The parasite muncher

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While they’re striking enough to look at, and have a quirky swimming style, the cigar-shaped characins of Anostomus have another surprising trick up their sleeves.

In the English translation of Baensch’s Aquarium Atlas there is a most curious statement that had always intrigued me: “Anyone who has not kept this fish cannot be called a true aquarist.” The fish in question is Anostomus anostomus, known colloquially as the Striped anostomus, a South American characin that’s neither particularly rare nor notably difficult to keep. So, what is it about this fish that makes it so special?

To be honest, I’m not entirely sure. It’s slightly more difficult to keep than the average community fish, but nothing that even a moderately experienced aquarist can’t handle. 

True, it’s an oddball in terms of appearance, but still attractive and colourful, with distinctly quirky behaviours that make it a fun fish to watch. In short, it bundles a pleasing mix of aesthetics and personality that anyone interested in fish biology is going to find charming.

Species checklist

Five Anostomus species are currently recognised, but only two are traded on a regular basis: Anostomus anostomus and Anostomus ternetzi. Other species have been included in the genus Anostomus in the past, but these have now been reassigned as the evolutionary history of the group has become more clearly resolved. 

For example, the Banded headstander is sometimes imported as Anostomus taeniatus, but is more properly called Laemolyta taeniata.

In any event, by far the most commonly sold species is Striped anostomus, Anostomus anostomus, imported from Suriname and Guyana since the 1930s. 

It’s said to be more abundant in the Orinoco river basin than the Amazon, and so it’s not as frequently seen in shipments out of Brazil or Venezuela. Adults are 12-15cm in length, with bold black and yellow stripes along their bodies and red patches on their unpaired fins.

Anostomus ternetzi is less commonly seen. Compared with Anostomus anostomus, this species is a little smaller and has less red on its fins, but is said to be less prone to aggression. Still, in twos and threes you’re still likely to see a certain amount of territoriality, so like the Striped anostomus, Anostomus ternetzi is best kept either singly or in groups of five or more specimens.

There’s a fair amount of variation within both species, which has led some authors to theorise that we may be looking at species complexes rather than just the two species listed here. But is there any significance to their striking colouration? It’s been speculated that these fish evolved their cigar-like shape and striped colouration to confuse potential predators. Both ends of the fish look the same, and the eye is well hidden, by the horizontal banding, which makes it difficult for a predator to know which way the fish will swim
if startled. It’s a trick we’re more familiar with in fish that have eye-spots (called ocelli) on their tails, misleading predators into attacking the ‘wrong’ end of the fish.

One thing to look out for is what happens at night. Both species change their colours after dark, becoming much paler. This is something aquarists will have seen
if they’ve kept pencilfish, and presumably helps them hide from nocturnal predators more effectively.

Turned on their heads

What all Anostomus have in common is a small, upturned mouth (what we technically call a ‘superior’ mouth). Normally, fish with upturned mouths feed from the surface — think about the likes of guppies, hatchetfish, gouramis and halfbeaks. 

But that isn’t what’s going on here. Instead, Anostomus swim about slowly in all sorts of unusual configurations, but usually head-downwards. This is why they (and their close relatives such as Abramites and Leporinus) are sometimes referred to as ‘headstanders’.

This way of swimming seems to be related to their habitat. In the wild they live in places with strong water currents and rocky substrates. Their long, narrow shape is streamlined for when they need to swim quickly, but it’s also well adapted to poking around in narrow crevices. The aquarist can encourage this foraging behaviour by putting lots of rocks or bogwood in the tank, but the fish will poke about the roots of plants (even floating plants) just as readily.

Should they need to pick up the pace a bit, they’ll level out into a more typical orientation, and can swim remarkably fast if they have to. Still, their preferred swimming orientation is head-downwards, and they look best when kept in tanks at least 45cm in depth.

Decent filtration is important. Clear water allows light to penetrate water better, and this promotes the growth of the green algae these fish like to eat. Robust filtration also creates the strong (though not turbulent) water currents they enjoy. Water quality should be good (free of ammonia and nitrite, with minimal nitrate content), of course, but Anostomus are not delicate fish by any means.

In good conditions they can live a long time, well over ten years.

Water chemistry should tend towards the softer end of the range; 5-12˚dH, pH 6.0-7.5 works well, but they can handle slightly harder and more basic water if they must. They aren’t too fussed about water temperature, and somewhere around 25˚C is fine. If you must keep them a degree or two cooler or warmer than that because of their tankmates, that won’t bother them at all.

Read the rest of the feature in the January 2022 issue. Buy the latest digital edition and read instantly on your computer, mobile or tablet device.


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