Hey, Four-Eyed Fish!

Sure, those of us who wore glasses when we were younger may have been called “Hey, four eyes!”.  But I wonder if anyone ever took offense to the level of “Hey, you four-eyed fish!”.  ‘Cause that would be combining two insults, the discrimination against an ocular disability and the idea that you were kind of cold…or wishy-washy…well, anyway.  I sometimes get to share the fact that I once caught a Four-eyed Fish, and recently I found out that the species belonging to the Genus Anableps that I caught is rather rare, so I feel even more special!

(Imagine me affecting a British accent here, to make my story sound more adventurous).  “There I was, standing in the river with my doughty crew, when one of the young stalwarts excitedly shouted “Quatros ojos, quatros ojos!””.  Yes, just a few feet away from me cruised the rare and dangerous (dangerous if you’re an insect, that is) Pacific Foureyed Fish Anableps dowei!

In 1999 I accompanied members of my church on a mission trip to the area of Siguatepeque, Honduras, to assist in building cement block housing for victims of Hurricane Mitch (in 1998 Mitch was responsible for the death of at least 11,000 people in Central America) that caused a flood perhaps 40 feet deep in a valley near Siguatepeque.  After the rest of the mission left I stayed behind to travel to the Pan American School of Agriculture near Tegucigalpa, where the fisheries instructor there graciously allowed me to accompany them on trips to waters near the school.

The streams we sampled were the mainstem and tributaries of the Rio Choluteca, the major river on the Pacific slope of Honduras that winds through mountainous terrain until it empties into the Gulf of Fonseca, an estuary shared by El Salvador, Honduras and Nicaragua.  At a site on the Choluteca, near the village of Zamorano, the school’s students and I seined up the Pacific Foureyed Fish (Anableps dowei).  This was a species I’d read about prior to making the trip, so when I heard the student’s cry I became quite excited!

The species is named for a Captain J. M. Dow, who skippered the steamer “Guatemala” of the Panama Railway Company.  Captain Dow collaborated with two associates to send over specimens from over 1500 samples in Central America to the U.S. National and the British Museums.

The reason for the Four-eyed Fish’s common name is the presence of two pupils in each eye, one in the upper and one in the lower half, separated by a band of tissue. This enables them to see above and below the water while they cruise at the surface of the water body and makes the Four-eyed Fish extremely difficult to catch with a seine: they are able to see you (or an eagle, or other bird of prey) coming from a long ways away. They are known to leap right over a seine and like fish in another family, topminnows, they dive down to the bottom to avoid capture.  An effective method of capture is described as using a group of fishermen to drive a school of quatros ojos toward a concealed individual waiting with a cast net that is thrown over the school, ensnaring a “bushel full” of the prey.

The Anableps‘ eye is flattened on the top and rounded on the bottom half, with a thickening of the lens from the bottom to the top to adjust for the refractive differences in the two mediums.  The upper pupil casts the terrestrial image through the lens on the lower retina, while the lower pupil’s image is reflected on the upper retina.  The Four-eyed Fish’s eye recently inspired at least one contact lense company to develop lenses that work extremely well both out of and in the water.

 

1. Underwater retina 2. Lens 3. Air pupil 4. Tissue band 5. Iris 6. Underwater pupil 7. Air retina 8. Optic nerve

Swimming at the surface with the head exposed is relatively unusual for fishes in general, but species of this genus show other oddities as well.  Not only do the quatros ojos leap out of and skip along the surface of the water, but when they see terrestrial insects on the banks they will actually leap onto the shallow, inundated bank side areas to capture their prey.  These fish have been observed lying in the sun, sometimes for several minutes, before pushing their way back into the water.  Once they’re out of the water their mobility is severely limited since unlike eels they cannot locomote with a wriggling motion, nor can they push off with their tails to leap forward on land. Unlike mudskippers and the “walking” catfish their pectoral fins are unsuited to pulling themselves along.  So, although they may push themselves along with their tail and pectoral fins to chase their prey, the extent to which they are able to do so is severely limited.

Another anomaly that characterizes anablepids is that their genital organs are oriented either to the left or right, thus they can reproduce only with mates having compatible organs.  They share this character with the group of species to which they are said to be most closely related, the “One-Sided Livebearers”, or Jennysina.  The functional significance of this anomaly is not known.  Anableps species are viviparious, meaning the young are birthed live rather than from an egg deposited in the water.  The eggs are carried to term inside follicles in the female’s ovary at which point they hatch and are extruded from the genital pore.  The male of the species has a gonopodium, a modified anal fin ray that develops as the males mature and facilitates placement of the sperm into the oviduct, fertilizing the female’s eggs.

At present three species of Four-eyed Fish are recognized: Anableps anableps, the Largescale Foureyes, is found in South America from the island of Trinidad and Tobago, and Venezuela to the Amazon Basin of Brazil.  Anableps dowei, the Pacific Foureyes, has the most limited distribution of the three species, occurring in Central America from southern Mexico to Nicaragua. Anableps microlepis, the Foureyes, is the most salt tolerant species of the three. They are found in open marine areas in full seawater (also from Trinidad to the Amazon Basin in Brazil) and follow tidal rhythms, moving up into sheltered lagoons and further upstream with the high tides, and back out into open waters as the tide wanes.

Anableps congregate in schools of up to 200 or so as juveniles, with their gregariousness decreasing with age until at adulthood they are as likely to be found as individuals as in small groups.  Some of their known fish associates include characins, pimelodid catfish, poeciliids, atherinids, eleotrids, flatfishes and cichlids.

If you are looking for an unusual fish for your aquarium the species that is most commonly available from suppliers (there are several that raise their own stock), the Four-Eyed Fish, is moderately hardy, but they are comparatively large in size, growing to around a foot in length.  Since they are surface swimmers they do best in a long, relatively shallow tank in fresh to moderately brackish water (depending on the species).  They are gregarious so it is best not to keep them singly or in pairs.  They will probably do well with Sailfin Mollies, bottom-dwelling Gobies, Mudskippers, and even Orange Chromide Cichlids, Archer Fish and Monodactylus.

The Family Anablepidae is placed within the Order Cyprinodontiformes (and, the Pacific Foureyed Fish attains the largest size of any species in that order).  That order contains a bounty of fascinating forms, with a wide variety of reproductive types, a plethora of adaptations to environments, and high importance in terms of biogeography.  My next post will portray some of those very diverse species.

About the Author: Marc Kibbey is Associate Curator of the Fish Division at the Museum of Biological Diversity.

*** Have you ever seen a four-eyed fish? Let us know, leave a comment ***