An armorlike arapaima fish scale resists being fractured by a piranha tooth that is slowly pressed into it. In fact, it is the tooth that fails. Credit: Meyers Group: Credit: UCSD Jacobs Sch. of Eng.

If you ever watch cable TV’s River Monsters (and, honestly, who doesn’t!), you might be familiar with a large Amazonian “living fossil” fish that goes by the name arapaima.

The arapaima have a reputation for being one of the few animals in the Amazon that hungry piranha don’t bother. Why? Apparently it is because evolution has draped the arapaima in a flexible skin of “armor” that is effectively impenetrable to the piranhas’ teeth. The fish has “scales,” but there aren’t many other species among fish that come close to the defenses of the arapaima.

Researcher Marc Meyers, an expert on bio-inspirational design and a professor at the Jacobs School of Engineering at University of California, San Diego, knew of this reputation and speculated that if the arapaima are indeed protected from piranhas’ bites, maybe insights from the structure of the arapaima’s scales could provide ideas for engineering new materials, such as flexible ceramics.

(Meyers’ name may be familiar to some. He was one of the stars in one of the episodes of Nova’s “Making Stuff” TV series, in which he discussed the strength of mullosk shells.)

As can be seen in the above video, Meyers, along with his students and colleagues, set up a simple desktop test. They attach an arapaima scale to a soft rubber base (to simulate the fish’s soft muscle and tissue under the scale) and mounted a single piranha tooth in a press. The tooth is then pressed into the scale. In each case, the tooth can partially penetrate the scale, but the tooth cracks before the scale suffers a total fracture.

Credit: Meyers Group; UCSD.

Meyers says in a UC San Diego release that the structure of the scale is combination of a mineralized outer layer with a clever and tough internal design (see diagram from the press release). This tough inner layer has collagen fibers stacked in alternating directions “like a pile of plywood.” He says the mix of materials in the scale is similar to the hard enamel of a tooth deposited over softer dentin.

Implications? Meyers says that the arapaima’s design should serve as bioinspiration for lots of things that need to be both tough and flexible, for example body armor, fuel cells, insulation and aerospace designs.

Some lessons for engineers are from this work are:

  • Combine hard and soft materials
  • Stack the materials in the underlying layer with different orientations
  • Texture is key, and a varying surface provides more capability.

On this last point, Meyers notes that each scale has an exterior that is “corrugated.” According to the story, “the corrugated surface keeps the scales’ thick mineralized surface intact while the fish flexes as it swims. Ceramic surfaces of constant thickness are strained when forced to follow a curved surface. The corrugations allow the scales to ‘be bent more easily without cracking,’ Meyers said.”

Meyers says he will also be studying the scales of another unusual fish, the alligator gar whose scales were reportedly used as arrow tips.

Corrugated texture of arapaima scales. Credit: Meyers Group; UCSD Jacobs Sch. of Eng.

Meyers et al. have written about their studies in the The Journal of the Mechanical Behavior of Biomedical Materials in the paper, “Biological materials: A materials science approach” (doi:10.1016/j.jmbbm.2010.08.005).