Who You Calling Four Eyes...?

The bizarre and unique looking four-eyed fishes have eyes raised above the top of the head and are divided in two different parts, so that they can see both below and above the water's surface at the same time! Four-eyed fish are a genus, Anableps, and originate from the lowlands in southern Mexico to Honduras and northern South America. These hardy fish predominately inhabit fresh and brackish waters, but are also known to venture into full marine water. There are currently three recognised species in this genus, where the maximum length is approximately 30 cm. Of the 3 species, the most commonly seen in aquaria is Anableps anableps (the Large scale four-eyes). The other 2 species are Anableps dowei (the Pacific four-eyed fish) and Anableps microlepis (Four eyes).

Despite their name, four-eyed fish only have two eyes, but over millions of years their eyes have evolved to be specially adapted for their surface-dwelling lifestyle. The eyes are positioned on the top of the head, so as the fish floats at the water's surface the eyes are clearly seen, with only the lower half of each eye being underwater. The two halves are divided horizontally by a band of tissue and so each eye has two pupils, connected by part of the iris. The upper half of the eye is adapted for vision in air (indeed their scientific name means 'to look up' in Greek), and the lower half for vision in water. The lens of the eye also changes in thickness from top to bottom, to compensate for how light is refracted differently in air to that in water.

As they spent all of their time at the water's surface, their diet mostly consists of insects which are readily available at the surface. However, they also consume other foods such as other invertebrates, algae, and small fishes. These fish will group differently depending on the species, for example, A. anableps commonly congregate in schools of high numbers, whereas A. microlepis schools are restricted to numbers of about a dozen individuals and can sometimes also be found singly or in pairs. 

Apart from their unique eye structure, there are 2 other interesting facts relating to four-eyed fish. Firstly, A. anableps is also known for their ability to survive out of water for some time, such as during low tide, as long as they don't dry out. Secondly, all 3 species are livebearers that only mate on one side; 'right-handed' males with 'left-handed' females and vice versa. To date, no studies have discovered why this is so and what advantages it possibly has? Once impregnated, a female is gravid for about 12 weeks after which up to a dozen perfectly formed young of about 4 cm in length are born. In fact, their reproductive organs are unique to their subfamily and this characteristic is only shared with the related genus, Jenynsia, the 'one-sided livebearers', but we'll save that for another time…