Sunday 28 August 2016

Four-eyed Fish or Anableps

Two quite different fishes are sold as four-eyed fishes, the three livebearing Anableps species and the Indian mullet Rhinomugil corsula. Although only distantly related, these fishes are celebrated examples of evolutionary convergence: the way different animals can evolve similar bodies to live a similar lifestyle.

Species

There are three known species of Anableps, one from the Pacific coastline of tropical South America, Anableps dowei, and two from the Atlantic coastline, Anableps anableps and Anableps microlepis. Most of the fish seen in aquarium shops are collected from localities along the Atlantic coastline, but Anableps anableps and Anableps microlepis seem to be about equally widely traded. Telling them apart is very difficult and depends upon counting the number of scales along a certain part of the flank. Since this isn’t really practical for most aquarists or for that matter fish collectors, wholesalers and retailers, it is wisest to assume that any fish traded as Anableps anableps could be Anableps microlepis, and vice versa.

All three species get to about 30 cm in length and share the same basic colouration.

Habitat

Anableps inhabit both freshwater and brackish water habitats, but always in very shallow water. They occupy a very specific ecological niche, foraging on the muddy and sandy areas covered with water as the tide moves forwards and backwards. The feed by diving down and skimming algae and whatever organic detritus they can find on the substrate. To avoid being taken by predators, they periodically rise to the surface, using their distinctive split eyes to look out for both aerial predators (such as seabirds) and aquatic predators (such as bigger fish). When they aren’t feeding, these fish fish will rest on the substrate, often in such shallow water that their backs are barely covered. This keeps them relatively safe from aquatic predators, but should they see an aerial or terrestrial predator coming towards them, they will quickly dart back into deeper water.

Anableps rarely stray far from their favoured river delta environments, and this will need to be considered when these fish are being maintained in captivity.

Maintenance

Any aquarium designed for Anableps will need to offer a sandy area for them to root about in, a flat surface upon which they can rest, and some space above the waterline so that their eyes are not too close to sources of heat or bright light. In very large systems, such as those in public aquaria, Anableps can be combined with mudskippers, the two species having much in common in terms of maintenance requirements, but hobbyists will likely find a simpler system designed exclusively for Anableps somewhat easier to build.

The classic Anableps aquarium is a large tank (at least 200 litres) equipped with a strong filter and about half to two-thirds filled. This leaves some open space at the top of the tank. A mixture of silica, river, and/or coral sand can be used to create a substrate deep enough to look attractive but not so deep it is difficult to clean. The addition of a few Melanoides livebearing snails might be useful to keep the sand clean and well oxygenated. The water needs to be brackish, but the precise salinity isn’t critical, and anything in the low to mid range should be adequate; 1.003 to 1.010 at 25 degrees C is suggested.

A resting shelf will need to be provided in the tank. This is where the Anableps can ‘haul out’ when they need to rest. The simplest arrangement would be a smooth flat rock, such as a slate, balanced on a couple of clay flowerpots. Creative aquarists could build equivalent structures using sheets of perspex and silicone sealant, perhaps at one end of the tank so that there was more swimming room. Alternatively, commercially made structures designed for terrapins (turtles) could be used instead. It really doesn’t matter what you use, so long as there’s a nice flat, smooth structure big enough for your Anableps to rest on positioned just deep enough the fish can swim on and off easily.

Water temperature isn’t critical but should be fairly warm; anything in the range 25-30 degrees C seems to work well. Make sure the heater isn’t positioned somewhere that it might be used as a resting shelf. Ideally, put a heater guard around the heater, or else position it outside the tank, for example in the sump.

A lid is essential! These fish are notoriously good at jumping, and will throw themselves out of their aquarium if alarmed.

Diet

Healthy Anableps are voracious and will eat good quality pellets without complaint. This diet can be rounded out with krill, live brine shrimp, bloodworms, livebearer flake, etc. Floating foods are particularly favoured, and sometimes they ignore food on the substrate, so take care to clean away uneaten food promptly, for example by using a turkey baster to pipette it away.

Social behaviour

Anableps are social and should be kept in groups, preferably five or more specimens. They coexist reasonably well with other fish, provided their tankmates are peaceful and non-territorial. Mollies in particular make excellent ‘dither fish’ encouraging the Anableps to settle into the aquarium more quickly than otherwise. While Anableps are not predatory as such, very small fish may be snapped at (and this includes their own offspring).

Breeding

Anableps are livebearers, the males using a modified anal fin to direct sperm into the vent of the female. One problem is that the anal fins of males either bend to the right or to the left, and the vents of the females seem to have a similar degree of handedness. So not all males are compatible with all females, and a reasonably large group of specimens will need to be bought if you want any chance of securing a compatible pair.
Another problem with Anableps is that pregnany doesn’t progress as easily with them as it does many other livebearer. Miscarriages are common, and some aquarists have reported adult Anableps attacking and killing newborn fry. Isolate newborn fry when found and rear them in their own aquarium.

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