Fishes, like other vertebrates and many invertebrates, have developed mechanisms to perceive light, which quickly disappears with depth. Let’s see vision in fishes.
VISION IN FISH: THE WORLD FROM THE EYES OF A FISH
Vision consist on the perception of the light in the environment around us. Because of fishes live in the aquatic environment, the light is quickly extinguished. In addition, because they live in very different habitats, the system to perceive the light varies considerably between species. For details of the vision in general, you can read how animals see the world.
LIGHT UNDER THE WATER
Before beginning to speak of vision in fish, it is important to understand the light pattern with the increasing depth.
As mentioned, light rapidly disappears with depth, but not all colours do the same: red light is absorbed in the first 10 meters; orange and yellow, in the 30 m; green in the first 50 m, and blue at 200 m. For this reason, when we dive we see starfish black!
The amount of light in the water column that led oceanographers to distinguish two zones: the zone in which there is light is called photic and where light lacks is known as aphotic (from 1,000 meters). The photic zone can be subdivided into:
- Euphotic zone: is the most superficial layer and where photosynthetic organisms can carry out photosynthesis. Although it can vary, it is usually considered that reaches 200 meters.
- Oligophotic zone: the area that receives enough sunlight to permit the organisms to see, but not enough to carry out photosynthesis (between 200 and 1,000 m).
THE EYES OF FISHES
The organisation of eyes in fishes is similar to mammals, but they have their singularities.
The lenses of bony fishes are spherical, while elasmobranchs have slightly flattened lenses, and have a high refractive power because the cornea is in direct contact with water. In addition, to focus the images do not change the shape of the lens, but move them forward or backward. This mechanism is also carried out by snakes.
Another curiosity of the optical system is that, in many fish, the iris cannot be contracted, so that the pupil cannot close if the light intensity increases. To avoid overexposure, the rods and cones (photoreceptor cells, the first detect colours and second not) change their shape and melanosomes (pigment organelles) are arranged so that make “shadow”. The opposite process happens when light is scarce.
Fish can have up to four different types of cones, one of which detects ultraviolet light. Ultraviolet cones are used to detect plankton, although not all have. Some species only have them when they are larvae and others only during certain stages of adulthood. For example, the rainbow trout (Oncorhynchus mykiss) have ultraviolet cons only when living in the freshwater.
On the other hand, some fishes, such us elasmobranchs and deep fishes, only have rods. Thus, they cannot see colours.
Another notable difference is that in the bony fishes, eyes grow throughout life and, therefore, so does the retina. In addition, the retina has the ability to regenerate in case of getting hurt.
Finally, some nocturnal fish and sharks, among others, present tapetum lucidum behind the retina, the function of which is to return light rays that have escaped from the retina to the retina to improve vision. It is also present in some mammals, such as cats.
CHANGES IN EYES IN MIGRATORY SPECIES
The adaptive capacity of fish is so great that some migratory species change the eye’s anatomy according to the environment. The lampreys, for example, are fishes that migrate from rivers to the sea. They have a different pigment for each environment: in freshwater is porphiropsin (red) and in the sea is rhodopsin (blue).
Eels also change the habitat and also change their eyes. When they are ready to begin the migration to the sea, the diameter of the eye doubles, the lens size increases and the number of cones increases significantly (only 3% of the photoreceptors before the migration), among others changes.
VISION IN DEEP-SEA FISHES
Deep-sea fishes have a set of adaptations to live in the deep ocean. Vision also have its adaptations.
The mesopelagic fishes (living in the oligophotic zone) are characterised by big eyes, with wide pupils and big lenses. Some species, such as the telescopefish (Gigantura) also have tubular eyes.
The barreleye (Macropinna Microstoma) also presents tubular eyes, which are usually directed upwards to identify the silhouettes of fishes. Unlike other fish with such eyes, it can rotate them.
Batipelagic fishes (living below 1,000 meters) usually have either small eyes or degenerate. In this case, the eye lenses are very large compared to the rest of the eye, which does not allow them to create clear images and, moreover, they can only detect objects close to them.
ADAPTATION OF VISION TO DARKNESS
When a fish moves from a lit area to another dark, adaptation to the second condition is done in two phases: the first phase is mainly due to the sensitivity of cones, while in the second phase dominate rods.
In the zebrafish (Danio rerio), for example, the first phase lasts 6 minutes and the sensitivity is mainly due to cones. After this time, the sensitivity is mainly for rods. However, rods need 20 minutes in the dark to work at their maximum efficiency.
OTHER CURIOUS ADAPTATIONS IN THE EYES OF FISHES
Some fish species have strikingly curious adaptations in their eyes. We are giving you some examples.
Limnichthys fasciatus is a small fish that lives in shallow, well-lit water, which buries in the sand and only leave the eyes outside the sand. The retina is very thick, but at one point presents a sharp narrowing of the retina, which magnifies images on this point. In other words, this fish has telescopic vision.
Most fishes have adapted eyes to underwater vision. However, some species, such as the Atlantic flying fish (Cypselurus heterurus) has also adapted to the air. To get a good view out of the water, the cornea, rather than being spherical, has a triangular shape with three flattened zones.
A fish with an extreme adaptation to the aquatic and aerial view is the four-eyed fish (Anableps anableps). This freshwater species swims with the upper half of each eye out of the water and the other half in the water. Both lenses and the whole eye are extremely asymmetric, so that they can perfectly see inside and outside of the water. If you want to see this fish swimming with its eyes half-submerged in the water, you can watch this video:
As you have seen, the vision in fish is a great deal more complex than it seems, since the water largely determines the anatomy of the eye and its adaptations. Do you know some other curious case of vision in fish? Leave your comments in the article!
REFERENCES
- El mar a fondo: La luz en el mar
- Farrell, A (2011). Encyclopedia of Fish Physiology: From Genome to Environment. Volume 1: The senses, supporting tissues, reproduction and behaviour. Academic Press. 2266 p.
- Hara, T. & Zielinski, B. (2006). Fish Physiology: Sensory Systems Neuroscience. Academic Press. 536 p.
- Hill, Wyse & Anderson. (2006). Fisiología Animal. Editorial Médica Panamericana. 916 p.
- Rhodes University: Fish Sensory System
- Cover picture: Forum Acvarist
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