You’re not seeing double, but Anableps anableps is.
With its bulging, orb-like eyes that can simultaneously see above and below the waterline, “four-eyed fish" is an understandable nickname for this curious creature—but it’s not actually accurate. Individuals only have two eyes, but each eye is divided by a thin band of tissue into dorsal (upper) and ventral (lower) sections, each with its own cornea and pupil to capture light from above and below the surface. A. anableps likely evolved such unusual optical anatomy to help it hunt both terrestrial and aquatic prey. Talk about the best of both worlds!
Range, distribution, and habitat
A. anableps inhabits the coastlines of the Caribbean island of Trinidad and northern South America, from Venezuela to the muddy mouth of the Amazon. It prefers the brackish (somewhat salty) water of coastal mangrove forests and estuaries, but can also survive upstream in freshwater creeks and rivers.
This fish goes with the flow—literally: When the tide is high, it ventures into intertidal channels to forage among the inundated mangroves, and when the tide is low, it returns to the shallows of the permanent water channels.
In the wild, high tide means mealtime for A. anableps. The tangled mangrove root systems host a buffet of goodies, from red macroalgae, the main component of the fish’s diet, to insects, crabs, amphipods, snails, mussels, and worms. How does a fish feast on terrestrial prey? This is where those split-screen eyes come in: Because A. anableps is able to see above the surface, it can launch itself out of water to grab an unsuspecting meal. It can even haul out onto mudbanks to pursue prey!
At the Academy, A. anableps are fed twice a day typically around the same time to mimic the schedule of the tides in the wild. A wide variety of foods are on the menu, including vitamin-dusted crickets, earthworms, krill, floating pellet food, spirulina, and flaked red nori.
A. anableps is diurnal, meaning it’s primarily active during the day. Groups of 100 or more often congregate in the shallows together—there’s safety in numbers. Despite being skilled ambush predators, they are extremely skittish, bolting across the surface of the water or diving for shelter in crevices or under submerged debris at the first sign of danger.
Thanks to their “four” eyes, A. anableps are highly sensitive to out-of-water activity—including passing museum guests! When observing these fish at the Academy, take care to avoid sudden movements and you may be rewarded with an up-close encounter.
They say opposites attract—but in the case of A. anableps, it’s a biological requirement. The sex organs on males and females are located on either the right or left side depending on the individual, so mating can only occur between “right-handed” males and “left-handed” females, and vice versa.
If the amorous encounter is successful, the female four-eyed fish will give birth to miniature, fully formed live young (viviparity) about three months later.
Conservation and regeneration
While we don’t have enough information about the relative scarcity or abundance of A. anableps, another species in the genus, A. dowei, is classified as “Least Concern” by the IUCN.
Unfortunately, Anableps are only as safe as their preferred habitat—coastal mangrove forests. These superstar ecosystems are home to thousands of animal species, protect coastal communities from punishing storms, filter polluted water and runoff, and absorb carbon dioxide, but are disappearing at an alarming rate due to sea level rise, aquaculture, erosion, and coastal development. In order to ensure weird, wonderful creatures like the four-eyed fish survive in the future, we must make mangrove conservation and regeneration a priority in the present.
"One of my favorite parts about caring for this species is that they are so vision-centric. They really are watching everything, and they respond accordingly. Unfortunately, this makes them a very flighty species that spooks easily, often dashing across the water's surface in an attempt to get away. This poses a problem whenever I need to take the top off to enter the habitat, so I whistle every time before I feed or clean around their habitat just to let them know of my presence. Now they come to me instead of fleeing before feeding."
 Republished with permission of The Royal Society (U.K.), from Perez, Louise N., Jamily Lorena, Carinne M. Costa, Maysa S. Araujo, Gabriela N. Frota-Lima, Gabriel E. Matos-Rodrigues, Rodrigo A. P. Martins, George M. T. Mattox, and Patricia N. Schneider. 2017. “Eye Development in the Four-Eyed Fish Anableps Anableps: Cranial and Retinal Adaptations to Simultaneous Aerial and Aquatic Vision.” Proceedings of the Royal Society B: Biological Sciences, no. 1852 (April): 20170157. https://doi.org/10.1098/rspb.2017.0157; permission conveyed through Copyright Clearance Center, Inc.
Common name: Four-eyed fish
Scientific name: Anableps anableps
Size: Up to 12 inches
Range: Trinidad to the Amazon delta
Habitat: Freshwater and brackish estuaries, mudflats, and mangrove forests
IUCN Red List status: Unknown; likely Least Concern