Sight is a vital sensory system for most living things that enables them to process and visualise information from their surroundings. What to eat, and where to find it? Who to flee from, and who to mate with? Survival and procreation are some of the day to day necessities that would be impossible, for most, without the sense of sight.
For something that is so basic and vital to all living creatures, it is marvellous how diverse and variable the organ responsible for such a process is. The eye. From the complex human eye, to the multifaceted compound eyes of insects, to simple primitive “eye-like” organs that certain jellyfish possess; they all serve the same function. To allow sight and the ability to receive and process visual information. The fish eye is particularly interesting, and like all animals, is highly varied and has evolved to suit the needs of each particular species, or family, living in a wide array of habitats.
Fish eyes are rather similar to other terrestrial animals such as mammals and birds, but have evolved differently for function under water. Unlike their terrestrial counterparts, fish possess a more spherical lens. Some species, like those in the genus Paracheilinus, have eyes that incorporate a double lens. The photo above shows an eye belonging to Paracheilinus attenuatus. The left portion allows for close up focusing on prey, in which it picks up amongst floating zooplankton. The right portion, or the main portion, allows for a wide-angle view of its surroundings.
This allow the fish to multi task, picking up small pieces of food from a plankton soup, yet maintaining caution and keeping an eye out for danger at the same time. Throughout this article we will explore a few other adaptations that fishes use to heighten their visual strength in the corresponding habitats they inhabit.
Like mammals, fish eyes possess rod cells and cone cells. Rod cells are photoreceptors in the retina that function in less intense light, and are almost entirely responsible for night vision. To put it simply, the pigment rhodopsin, or visual purple, is present within rod cells and is highly sensitive to light. Rhodopsin allows for vision in very low light conditions. Being very sensitive, rhodopsin bleaches very easily upon contact with intense light, and this causes temporary night blindness. Imagine yourself squinting in the dark when suddenly a car drives by and shines its headlights at you. You go blind temporarily right? This is the rhodopsin bleaching, and it takes awhile for you to regain back your vision.
The amount of rod cells differ between species to species. Deep-water fish that live in dimmer environments have more rod cells as compared to shallow water species. During Dr. Luiz Rocha’s key note address in MACNA 2014, he mentioned that Acanthurus reversus, a sister species of A. olivaceus, has evolved to live in murkier waters of the Marquesan islands. This environmental pressure started a series of evolutionary processes that allowed A. reversus to speciate from A. olivaceus, and as a result of the murky water conditions, developed more rod cells for better vision.
Cone cells are for colour vision, and are more abundant in shallow water reef fish. They function well in bright light, and are able to detect finer detail and faster changes in images. Reef fish that live in shallow water coral gardens have eyes with more cone cells, and are better equipped at seeing and interpreting the changes and cues in their habitat than deepwater fish, and vice versa.
Not only can fish see colour and in the dark, many are able to see in the ultraviolet (UV) spectrum as well. Their ability to see UV is mediated by four visual pigments that absorb various wavelengths of light. Each pigment is constructed from a chromophore, and a protein known as opsin. A wide range of fish species possess UV vision, and this is speculated to help in foraging, communication and mate selection.
In a study conducted by the University of Queensland, they found out that certain reef fish such as P. amboinensis possess UV vision which allows them to see certain things that are not discernible in regular lighting. The facial patterns of P. amboinensis for example, is only obvious under UV lighting, and this allows conspecifics to identify each other for procreation or for territorial disputes.
In a coral reef with a dizzying array of colour, what you interpret as a chaotic rainbow spectacle is an entirely different world to its inhabitants. For example, brightly coloured reef fish may possess “secret colour codes” that are only visible with UV vision. One must also consider the wavelength absorption under water. The peppermint angelfish may look like a striking barbershop pole to you, but deep under water, red is the first wavelength to be absorbed and disappears, appearing black instead. This helps it to blend in with its surroundings and go unnoticed.
Another difference between fish eyes and eyes belonging to terrestrial animals are the environment in which they occur in. Fishes live in an aquatic environment, which shares roughly the same refractive index as aqueous humour (the fluid between the cornea and the lens). Therefore their spherical and denser lenses have to do majority of the refraction and this creates a refractive index gradient, allowing for sharper and clear images free of spherical aberrations.
Fish have evolved to dominate a wide variety of habitats. From nearly land dwelling mudskippers in the mangrove flats, to murky turbid seagrass beds, pristine coral reefs down to the mesophotic and abyssal regions of the ocean. Some live in caves where light is so scarce that their eyes have completely atrophied, forcing them to evolve and rely on other senses such as smell, or electroreception. With a plethora of habitats comes the corresponding ocular evolution that accompany it. How have certain reef fishes evolve to suit the conditions they live in?
The tapetum lucidum is a membranous layer consisting of reflective tissue lying just behind the retina. It is made of iridescent leucophores which reflects visible light back into the retina, increasing the amount of light available to photoreceptors. The presence of a tapetum lucidum contributes highly to the superior night vision seen in many nocturnal animals and deep sea fish. In terrestrial animals, the tapetum lucidum is what gives an animal its characteristic “eye-shine”, most commonly seen when light is shone directly into the eyes of said animal, such as a “deer in headlights”.
Eye shine is most evident when a light is shone into the eye at low light conditions. Like a cat or a dog, the eye shine that arises from tapetum lucidum reflection is iridescent. This means that under certain angles, the coloration changes or disappears altogether. Many species of deeper water fish possess a tapetum lucidum as well. Deep water fish living in very low light conditions, or cryptic fish that skulk in the shade are some examples of species that may possess this. In the photo above, the green reflection seen in the eye of a Plectranthias pelicieri is caused by its tapetum lucidum reflecting back the camera flash.
When the light is shone at a specific angle, the tapetum lucidum is not able to reflect back the light at the camera lens, and thus the same eye belonging to the Plectranthias two photos above appears black. The tapetum lucidum is just one of various adaptations for living in a dimmer more dreary environment. Some fish do not possess a tapetum lucidum, but instead, possess very large eyes to compensate for the lack of light. In the case of Pristigenys, a tapetum lucidum and a huge eye allows for two times the efficiency in adaptation to dark waters.
Squirrel fish for example, also have huge eyes with large pupils. The larger pupils allow for more entry of light, and the larger eye possesses a heavier amount of rods for heightened vision in the dark. Many deepwater and deep sea abyssal fish have large eyes to pick up even the most minute traces of light. Some are able to discern varying shades of grey, and are able to spot prey floating in the water above just by the silhouette.
Some fish go the extra mile in terms of adaptation to dark water, and possess light emitting organs. Organs called photophores are complex structures found in some species of deepwater animals. They can be simple, or highly complex organs that possess shutters, reflectors, colour filters and a whole assortment of other related structures.
Photophores emit light from and for a multitude of reasons. The bioluminescence can be produced by digestion of certain prey, or they can be produced independently by cells themselves called photocytes. Most commonly though, it is through symbiosis of light-emitting bacteria that live within the photophores. Fish use bioluminescence for various reasons. Some use it to attract prey in the dark waters of the deep, while some use it to confuse predators. Some even use it to communicate with each other.
Photoblepharon palpebratus is an example of a reef fish that uses bioluminescence. In the photo above, the light producing photophore is seen just below the eye, and it glows in the dark. The fish is able to control when and where it wants to show the light, by means of an accessory sheath. Remember when we mentioned that photophores can be complex, possessing multiple secondary structures? In this case, a sheath, or shutter is present and can be retracted to shield the photophore when not in use, as seen in the photo directly above.
In the dark, as in directly above, the sheath is unfurled and the photophores glow. This helps to draw prey toward the fish and thus, enabling them to catch their prey easily in low light conditions. The controllable sheath allows the flashlight fish to glow whenever it wants, and can “blink” in the dark as well, suggesting this could be a kind of communication between its peers. Nonetheless, a wonderful adaptation to the dark.
Accommodation is the process of focusing on an object as it moves further or nearer away, and is another adaptation that is unique for aquatic animals that are not seen in their terrestrial counterparts. Unlike birds and mammals that deform the shape of their lens for accommodation, fish do not. Fishes instead move the entire lens further or closer away from the retina by means of a special muscle, called the retractor lentis. For close up or near vision, the muscle is relaxed.
It is noteworthy that only boney fish relax this muscle for near vision. In cartilaginous species such as sharks and rays, the muscle is protractor lentis, and is relaxed for far vision instead. Therefore boney fishes accommodate for distance vision by moving the lens further from the retina, while cartilaginous fish do so by moving the lens closer to the retina.
A fish’s eye is indeed a fascinating organ that has adapted and evolve to suit the need of each particular species’ habitat. To think that even amongst fish, the ability of the eye to diversity in so many ways is pretty amazing. Truly, a circus freak show with glowing, reflecting bulging eyes of all shapes and forms.
Which kind do you like best?