By Dr Susan Theiss
From freshwater rivers to bustling coral reefs and down in the darkness of the ocean’s depths, sharks are found in nearly every aquatic environment on earth. With such vast differences present between these habitats, one might wonder how sharks are adapted to perceive their particular environment in order to survive in it. For most people, vision is considered to be the most important sensory system. Visual perception plays a major role in several day to day activities, such as choosing what to eat, getting safely from one point to another, deciding who to ask out on a date and picking your best friend out from the crowd. While we might not give these events much thought, sharks depend on vision, and a variety of other sensory systems, for similar basic survival strategies, such as prey detection, predator avoidance, mating and communication. Some of these senses are similar to those found in humans, such as taste, touch, vision, olfaction (smell) and audition (hearing), but sharks also have two extra sensory systems, electroreception and the mechanosensory lateral line, which are exclusive to aquatic animals. Electroreception is the ability to detect weak electric fields, which all marine organisms emit, and the lateral line system is used to detect water movements. Some species of shark have also been known to geonavigate over long distances, possibly through the detection of the earth’s magnetic fields, but the mechanism for this is poorly understood. All of these sensory systems are beneficial for different aspects of a shark’s biology, and the reliance on a particular sensory system, or systems, may vary between species depending on their specific ecological needs.
Originally, sharks were considered to have quite poor vision but in the last 50 years, several studies have shown that shark vision is actually much more advanced than traditionally thought. The basic structure of all vertebrate eyes is similar; however, vertebrate animals span a wide diversity of physical environments. Therefore, the visual system is often modified to best suit a particular habitat and hence several adaptations have evolved to deal with life underwater. For example, the cornea (the transparent portion at the front of the eye) is the major refractive component of the terrestrial vertebrate eye. However, the refractive index of the cornea is nearly identical to that of seawater, which renders the cornea essentially useless underwater. In order to compensate for this and focus light on to the retina, aquatic vertebrates, including sharks, have evolved a spherical, or near-spherical, lens with an elevated refractive index compared to the lenses of terrestrial vertebrates.
To fully understand some of the visual specialisations found in sharks, a brief introduction to the visual process is needed. Light enters the eye through the cornea and pupil and passes through all optical components before reaching the retina, which lines the back of the eye. Photons of light are absorbed by photoreceptor cells in the retina and converted into a biochemical signal that passes through the rest of the retina to the ganglion cells. The axons of the ganglion cells form the optic nerve, which transmits the visual signals to the brain where an image is perceived. An important structure located behind the retina in sharks is the tapetum lucidum, which is also found in many other, mainly nocturnal, vertebrate animals. The tapetum functions like a mirror to reflect any light that has not been absorbed back onto the photoreceptor cells. This helps to maximise light capture and increase visual sensitivity in low light conditions. A larger eye will also allow for enhanced visual sensitivity as more wavelengths of light can reach the retina. This would be beneficial to species relying on vision as a primary sensory system, or species living in habitats with minimal light penetration. In fact, using relative eye size as an indicator for the importance of vision in sharks has just been investigated, and the sharks with the largest eyes are pelagic species, such as the blue shark, and benthic or bentho-pelagic species found at deeper depths.
As the light detecting cells in the retina, the photoreceptors of sharks have been of great interest, particularly in recent years. Like other vertebrates, sharks have two types of photoreceptors, rods and cones, which differ in morphology and physiology. Rod cells are used during dim light conditions (e.g. at night) and provide good contrast vision. Cone cells, on the other hand, operate under bright light conditions (i.e. during the day) and are responsible for colour vision. A major misconception, however, is that the mere presence of cones means that an animal has colour vision, and this is, in fact, incorrect. Both rods and cones contain visual pigments, which are the compounds responsible for the absorption of light. There are several different types of visual pigments and each one is tuned to absorb only a small portion of the wavelengths of light in the visible spectrum. Therefore, for an animal to have colour vision, it must have at least two types of cones, each with a visual pigment absorbing in different regions of the light spectrum. This facilitates a comparison of the wavelengths of light in the two different areas of the spectrum, and hence allows for discrimination between colours. To put this in perspective, humans have three different cone types, a blue, green and red cone, whereas most other mammals have only two cone types. This is the reason you might have heard that dogs are ‘colour blind’ or only see in black in white. Dogs most certainly can see in colour, but they are not able to discriminate as many colours as humans. Similarly, human red-green colour blindness is the lack of the green cone type, which also prevents those affected from distinguishing certain colours. While multiple cones types in fish, reptiles, birds and mammals are common, rod cells are normally present as only a single type.
Determining how many cone types are present in sharks fills an important gap in the evolution of colour vision in vertebrates. The presence of cones in sharks was first documented over 100 years ago, and every species examined since, save a handful of deep-sea sharks, have been found to have cones. However, essentially nothing is known about the cone visual pigments in sharks. In contrast, rod visual pigments are fairly well understood and appear matched well to the wavelengths of light available in a particular environment. For example, longer (‘red’) wavelengths of light are filtered out quickly in the ocean with depth, and thus rod visual pigments in deep-sea sharks are shifted to absorb the shorter (‘blue’) wavelengths of light that dominate in deeper waters. One of the problems with studying shark cone visual pigments is that sharks have fewer cones in relation to rods and, therefore, cones are much harder to find within the retina. The proportion of rods and cones is related to habitat and behaviour, such that diurnal species, or those species living in brightly lit environments, have a higher proportion of cones than nocturnal species and species occupying dimly lit habitats. The deep-sea piked dogfish, for example, has 50 times as many rods than cones whereas great white sharks have only four times as many rods.
Behavioural experiments from the 1960’s and 1970’s sought to prove the existence of colour vision in sharks by training them to associate with different coloured targets. The experiments proved inconclusive, however, as it could not be determined whether the sharks were responding to a change in colour brightness or hue. In recent years, a new technique has been used to assess visual pigments in a variety of marine creatures, including sharks, and this has proven to be a very successful and accurate method for analysing visual pigment absorbance. Studies are still underway on sharks, but it’s been shown very recently that some species of the closely related rays, the giant and eastern shovelnose rays and the blue-spotted maskray, have three different cone types. This means they have colour vision comparable to humans, which is a very exciting finding! While the jury is still out on whether or not sharks see in colour, there can be no doubt that the visual systems of these fascinating creatures are adapted remarkably well to their particular lifestyles and habitat preferences. There is still a wealth of information waiting to be uncovered on the visual system of sharks, and in a time where understanding the biology and ecology of sharks is critical to their survival, sensory neurobiological studies offers another method for predicting such bio-ecological factors as predatory strategy, habitat preference and behaviours.