Pacific four-eyed fish

Geographical Origin and Habitat: This species is native to the Pacific slope of Central America, ranging from southern Mexico to Nicaragua. It inhabits brackish coastal lagoons, mangrove swamps, estuaries, and slow-flowing river mouths, swimming almost exclusively at the water surface in search of food.

Taxonomy and Morphology: Belongs to the family Anablepidae, genus Anableps. The species is scientifically accepted as Anableps dowii, though it is frequently spelled Anableps dowii. It features an elongated cylindrical body, a flattened head, and protruding eyes divided horizontally into two lobes, each with an independent pupil and retina. Males reach 22 cm, while females can reach 34 cm.

Social Behavior: A gregarious and active species that lives in schools swimming constantly at the water surface in parallel alignment. It is peaceful yet highly vigilant, capable of detecting aerial and underwater threats simultaneously. It requires large surface areas free of obstructions.

Coloration and Sexual Dimorphism: Displays an olive-grey or brownish body with a pale longitudinal stripe along the sides and a white belly. It shows pronounced sexual dimorphism: males are smaller and possess an anal fin modified into a tubular copulatory organ called a gonopodium, which is asymmetrical (oriented to the left or right).

Aquarium Setup: Requires an exceptionally large tank with a minimum capacity of 300-400 liters, prioritizing water surface area over depth. Water depth can be shallow (around 25-30 cm), but the surface dimensions must be maximized. A secure lid is mandatory as they are skilled jumpers. Leave at least 10 cm of air space below the cover.

Diet and Feeding: A specialized surface-feeding carnivore. In the wild, it feeds on terrestrial insects, small crustaceans, and small fish. In captivity, feed floating foods such as live insects (crickets, fruit flies), mysis, shrimp, and floating dry pellets or sticks. They will not consume food that sinks to the bottom.

Water Quality: A brackish species that requires the addition of marine salt to reach a specific gravity SG of 1.005-1.015. Optimal parameters are: temperature between 24 and 28 °C, pH stable between 7.5 and 8.5, and high hardness. Powerful filtration is necessary to handle the biological load of these large fish.

Compatibility and Cohabitation: A peaceful fish that can cohabit with other medium-to-large brackish species such as archerfish (Toxotes), monos (Monodactylus), or scats (Scatophagus). Avoid small fish that could be swallowed, and aggressive species or fin-nippers that might damage the goby's delicate protruding eyes.

Aquarium Breeding: Viviparous (internal fertilization via the gonopodium). The female gives birth to fully formed, unusually large fry (up to 5-6 cm long) after a gestation period of about 8 weeks. To protect the newborn fry from adult cannibalism, it is advisable to transfer them to a shallow nursery tank.

Risks and Diseases: The primary risk is the drying out of the upper half of the eye, which remains exposed to the air; to prevent this, the humidity of the air space above the water must be kept high by tightly covering the tank. They are also prone to bacterial and fungal eye infections if water quality declines.