The fish with ‘four eyes’ (Anableps)
by Russell Grigg
One of the strangest fish in the world is Anableps anableps, commonly called
the ‘four-eyed fish’ because of the unique configuration of its eyes.1 These are large and bulging,
like those of a frog, and are located on the top of its head so that it swims with
its eyes half in and half out of the water.
At the waterline, the eyes are divided by a band of epithelium (tissue) into upper
and lower parts, with separate corneas and retinas, which function for aerial and
underwater vision respectively.2,3 The lens is more egg-shaped
than convex and can focus two images simultaneously, one from above the water and
the other from below (see diagram). This means that the fish’s two eyes actually
function as four eyes; hence its name.
When Anableps is looking out into the air, the light rays pass through
the shorter width of the lens, which gives good distance vision for locating its
prey of insects; it also means that the fish is difficult to catch, as it can see
fishermen approaching! From under the water, the light rays pass through the full
length of the lens, so that the fish is near-sighted under water.4 One is reminded of a bifocal lens in spectacles.
This configuration of the eyes enables Anableps to search for food in one
habitat (air) and at the same time watch out for predators in another habitat (water).
When it sees a predator approaching through the water, it escapes by leaping out
of it, which is a reason why this species is not very suitable for keeping in a
small home aquarium. Another unusual habit is that when swimming it continually
ducks its head under the water. This is because, unlike most land animals, it does
not have a tear duct to keep the eye moist and so must duck to prevent its eyes
from drying out.
How the eye of the four-eyed fish works
So is the four eyed fish evidence for evolution or creation?
The Encyclopaedia Britannica describes Anableps as ‘an extreme
example of adaptation to life near the air-water interface’.5 If this is the case, how did it develop its multi-functional
eyes, beginning as an ordinary fish with normal, single-function eyes? It is extremely
difficult to see how something as complex as this could develop by gradual stages,
and the more parts that have to develop, the greater the difficulty. This includes
the complex brain setup needed to simultaneously decode the double images being
received by the two retinas. How did all this evolve until it was fully functional?
There is, in fact, no evidence that Anableps evolved.
By far the simplest explanation of the evidence is that Anableps was created
on the fifth day of Creation Week by the Lord God, who created ‘every
living creature that moveth, which the waters brought forth abundantly, after their
kind’ (Genesis
1:21). This accounts for the formation of all the parts of the eyes
and the needed complementary parts of the brain, with everything functional from
the beginning.
References and notes
- Four-eyed fishes are classified as order Atheriniformes, suborder
Cyprinodontoidei, family Anablepidae, genus Anableps. They live in shallow,
muddy freshwater streams in Central America and Mexico, are born live, and grow
from 4–6 cm (1.5–2.5 inches) at birth to 15–30 cm (6–12
inches). They usually travel in schools, cruising the surface to feed on insects
and other small invertebrates. Return to text.
- ‘The cornea is divided by a horizontal band of pigment,
separating an upper, strongly convex part from a lower, flatter division. The iris
has a pair of projections, partially dividing the pupil into two.’ Encyclopaedia
Britannica, 19:252, 1992. See also ibid 1:668.
Return to text.
- Using a camera as a rough analogy, we can think of the cornea
as the ‘window’ in front of the lens, and the retina as the surface
on which the image is focused, somewhat like the film in a camera. From there, electrical
signals send this visual information to the brain, where the signals are processed
into mental images. Return to text.
- Edward Migdalski and George Fichter, The Fresh and Salt
Water Fishes of the World, Bay Books, Sydney, Australia, 1976, p. 186.
Return to text.
- Ref. 2. Return to text.
(Also available in Russian)
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