More information about Four-Eyed Fish
Physical description: 

Does this fish really have four eyes?  The generic name "Anableps" is derived from the Greek for "up-looking." As you can see from the picture at the left, the fish appears to be gazing meditatively upward, as though it were trying to remember how to multiply 9 x 16.  

The fish does not actually have four eyes, but the eye is divided to allow the fish to see both above and below the waterline.  A narrow band of epithelium divides the upper and lower halves of the eye.  Each half of the eye has a separate pupil, iris and cornea, but the retina is divided. Both halves of the eye use the same lens, with the upper light path traveling through the short axis of the lens, while the lower light path travels through the long axis. This dual use of the lens corrects for the different behavior of light in air and in water, with the underwater lens face more strongly curved. The underwater half of the eye projects an image to the upper half of the retina, while the part of the eye above water projects to the lower retina. 

The upper eye must be occasionally wetted to prevent dehydration, but when the fish is completely submerged, the image from the upper half of the eye is out of focus. Click here for more information about the Anableps eye.

General Information:  

There are three species in the genus Anableps.  The species found in the James R. Record Aquarium is Anableps anableps.  The genus belongs to the order Cyprinodontes, small, often colorful livebearing fish with complex behaviors.  The famous desert pupfish is also in this order.  Anableps is found in coastal waters from the Yucatan peninsula to the equator. 

Special anatomical, physiological or behavioral adaptations:

Anableps seems to be designed for asymmetrical mating.  In females, a single large scale covers the cloaca and opens either to the left or to the right.  In males, the anal fin is adapted as a funnel for sperm transfer, and similarly extends either to the right or the left of the body.  

Fertilization is internal, and males and females have to be appropriately matched.  Right-handed males outnumber left-handed males about 60: 40 with about the same proportion of the appropriately structured females.   

Development is viviparous, with the young nourished by nutrient diffusion across the reproductive tract of the female.  The fry are retained until they reach a length of about 45 mm.    Probably because of the size of the young fish at "birth" only 14 to 20 embryos develop from a brood.  Sexual maturity occurs at about 9 months. 
Anableps is primarily a surface feeder and its view of surface activity makes it difficult to catch, except with long-handled nets. 

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