User:Lubovmyra/Four-eyed fish

Article Draft by lubovmyra kopczuk
The four-eyed fishes are a genus, Anableps, of fishes in the family Anablepidae. They have eyes raised above the top of the head and divided in two different parts, so that they can see below and above the water surface at the same time.

Like their relatives, the onesided livebearers, four-eyed fishes mate only on one side, right-"handed" males with left-"handed" females and vice versa.

These fish inhabit fresh and brackish water and are only rarely coastal marine. They originate in lowlands in southern Mexico to Honduras and northern South America.

Species
There are currently three recognized species in this genus:


 * Anableps anableps (Linnaeus, 1758) (Largescale foureyes, found in the coast and islands of northern and northeastern south america)
 * Anableps dowei T. N. Gill, 1861 (Pacific foureyed fish, found in the coast and islands of northern and northeastern south america)
 * Anableps microlepis J. P. Müller & Troschel, 1844 (Foureyes, found in the pacific coast of central america)

Physical characteristics
The maximum length of four-eyed fishes is up to 32 cm TL in A. microlepis, making this species the largest in the order Cyprinodontiformes.

Four-eyed fish have only two eyes, but the eyes are specially adapted for their surface-dwelling lifestyle. In early development, the four-eyed fish’s frontal bone expands dorsally allowing the eyes to be positioned on top of their head and appear bulging. This allows the fish to simultaneously see above and below the water as it floats at the surface. The eyes are divided into dorsal and ventral halves, separated by a pigmented strip of tissue. Each eye has two pupils and two corneas filtering light onto one lens, refracting onto separate hemiretinas and processed through one optic disc. The upper (dorsal) half of the eye is adapted for vision in air, the lower (ventral) half for vision in water. The lens of the eye also changes in thickness top to bottom to compensate for the difference in the refractive indices of air versus water. The ventral hemiretina is characterized by thicker cell layers containing more sensory neurons and an increased visual acuity compared to the dorsal hemiretina. There are six stages when it comes to their eye development. In the first two stages of the eye development, the larvae have eyes resemble a normal vertebrate eye. Vertebrate eyes being eyes like peoples. During stage 3, the cornea and pupil of the anableps eye begin to duplicate, followed by the longitudinal constriction of the pupil, a conspicuous feature of the eye. At stage 4, the dorsal portion of the eye begins to display chromatophores, and continues to develop. During stage 5, the eye continues to develop, and at stage 6, partial duplication of the eye is complete. When it is fully developped the eye has a iridial flap and a pigmented band of corneal epithelial tissue that seperates the dorsal and ventral half.

Anableps have a single pyriform lens, whose dorsal half is flat, with the ventral half being round (Schwab et al., 2001). The dorsal and ventral corneas also differ in their axis curvature and epithelial thickness, the dorsal half being much thicker (>20 cell layers thick) and flatter (~1.94 radius curvature) than the ventral half. The retina is a single structure that is subdivided into a dorsal and ventral region with the light from the aerial stimuli being projected by the dorsal lens onto the ventral retina and light from aquatic stimuli being projected by the ventral portion of the lens onto the dorsal retina. Each half of the retina in reality is connected to a single optic nerve where the information gets relayed.

When it comes to the visual streght of the eyes the optomotor response or OMR has been used as a test to investigate potential differential visual processing in Anableps on normal versus ‘blinded’ fish (the eyes are actually covered—not physically blinded). It was found  that the OMR does exist in Anableps and that the strength of this response is dependent on the visual field being tested—a stronger OMR was seen as a result of visual stimulation from the aerial environment.

Four-eyed fish are livebearers. Along with their sister genus Jenynsia they mate on one side only, right-"handed" males with left-"handed" females and vice versa. The male has specialized anal rays which are greatly elongated and fused into a tube called a gonopodium associated with the sperm duct which he uses as an intromittent organ to deliver sperm to the female. Anableps are viviparous with internal fertilization. It is noted that when comparing a male and a female Anablep that the male is noticeably smaller than the female; this is believed to be due to reproduction and sexual selection.

Behavior
Over the last glacial maximum, the exposure of the Atlantic shelf is believed to have drastically impacted the ancestors of the Anableps today. It is believed that it presented the opportunity for the dispersion into an Amazon ‘freshwater sea’ or even along coastal lagoon environments. The four-eyed fish are a well-spread species that as mentioned live in freshwater, only able to adapt to narrow ranges of salinity but can thrive in oceanic shores, estuaries, and freshwater streams. Generally, preferring to reside in estuaries where freshwater meets the ocean.

Four-eyed fish spend most of their time at the surface of the water. Their diet mostly consists of terrestrial insects which are readily available at the surface, however they may consume other foods such as other invertebrates, diatoms, dinoflagellates, and small fishes. These fish are known for beaching themselves during tidal movements and feeding on what is found in the sand and mud. They may also be seen jumping onto exposed banks to catch insects. While during high tide they can be found mainly in the first channels either praying on organisms that come close to the water or eating algae found at the bottom.

The fish will group differently depending on the species. A. anableps commonly congregates in schools. A. microlepis also is gregarious, but restricts its schools to about a dozen individuals; it is also recorded to be found alone or as couples.

A. anableps is also known for the ability to survive out of water when exposed to air, especially during low tide.