Predators: The Butterfly fish

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Not all butterflies flit gently. Nathan Hill homes in on a bizarre waterborne namesake that moves with lightning reactions.

Beneath the surface of still waters a wide, wedge-shaped brown fish hangs motionless. It views the world above and below with diligence, ready to pounce on food or launch itself like a missile at the first hint of trouble.

This is the Butterfly fish (Pantodon buchholzi) and a common sight in any store professing to know anything about curiosities. Its ease of keeping makes it popular with outlets catering for beginners.

At first glance it’s hard to work out which way the fish is pointing. In some cases, newcomers may fail to identify it as a fish at all.

Different league (picture by Isidro Martínez, Creative Commons)

Two anatomical features put Pantodon in a different league: the pectoral fins and the eyes.

From above, massive and triangulated pectorals look more aerodynamic than hydrodynamic, hence the fish’s common name. The fins look beyond sensible proportion, but this is not recent evolution, Butterflies being examples of older teleost fishes.

The fins are part camouflage, part escape route. Spread wide they emulate fallen foliage and with graceful movements the fish can saunter without giving their position away to potential predators or prey.

However, in times of crisis those fins can become ailerons as the fish launches from the water, rising as much as twice its own body length high, before linear flight brings it down and away from the danger.

The pectorals do the catapulting, while most other fish use their caudal fin — the tail — as the main driver.

The eyes are possibly even more spectacular.The internal configuration is genius and, had this fish been created by man, it would have been a combination of Japanese innovation and German engineering.

Pantodon have simultaneous air and water vision. With multiple retinal hemispheres in the eye, a dedicated portion of the visual angle is directed upwards, scanning for insects. Part of the eye is dedicated to viewing straight into the water column and a third section views predators below in a most remarkable way.

Rather than dedicate some of the eye downwards, as in the case of the Four-eyed fish (Anableps anableps), the Butterfly has a further region of retina directed to the water’s surface at such an angle that it views a reflection of what is beneath it. It makes a mirror of the very water it is looking out of.

Pantodon have evolved extra neurons and pathways in the brain to cater for this remarkable ability and such clusters of neurons are unknown in any other fish species.

These eyes, combined with those impressive launching abilities, combine with a response time to danger that we humans can only imagine.

The fish will register a hazard within five milliseconds (ms). Just ten ms later its fins respond, giving a total reflex time of 15ms.  A typical human eye blasted with a puff of air generally takes 30-50ms before the stimulus registers.

This fish has also developed a kind of mechanoreception, currently known in only one other surface-feeding predator.

A Butterfly can detect the distance of prey hitting the water’s surface with pinpoint accuracy. If an insect touches the top of its aquaria anywhere between 5-20cm/2-8” away, it can lunge that exact distance.

Detection waves

It does so with detection performed by the lateral line, and the first few waves in a train that reach it are enough to give a location.  This mechanism stays active at night, and in the aquarium they manoeuvre in the dark towards droplets of water.

Pantodon can be tricked in a laboratory by producing wave frequencies from a surface probe that represent a different distance of a falling object. The fish will base its strike distance on the information received and not the location of the probe.

Pantodon have almost stopped evolving. Scientists revealed that two geographically separated populations split over 57 million years ago and since then had indulged in no change to their phenotypes —the physical body.

It would appear that whether isolated in the Niger basin or the Congo basin, there’s no further requirement to tweak the body to fit in any more successfully.

However their genotypes – the gene sequences – have been subject to massive change, totaling nearly a 15% difference in their mitochondrial genomes.

By organism standards this is huge, and it’s not yet known whether the two populations can actually interbreed. Scientists generally say ‘no’ but anecdotal evidence may hint at the contrary.

Taxonomically, Pantodon buchholzi was always the outsider, with only one species found within its solitary genus. From a categorisation perspective, this is quite the Duck-billed platypus.

These fish can even breathe atmospheric air, expelling and gasping in one sudden movement. Unlike some fish that pass air through a vascularised stomach, Pantodon breathe through their swimbladders, having to exhale exhaust gases. These breaths will take place typically at around 10-30 times every hour.

Atmospheric breathing is a problem for domestic aquarists, increasing opportunities for airborne toxins to enter the fish as well as compounding issues in cold environments.

Pantodon aren’t fans of cold air and a chilly fish house with lukewarm aquaria but an arctic atmosphere can spell trouble.

How to set up home for an oddball of your own

Butterflies are among the easiest oddballs to keep, and setting up a harmonious tank is a project that every inquisitive aquarist should try at some stage.

Found from a range of waters over Cameroon, Gabon, Nigeria, Sierra Leone, Benin, and the Democratic Republic of Congo, their typical habitat will often be generic, although those fish they share their homes with may vary from place to place.

These fish inhabit either very slow moving or static waters, sometimes with abundant surface cover and sometimes with sparse. Decoration and planting is not critical, as they are far more concerned with what happens above the surface than beneath it.

Even if the fish pay no attention, a biotope can still be effective and décor should be steered toward a riverbank with steep inclines, tangled wood and plenty of vegetation.

Anubias species make good choices, as do Crinum natans and Bolbitis plants. Floating plants should be considered, although true African species are rarely seen in the hobby. Opt for some invasive Salvinia and keep surface movement minimal to see Pantodon at their most natural.

Pantodon like substrates to be dark. Coloured gravels are anathema to them and shouldn’t be considered.

Lighting should be dull. Pantodon are crepuscular fish, tending to do nothing when the sun is intense. In a bright tank they will usually stay dormant for long periods, so shading and an abundance of floating plants will help to increase roaming.

Given that bright lights are out, garish tank mates are redundant in the Butterfly tank. Congo tetras are often used, as are Brycinus longipinnis and Micralestes tetra, but the range can be so much more interesting. More fascinating companions can include elephant-nose mormyrids, African knifefishes, Ctenopoma species, and smaller, docile Synodontis catfish.

Small tetra and shrimp are not viable. Pantodon mouths are upward-slanting black holes, capable of fitting in astonishingly big fish and a brave tetra or a shrimp venturing high in the water will be consumed.

Pantodon are not critical of temperature, sitting cosy between 23-30°C/73-86°F. They like water soft and slightly acidic, but not the hostile and corrosive levels of some tetra: 6.2 to 7.2 pH keeps them happy and they approve of tannins.

They don’t get too big, reaching a maximum 12cm/4.7” in length, so a tank of 60cm/24” or more for one for much of its life is fine.

Sexing is easy, but breeding less so. Gender is identified by the shape of the rear edge of the anal fin. In females this is straight, but males are more convex in shape.

Males have a unique copulatory organ with folded structures in pouches covered by a bony plate. After chasing the female, he positions himself above her. Fertilisation is believed to be internal and eggs laid into the water. These clear, floating offspring rapidly turn dark and the female may produce them over several days — and up to 100 eggs per day.

The fry are difficult to raise, initially requiring tiny infusoria. Producing the food isn’t a problem, but getting it in the right place is. Unless it’s at the surface, the fry won’t be interested.

The fry should be removed, lest the parents eat them.

There’s no need to wean adult Pantodon on to dried foods, as they will take them from the offset. However, ensure any food remains on the surface as these fish will not drop into the water column to fetch it. Insects are taken without a thought.

Anything is accepted, with flies, spiders, woodlice and beetles all going smoothly down the hatch.

A true flyer? (picture by Toniher, Creative Commons)

Some scientists believe Pantodon is capable of more powered flight, given the anatomy of the muscles on the trunk and the connections to the pectorals.

However, as much as the fish may kink mid-glide, and though the fins may be tilted and twisted to aim, this cannot be considered flying in the true powered sense.

See some of the other articles in this series:

The knifefishes

The Striped pike characin

Hoplias, wolves of the water

The leaf fish

Clarias catfish

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