Anableps anableps – Four Eyed Fish

Anableps anableps, called the Four Eyed Fish, their adapted eyes enable them to look up and down at the same time!

Anableps anableps eyes explained

Anableps anableps - gelukkige school

A school of happy Anableps anableps

Our love affair with Anableps goes back to March of 1981 when we were lucky enough to obtain a group of six of these charming rascals from our favourite local pet store. They were about 3 inches (8cm), (TL). These fish had been, like almost all Anableps to this day, captured in the wild and transported from somewhere on the coast of South America or the east coast of Central America to a wholesaler and then to our pet store. They don’t travel very well and were abnormally dark in colour, obvioiusly stressed, but alive. These live-bearers are called “Four-eyed fish” for rather obvious reasons.

Anableps anableps - De vieroogvis

Anableps anableps – The “Four-eyed” fish

We knew nothing about our Anableps beyond what we had gleaned from a couple of excellent articles by Edward Taylor but, fortunately, that was enough to get us started. We’ve been learning ever since. The common name, “Four-eyes”, is a slight exaggeration. There are not four distinct eyes, but rather two divided eyes. Still, the separate functions of the two divisions and the beauty of the eye’s adaptation to the necessities of the Anablep’s life is quite remarkable.

Anableps anableps - een vreemde vis

Anableps anableps – A strange fish indeed

Bear in mind, as you read and examine the drawings and photographs that Anableps are surface dwellers. It is true that they can leap quite remarkable distances above the water and, if required plunge to the bottom for a while, but they much prefer cruising along the surface. This makes them fairly easy prey for birds to spot. It is also likely that predatory fish could, in spite of the Anablep’s whitish bellies, spot them and sneak up from underneath. For best survival chances the Anableps must be able to spot these attackers in time to take evasive action. Their very special eyes provide the means. A band of pigment divides the eye horizontally just at the waterline. At least Anableps ordinarily swim such that the waterline coincides with that band. Just behind and avove that band is a golden iris flap to shield the upper pupil from the glare at the water’s surface. The lens behind this upper pupil is flattened to provide an undistorted image of what’s going on above water, (in air), for the lower retina. See the simple human eye diagram below for comparison.) Anableps really see their keepers very clearly!

Anableps anableps oog links - Menselijk oog rechts

Anablep eye at top-left, human eye top-right

Anableps anableps - De onderwater lens

Anableps anableps – The below water lens

The part of the lens that is behind the lower pupil is rounded as in an ordinary fish eye. This provides the best image of the underwater scene to the upper retina. As with human eyes, all images are inverted. It’s up to the brain to turn them around and right side up.

As you can see the Anableps is abel to keep track of what is above and what is below at pretty much the same time. It’s likely that they can only attend to one image at a time but I’ll gladly leave it to some scientist to determine how quickly they can flip back and forth. By the way, these eyes aren’t quite as delicate as they appear. They are well protected by a stron bony socket.


Anableps anableps van boven gezien

Anableps anableps – Photo: E.Naus

Anableps' paring van boven bekeken. Vaak beschreven als S-en. Dit jonge mannetje laat zijn enorme bereik zien.

Anableps’ mating seen from above – often called “s”ing. This young male exhibits amazing reach

Back in the early 1700’s when the first Anableps were sent back to Europe, Artedi, the most famous icthyologist of his day, made much of the unusual eyes, but did not mention that these fish were livebearers. I’m very certain that no one could have missed thta bit of information if any males had been captured. It came to light about half a century later. This oversight was likely due to the tendency for female Anableps to school together in the wild and, along with the fact that both males and females look alike when they are young, gave rise to the myth that male Anableps are very rare. When we first confided to a fellow hobbyist that we had six Anableps of which three were males, his response was, “Are you sure you have males!”

Statistically, I expect that about half of all Anableps born in the wild are males; in our tanks we have encountered the problem of having too many males and too few females at times. The myth of a scarcity of males needs to die, and yes we were sure! If you look at the pictures and drawings on this page you will easily see why.

Anableps man - let op wat Artedi nooit heeft gezien

Male Anableps – note what Artedi never saw

Close up van het flinke gonopodium van de Anableps

Close up of the Anableps’ fleshy gonopodium

Let op: het gonopodium kan slechts 1 kant op bewegen

Note that gonopodium can only move in one direction.

The maleness of Anableps is not subtle. What starts out as a normal-looking anal fin forms into a substantial gonopodium. No fragile-looking bony appendage such as you see on a guppy or swordtail.

Males can move their gonopodiums quite vigorously but, only in one direction. Some are right-gonopodiumed and some are left. There are no “switch hitters”.

If the female’s genital pore could be easily approached from either side, there would be no problem and no mystery.

Don’t make the embarrassing mistake that I originally did, and think that the anus is the genital opening. The female Anableps’ genital opening is covered by a “scale flap” called a foricula and, you guessed it, some have a foricula that hinges on the right and some on the left. As the picture and drawing on the lower right of a post-mortem mature female illustrates, the foricula, as pried up with the tip of my scalpel, is quite large and secure.

Een sterk licht kan je helpen bij het bepalen van de richting van de foricula

A strong light can help in determining the direction of the foricula.

Een onderzoek van de foricula na de dood

A post-mortem examination of a foricula.

What happens is this. When a right-gonopodiumed male approaches a left-foricula(ed) female along her left side and moves his member into position, she may, if she’s ready, lower the flap and permit him to inject sperm. That compatible situation is the one pictured here. If the male approaches a female with an incompatible genital arrangement, presumably he fails to impregnate her. By the way, females grow larger than males and may just turn, and drive them away if they’re not interested for any reason.

The mystery is why the process of natural selection over millions of years has not eliminated what appears to be an impediment to Anableps’ reproduction. It’s assumed that there is some overriding advantage to this arrangement that science has yet to discover. The problem for hobbyists when trying for compatible pairings is that while the orientation of males can be observed, that of females can only be guessed at.

All we can offer is our observation that the foriculas of young females are smaller and seem less secure. We think an incompatible male might sometimes succeed with a yougn female, but with an older one, it’s doubtful.

Anableps anableps babies

Anableps males seem to have two major interests – eating and mating. Compatible mating leads to pregnancy. Quite often the first reliable sign that a female is pregnant is a change in body shape. She loses her slender silhouette and becomes tubby in the abdominal region. Although this is very evident in the picture below, sometimes the change is not very noticeable and we have been surprised by a small number of babies from a female we didn’t think was pregnant.

Een zwangere Anableps vrouw

A pregnant female Anableps

As you will see, it’s important to understand the process involved in egg fertilisation and embryo development in order to provide the necessary environment for successfully raising the young. Anableps babies are sometimes aborted and each fetus always has the gut protruding out of a split-like opening in its belly. Even when the babies are born alive, the opening can still be seen with, sometimes, a bit of the gut protruding. Infection and the resulting death can easily happen if the tank water is less than ideal. I’ll try to explain the little that I’ve been able to find out about the internals of Anableps pregnancy. Keep in mind that this is the result of reading and deducing and not from any scientific research of my own.

Gezonde pasgeborene rust op een steen

Healthy newborn resting on a rock.

Consider follicles. The most common ones are found around hair roots. These are hair follicles like the ones I used to have on top of my head. The ovaries of animals, including female Anableps, have egg follicles which form a sack of cells around each egg. Apparently, mature eggs are not completely surrounded because impregnation by the male sperm happens within the follicle. This isn’t all that unusual and for some livebearing fishes the embryo then grows and develops using only the food supply in the egg. The gestation period for such fish is fairly short, (about a month), and the babies are quite tiny at birth. It’s somewhat different for Anableps.

The first part of the embryo to develop is the gut and tiny vascular protruberances on the follicle wall make contact with it and allow it to take i nourishing fluids. he gestation period is much longer – approximately 3 months, and it’s only at the end of that period that the rest of the body “catches up” with the development of the gut and the gut ascends into the body cavity. If all goes perfectly, babies from 1.5 to 2 inches (4-5 cm) long are born with only a slight opening or even just a line along the abdomen left as evidence of the process.

It’s an imperfect world ad probably due to conditions in our tanks which wouldn’t be present in nature, baby Anableps are sometims born before the gut has completed its journey into the body cavity. These pictures show that if, as we did, you provide sterile conditions, the necessary development will continue to a successful conclusion even after birth. We used an antibiotic, (Chloramphenicol), frequent water changes and only live food in this case. Five out of the seven premature babies survived. Fortunately, we ad many successful births where no such strenuous measures were needed.

Let op: de rode "darmen" zijn nog deels zichtbaar

Note the “red” guts still partly exposed

Anableps babys van onderen

Anableps babies from below

Anableps babys van onderen

Anableps babies from below

Een week na de geboorte - succes!

A week after birth – success!

Een netje vol Anableps babys

A netful with no problems

Twaalf van de dertien - een paar weken oud

Twelve of the 13 – a few weeks old

Camera staat klaar voor geboorte Anableps

Babies are being born here and my camera is ready

Nogmaals moeder "Bleps" ze wil gewoon niet meewerken bij het filmen van een geboorte!

Once again mother “Bleps” just won’t cooperate by letting me film a birth!

The babies look like miniatures of their mother at birth and, even though mother Anableps often grew to 10 inches (25 cm) in our tanks, (12 inches in nature), such large babies severely limit the number possible per spawn. Although one of our large females aborted 20 fetuses, (so we assume that many are a possibility), our most successful spawn was 13 live, healthy babies. Since then we understand that a fellow hobbyist, Carl Krajniak from Michigan, had a very large female which gave birht to 16 live babies. Spawns like this don’t seem like much compared to the 100 plus babies you can get from a swordtail or a Molly, but Anableps require more space and care. At one time we had 52 of them including adult, half-grown and babies from several generations and they made life very interesting and very busy in our fish room.

One of my goals has been to photograph an actual birth. I have come quite close, but not quite. Between us, Pat and I have witnessed 2 births, both of which were tail first. So far, although I’ve had my camera ready when births were taking place, mother Anableps simply wouldn’t pose where I could get a shot. I hope to try again in the future.

Care and Feeding

If male Anableps are only interested in eating and mating, females seem to have just one thing on their minds – eating. In nature they often inhabit the brackish waters in the estuaries of rivers along the coasts of northern South America and eastern Central America, so they probably live on a diet mainly of insects, worms, crustaceans, and plankton. In our tanks they appear to be constantly hungry and, if not given an ample quanitity of protein-rich foods, they usually develop spinal deforamties and show poor growth.

Anableps anableps taking brine shrimp for a baster

Anableps anableps taking brine shrimp for a baster

Ours have always loved freeze-dried plankton, thawed, frozen brine shrimp, live wingless fruit flies and our homemade concoction of fish, clams, beef heart, shrimp and veggies (Baby vitamins are stirred in just before feeding). With patience they can even be trained to take food from a turkey baster or a spoon. There’s very little that they won’t eat and, although floating food is desirable, if the tank isn’t too deep they will paddle down and scavenge from the bottom as well.

Bably “bleps” are large enough to eat almost all the same things as their parents. Sometimes we left them in the tank with adults and they survived but, although the adults don’t deliberately harm the babies, they don’t provide any parental care either. We usually set up the young with their own tank so they didn’t have to compete so strenuously for food. It was great fun to observe them carefully and even to “spoil” them with tasty bits like brine shrimp nauplii.

All of this feeding meant lots of cleaning and water changes to avoid any build-up of ammonia. The ideal tank temperature for Anableps seems to be 86 F (30 C), although some hobbysists keep them a bit cooler. Just keep in mind that they come from an area where the water temperature is consistently warm. Warm brackish water and ammonia make a deadly combination. In addition to good filtration and frequent water changes, we highly recommend that some ammonia-absorbing compound or resin be used. It can be as simple as adding it to a small box filter which can be set bubbling away in a corner of the tank, but don’t forget to renew it at least once a month.

When setting up an Anableps tank we always tried to keep a few things in mind. 1) Because of where they swim, surface is more important to them than depth. 2) They seem to like to lie about in very shallow water where they can rest while still keeping the water flowing over their gills. 3) Although in nature Anableps often swim some distance into the mouths of rivers and estuaries, they can’t survive and stay healthy for very long in completely fresh water. 4) They often spend time right out in the ocean so in addition to being salty, their water should be somewhat hard and alkaline. 5) Finally, they’re natural-born long-jumpers so a secure glass cover is a practical necessity.

Anableps - adding salt

A heater was adjusted to 30 C, some substrate from a healthy, established tank was added and the whole thing was left to age for a few weeks.

If you keep Anableps, make sure that the tank lid is quarter-inch glass or else weighed down with something. They are easily startled by any sudden movement or sometimes even a light coming on and many Anableps has met a sad end on a cold, hard, fish room floor. We always keep a low-wattage light on, (we call it the moon), in our fish room at night.

Anableps aquarium

A heater was adjusted to 30 C, some substrate from a healthy, established tank was added and the whole thing was left to age for a few weeks.

Anableps seem to be colour sensitive. Pat  had a tourquoise housecoat that inspired fireworks-like panic in a tank of Anableps. We assumed it must have meant “big predatory bird” in their lanugage!

Be prepared for surprises. Anableps are aware of the “out-of-tank” environment and may react strongly to the presence of strangers. This happens whe you want a visitor to see them.

When seriously frightened, Anableps usually tuck themselves into a corner or sit on the bottom of the tank. Theymay also turn quite dark, almost black. This colour, if it persists, can also be an indicator of poor health. They are also susceptible to bacterial skin infections which are hard to cure. Antibiotics may work, but prevention (excellent water quality), is always far better.

With luck we’ve had some that have lived 4 to 5 years and brought us joy.

Eerste publicatie: “The Scat” – St. Catherine’s Aquarium Society, Canada. Oktober1999

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Tom en Pat Bridges


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Tom en Pat Bridges


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