If you make one wrong move in parts of the Amazon River, you might find yourself becoming lunch for a gang of piranhas. However, this is not a worry for Arapaima gigas, a 3-meter-long (10-foot) Amazonian fish that’s evolved a novel method of warding off snappy predators: a natural armor of piranha-proof scales that's built like a stab-proof vest.
The scales of this monstrous fish are so strong, they have recently caught the eye of scientists from UC San Diego and UC Berkeley. Through studying the secrets of the scales, they hope researchers can develop stronger and more flexible synthetic armors for the military in the future.
"The structural design of these natural Arapaima fish scales really could inspire researchers to make tough materials in engineering and biomedical fields," Dr Wen Yang, study author and project scientist at UC San Diego, told IFLScience.
Arapaima gigas, known as pirarucu in Brazil and paiche in Peru, are undeniably freakish fish. They can be found in the Amazon Basin, although they have since been introduced to bodies of freshwater in Southeast Asia. They are among the planet's largest freshwater fish and fastest-growing vertebrates, reaching lengths of almost 3 meters (~10 feet). Stranger still, this fish breathes air. This species needs to come to the surface and breathe air every five to 15 minutes to obtain up to 95 percent of its oxygen, with the rest being supplemented by water through its gills.
Reporting in the Cell journal Matter, researchers closely examined the structures of the scales using scanning electron microscopy, revealing that the penetration-resistant scales closely resemble the structure of stab-proof vests.
First of all, the scales are as thick as a grain of rice and caked in a highly mineralized outer layer of collagen. Beyond this, there are also multiple layers of twisted mineralized collagen fibrils that help to absorb deformation. Unlike synthetic body armors, the layers and scales are not held together with an adhesive but are tightly integrated naturally since they all grew together. This flexible hierarchical structure means that any bite or stab will not break or shatter the scale, it will twist and deform it. The thickness of the scales, combined with the flexible overall structure, appears to be the key to being piranha-proof.
"It is tough because the outer mineral layer can protect from the tooth penetration, while the tough and flexible inner layer acts as a buffer to dissipate the energy," added Dr Yang.
"It is like putting concrete in a ceramic," she continued. "The ceramic is brittle but would have high resistance to the knife or compression force with the concrete inside. The combination is much tougher and stronger and even if you dropped it on the ground, it would not fracture. The advantage of the inner Bouilgand structure [a layered and twisted microstructure] is that the crack is not clearly propagated under force. Some of the collagen lamellae [layers] reorient towards the tension direction, and some can slide, stretch, and delaminate under the force."
The IUCN Red List states there is not enough data to accurately assess the species’ population numbers, however, they are known to be faced with severe threats. For one, their habit of surfacing regularly to gulp air makes them easy picking for fishers. A report from earlier this year found that Arapaima gigas scales are often used as a cheap substitute for leather.
In fact, if you bought a cheap pair of leather cowboy boots in the US before 2000, there’s a chance they are actually made from the scales of this bizarre fish.