THE cartilaginous fishes (sharks and rays) have appeared in the fossil record for upwards of 450 million years. Indeed, the ancestors of modern sharks and rays have been around for such a long time scientists believe they must have had to compete against some of the fiercest of marine dinosaurs like the ichthyosaurs and pleisiosaurs.
The reasons why sharks and rays were able to prevail against such worthy adversaries continue to be debated, but scientists agree that a couple of factors likely tipped the evolutionary balance in the favour of sharks and rays. One of these was their breeding patterns, particularly the birth of live young, which was less affected by environmental changes than the challenges experienced by the egg laying reptiles. Secondly, sharks and rays evolved to develop several new sensory systems including electroreceptors and an acute sense of smell, which may have given them the upper hand with key activities such as prey capture and navigation.
The highly sensitive sense of smell possessed by sharks and rays was one of the first aspects of their biology to be studied in some depth by early scientists. Because of this, up until recent times it was thought that these fish were nothing more than primitive ‘swimming noses’, and relatively little was known about other senses, including vision.
The information that was available until recently suggested that vision, which is highly developed in most bony fishes, was relatively poorly developed in sharks and rays. But in the last 10 to 15 years more scientists began to examine vision in sharks in detail, and to their surprise, they found that many species of sharks have eyesight that is at least the equal to that of bony fish from the same environments, including the potential for colour vision.
Rods and cones are the two types of photoreceptors in the eyes of both bony and cartilaginous fishes that allow them to view their environment. Rods are receptors that operate in low light conditions, whereas cones function in bright light only and are responsible for generating colour vision. Traditionally, most scientists thought that the eyes of sharks were dominated by rod receptors only, useful only at night and under low light conditions, generating vision in shades of grey with relatively poor acuity.
But now scientists have found evidence that the eyes of sharks and rays are similar to those of bony fish in that they possess both rods and cones, with species living in brightly lit environments and which are active during the day possessing a higher proportion of cones than those living in low light habitats and which are more active at night. Only a handful of deep-sea sharks have been examined, but all seem to lack cones entirely, suggesting they are well adapted to the constant darkness that occurs at depth.
It seems the sharks with the most effective eyesight live offshore in clear oceanic waters. Pelagic sharks such as the blue shark Prionace glauca have relatively large eyes which are likely to provide clear colour vision in shallow water, as well as the ability to adapt quickly to low light levels encountered during dives of up to 600 meters depth (which blue sharks perform naturally in as little as 10 minutes).
Other species of pelagic sharks, such as makos, have specialized blood vessels that keep the eyes up to 5°C warmer than the surrounding water, which presumably improves vision performance during deep dives below the thermocline where water temperatures can approach freezing. In contrast, sharks that live in the shallow turbid waters of estuaries have less well adapted eyesight and are thought to rely more heavily on their non-visual senses such as smell and electroreception for prey capture.
Most, if not all, of the sharks and rays examined to date possess the specialized reflective tapetum lucidum layer behind the retina. This layer reflects available light that passes into the eye back onto the photoreceptors, thereby essentially doubling the sensitivity of the eye at low light levels. This feature provides sharks and rays with a visual advantage over their prey during low light periods and at night. The tapetum lucidum (which is easily seen at night by the red or yellow reflection bouncing off the eye when using flash photography), is shared with many predatory bony fishes that also actively hunt at dawn, dusk or during the night. This optical feature is one of the main reasons why these periods of the day are prime feeding times for sharks, rays and predatory fish alike.
Dr Ben Diggles is Fishing World’s Marine Biology Editor.