do sharks see in color? Two recent studies use biological and behavioral tests to try to understand how sharks and rays perceive their world.
The retina of the eye of most vertebrates contains specialized cells, called photoreceptors, that detect light. The pattern of light falling on the retina is converted into a neuronal signal that is sent to the brain, and the brain then interprets the signal, changing it back into the “picture” seen by the eye. Light exists along a spectrum of wavelengths, and in the visible range these are perceived by the brain as different colors. The blue end of the color spectrum is made up of light of shorter wavelengths, while the red end of the spectrum is composed of light of longer wavelengths.
Most vertebrates have two main types of photoreceptors, rods and cones. Rods are highly sensitive cells that can detect very small amounts of light, but they do not discriminate color. Cones are activated by larger amounts of light, but they allow discrimination of different wavelengths of light, i.e. colors. True trichromatic color vision comes from having three different cone photoreceptors detecting wavelengths in the red, green and blue ranges. Specific colors are interpreted by the varying degrees of activation of these three photoreceptors.
Experiments by Hart et al tested the isolated photoreceptors taken from 17 different species of shark, measuring the wavelength of light each is able to detect. They used microspectrophotometry, which passes a range of wavelengths of light across a section of retinal tissue, and measures the wavelength(s) absorbed by the photoreceptors in that tissue. Each shark species was found to have only a single rod photoreceptor that captured a single wavelength of light, and no shark had more than a single cone photoreceptor. In fact, in 10 of the 17 shark species they were unable to find any cones at all! This means that that these shark species have little or no ability to discriminate colors. A complete lack of color vision is rare in terrestrial animals, which typically have two or three different cones and at least some degree of color vision. It may be common in the marine environment, however, as whales and dolphins are also thought to be color blind. It has been proposed that in some marine mammals a rudimentary sort of color vision may be achieved by comparing the wavelengths of one rod and one cone photoreceptor, but more data is needed to confirm this hypothesis.
Among the different species of shark, the wavelengths of light detected by their single cone photoreceptors varied across a wide range. The ideal wavelength for a given species is determined to a large degree by the depth at which they live. Longer (redder) wavelengths of light are quickly lost with increasing ocean depth, while shorter blue wavelengths penetrate more deeply. Shallow water species therefore often have photoreceptors detecting redder light, while deeper species have photoreceptors detecting predominantly blue wavelengths. The authors also note that deeper species might benefit from a blue range photoreceptor to help them see bioluminescent deep water animals, who usually emit a light in the blue range.
Additional work addressing this question comes from Van-Eyk et al, who realized that while laboratory techniques can determine a species’ ability for color vision, whether or not sharks are able to see and respond to colors requires behavioral assays. They tested the color vision capabilities of the shovelnose ray (Glaucostegus typus). In contrast to sharks, most ray species are known to have three different types of cones detecting different wavelengths of light. This is not the first time behavioral tests have been used to analyze elasmobranch color vision, but a confounding factor of previous studies was that they did not control for the differences in brightness among the colors tested. A bright yellow versus a dark blue, for example, could cause animals to react to the brighter target without discriminating its color.
To circumvent this problem, Van-Eyk designed a color discrimination test that also controlled for brightness. The rays were kept in a large training tank, and trained to touch a blue colored panel with their nose to obtain a food reward (Mmm, shrimp paste). Once trained, they were tested for their ability to identify that same blue panel against a background of gray panels of varying brightness. They were then presented with multiple targets, all blue but with a range of lighter and darker blue tones. The sharks therefore had to discriminate blue from grey, as well as different shades of blue from the original training blue. In both assays the rays performed significantly better than would be predicted by chance; they selected the correct target between 62% and 100% of the time.
These data provide strong evidence for color vision in the greater shovelnose ray, and likely in the many other ray species that have multiple photoreceptors. So why are most rays able to see color while most sharks likely cannot? Rays typically live a different lifestyle than sharks, inhabiting shallower water illuminated by light of multiple wavelengths. It may be that rays are better able to make use of color vision than sharks that spend much of their time at depth. It’s unlikely rays use their vision in feeding, they are largely substrate feeders who don’t find their prey by sight. The upper levels of the water column suffer from a significant degree of “flicker”, the ripple effect of light near the surface as waves move the water. So one advantage may come from the ability to more easily spot predators, who may use flicker as a sort of camouflage against species without color vision.
The papers are:
Microspectrophotometric evidence for cone monochromancy in sharks. Hart, NS, Theiss, SM, Harahush, BK and Collin, SP (2011) Naturwissenschaften 98:193-201
Behavioural evidence for colour vision in an elasmobranch. Van-Eyk, SM, Siebeck, UE, Champ CM, Marshall J and Hart NS. (2011) Journal of Experimental Biology 214:4186-92.