Anableps anableps. Photo by Wikipedia
Since just two eyes are visible, where does Anableps anableps’ common name of four-eyed fish come from? Being a surface-dwelling fish, it does have to beware of predators, such as birds from above, as well as larger fish from below. To cope with this, Anableps anableps has large eyes (half of which are submerged as the fish floats on the water surface) that are divided into two parts. The lower half of each eye contains its own pupil, which is adapted for perfect vision underwater, while the upper half of the eye with a second pupil offers clear vision of life above water. Hence, the common name of four-eyed fish is quite relevant. Its above-water vision is excellent for scanning the water surface for insects that fall into the water, which make up its preferred choice of food.
The four-eyed fish has quite a large range along the Atlantic Coast of Central and South America that even extends to the island of Trinidad. Its favored habitat is made up of the river estuaries and surrounding mangrove swamps, where the water is brackish, with salinity changing with the ebb and flow of the tide, which the four-eyed fish are well able to cope with. It can also sometimes be found in freshwater lagoons along the coast. Groups of mature adult female four-eyed fish tend to swim together in shoals along the water surface, with males following. Here, they usually feed on any terrestrial insects that hit the water surface. Although they are not really noted for diving down to chase food underwater, their diet also includes other aquatic creatures and perhaps small fish, since their large paddlelike pectoral fins give them a good turn of speed when needed.
At 12 inches, the four-eyed fish is a comparatively large fish for the aquarium. In order to keep them “happy,” keep them in groups of six or more, and never maintain them singly or even just as pair. They generally spend all their time at the water surface. These considerations dictate that the aquarium for four-eyed fish needs to be at least 48 inches long (90 gallons), but preferably 60 inches long (150 gallons) to accommodate six to eight four-eyed fish, as well as some other fish in the same aquarium. As they are surface-swimming fish, the water level in the aquarium needs to be dropped 12 inches from the top but should be 12 inches deep, dictating an aquarium with a height of 24 inches. Four-eyed fish are good jumpers, so the aquarium needs a tight-fitting cover.
While sea water has a specific gravity of around 1.020 to 1.024, brackish water with a specific gravity of around 1.005 is adequate for maintaining four-eyed fish. However, finding a hydrometer calibrated to check this specific gravity level might be difficult. Brackish water can be made by adding 2 level tablespoons of marine salt per gallon of water. Some hobbyists may elect to use aquarium salt or the even cheaper cooking salt, which is just sodium chloride and does not contain all the trace elements found in sea water. The water parameters of a brackish aquarium are generally alkaline and hard (pH: 7.5 to 8.5, dH: 12 to 20), which fortunately can be easily measured.
Additionally, regular partial water changes (20 percent) with fresh brackish water are needed every two weeks to remove accumulating nitrates to maintain good water quality. The canister filter return pipe also provides a nice current of water that is necessary in this tank. A heater thermostat or two is needed to maintain the water at a temperature of 77 to 86 degrees Fahrenheit. Adequate lighting can be provided by fluorescent tubes even if mangroves or macro algae are growing in the tank.
Breeding Four-eyed Fish
Here, each developing embryo is nourished principally through the embryo’s expanded belly sac, the surface of which is covered with delicate blood vessels, which can absorb nutrients and oxygen from the mother’s blood supply, while wastes, metabolites and carbon dioxide from the embryo are carried away by the mother’s blood supply, just as in humans. In the case of premature release of the fry before development is complete, this delicate belly sac area can rupture, leading to the fry’s gut protruding out of this opening in its belly. Sometimes, the fry may survive, with development continuing after their premature release if water conditions are perfect; but often these fry will perish.
The gestation period of about eight weeks is dependent on the temperature of the water and usually results in 10 to 15 large fry that are already 2 to 2.8 inches when the female drops them, usually tail first. The fry, which are miniature copies of the female fish, may be eaten by other fish in the aquarium, so they should be moved to their own aquarium that contains similar conditions to the main aquarium. The fry will grow quickly if they are well-fed with small live food, such as fruit flies, blood worms and Daphnia, as well as flakes and small floating pellets. Male and female young four-eyed fish are initially identical, but eventually the male’s anal fin develops into a gonopodium, while females grow faster and are eventually larger than males. The ratio of male to female fry is usually one to one.
A Last Look
Four-eyed fish are robust fish that are easy to maintain in an aquarium that is large enough to accommodate a group of six or more. With smaller groups, it might be best to have just one male with two or three females of the same orientation that are sexually compatible with the male. With just a pair, the male’s continuous attention to just the one female can be a bit of a harassing nuisance.
Four-eyed fish do best in brackish aquariums, but do not let this put you off as they are easily set-up; only a relatively small amount of salt needs to be added to the aquarium water. Maintenance is similar to that of a freshwater aquarium and does not need all the expensive equipment needed for a marine tank containing delicate fish and corals. Four-eyed fish in a brackish aquarium will add a whole new dimension to your fishkeeping hobby.
Iggy Tavares has been keeping fish since the early 1960s. He studies and breeds popular freshwater fish; his passion is breeding, photographing and writing about cichlids.