Picture the arch-nemesis of a fearsome piranha. What comes to mind? A giant, ferocious fish with teeth even more menacing than those of piranhas? A spiny, turtle-like fish that is as hard as a rock? Maybe something like an electric eel that could fight teeth with electricity? Whatever you're imagining, it's probably not the tiny, awkward, three-striped cory catfish, which can withstand bite after bite after bite from piranhas and then swim away. While cories do have some nifty armor, which consists of two rows of scutes – large, overlapping bony plates – on each side instead of typical fish scales, at only 2 cm long, cories are generally unassuming. A recent publication by a team led by Andrew Lowe of California State University, Fullerton, USA, reveals the secrets of the cories’ surprisingly impressive defenses.

To determine the strength of cory scutes, Lowe and his colleagues used a fancy hole poker, called a materials tester, which consists of a needle that moves slowly into a material as it records the resisting forces. They found that the catfish's scutes are remarkably tough and can withstand more than 4 N of force before being punctured, although the mighty arapaima, which is also renowned for its thick scales, can withstand 150 N. To put things into perspective, these scrappy little catfish are more than 10,000 times smaller than giant arapaima, but it only takes about 40 times less force to puncture cory scutes than arapaima scales. Pound for pound, I would take cory armor over arapaima armor any day.

In addition to testing individual detached scutes, Lowe and colleagues tested a series of overlapping but detached scutes and intact scutes on the bodies of catfish to see whether the configuration of the bony structures matters. They found that attached scutes take more energy to break than detached scutes. This is because of the way that the scutes in each row overlap, while the tips of the scutes from the upper row interlock with the tips of the scutes from the row beneath in a herringbone pattern. This overlapping pattern allows the force from the tip of a piranha's tooth to be spread out over a larger area. The overlapping and interlocking pattern of the scutes make them act as a single tough piece of armor, distributing the pressure from a piranha's sharp teeth, while remaining flexible and jointed to allow for easy swimming.

By poking scutes from both sides, Lowe and his team also found that the internal surface of the scutes is stiffer than the outside surface. This allows the scutes to function like a bike helmet, with a stiff layer to protect against scrapes and cuts and a softer layer to provide padding that reduces internal injuries. Furthermore, Lowe found connective tissue mixed in with these layers, making them less likely to catastrophically break even if they are punctured.

When people think of effective armor, they usually think of the biggest, heaviest, thickest outer protection, like that of medieval knights. However, different types of armor defend in different ways against different types of weapons, which is why modern soldiers wear Kevlar vests instead of plate armor to defend against bullets. Cory catfish armor may not be the bulkiest of defenses, but it works extremely well against piranha teeth by redistributing puncture forces to a greater area, in much the same way as chainmail protected knights from sword slashes.

A. P.
R. P.
E. M.
Scale performance and composition in a small Amazonian armored catfish
Corydoras trilineatus. Acta Biomaterialia