Species: Pantodon buchholzi
Habitat: still waters such as swamps and creeks in west Africa, very much doing its own thing
We’ve all heard the saying “If it ain’t bust, don’t fix it”. It turns out there’s an animal that takes it to extremes.
Tens of millions of years ago, the African freshwater butterflyfish evolved a distinct body and lifestyle, and it has steadfastly clung onto them ever since, despite dramatic environmental shifts and changes in its own genes. It is one of the most compelling living fossils – species that have stayed the same for millions of years.
To find a butterflyfish, you must go to the central Congo or lower Niger, and keep a close watch on a still body of water. Insects will be skittering over the surface, and every so often one of them will vanish with a “plop”. That is the butterflyfish at work.
It is a surface hunter, shaped by evolution to hover just beneath the boundary between water and air, where it picks off anything unlucky enough to catch its notice. To pull this off it has developed a host of tricks that have seen it through millions of years of tumultuous change.
At first sight it looks like an evolutionary mistake. Its pectoral fins, directly behind its head on either side, are enormous and resemble the wings of a butterfly. Slightly further back on its lower surface, the two pelvic fins each have four long threads dangling from them, rather like the tentacles of a jellyfish.
The butterflyfish is one of the few fish that can breathe air, which it does using its swim bladder, an internal organ filled with gas. Most fish with swim bladders use them to maintain their buoyancy, allowing them to stay at the same depth without effort, but the butterflyfish also uses it to take in oxygen.
Its eyes sit on the surface, half above and half below. As a result it can seeboth above and below the water, and monitor reflections on the underside of the water surface. This is trickier than it sounds, because light behaves differently in water and air, and the butterflyfish’s brain has an extra cluster of neurons to process the extra information.
What’s more, using sensors on its skin the butterflyfish can detect tiny wavesin the water produced by insects moving around on the surface, allowing it to work out how far away they are and in which direction. Once an insect has been captured it has little chance of escape, because the butterflyfish has a mouth containing three jaws: a lower jaw and two upper jaws. One will clamp onto the insect, perhaps by its leg, while another begins tearing it apart.
Sébastien Lavoué of the Natural History Museum in London and colleagues wanted to know how long the butterflyfish had been living its peculiar lifestyle, so they decided to look at how much it had evolved.
There are two major populations, which the biologists compared by looking both at fish body shape and genes. The two sets of fish were hard to tell apart based on shape, but 15 per cent of the genes they compared were different.
The genetic differences were so large that the two populations might not be able to breed with each other. They split from each other at least 57 million years ago, and possibly as far back as 100 million years ago when the dinosaurs were at their peak.
The butterflyfish is not closely related to any other species, but it belongs to a large group called the bonytongues, which contains a mishmash of primitive fish such as freshwater elephantfish and knifefishes. When they compared the butterflyfish to some of these relatives, they found that it had the slowest rate of change in body shape, despite a perfectly typical rate of genetic change.
Nobody knows why the butterflyfish has stayed the same for so long. It might be that it has become so well-adapted for its lifestyle that any change is harmful. If that’s the case, we can expect it to stick with what it knows long after humans are gone.
Journal reference: Proceedings of the Royal Society B, DOI: 10.1098/rspb.2010.1639 (in press)