The arapaima is probably the luckiest fish in the amazon. It’s scales have found to be impenetrable to piranha teeth. They have been described as “living fossils” as they retain lungs and therefore still need to breathe air. The enormous fish can be over 8 feet long and weight over 400 pounds.
In an experiment conducted by Mark Meyers (University of CA) and his colleagues,
To mimic an actual Amazonian chomp, they cemented a piranha tooth to a machine that delivered a bite to several arapaima scales. The fang didn’t just ricochet off the armor, it shattered, the team reports online in Advanced Engineering Materials.
To learn just how the arapaima does it, the researchers had to take a closer look at the scales themselves.
So the researchers took a closer look at the scales using a scanning electron microscope and similar tools. The scales, they discovered, are double layered. On the inside, they’re made from sheets of collagen, a tough-yet-springy material found in joints and bones. But their exteriors, or the sides exposed to piranha bites, are rock hard. These surfaces are also made from collagen, the team realized, but those fibers have been cemented together with a mineral also popular in bone: calcium.
Arapaima scales are compared to our teeth. They may be covered by enamel, but the core is dentine and more pliable. This sort of cushioning can stop the formation of cracks produced by some sort of pressure.
Unrelated to the scales, I wonder if the retaining of lungs by the arapaima has anything to do with little predatory pressure. I’d be interested to see if, perhaps there were some sort of decline in other available food sources, the piranha could evolve to take advantage of the arapaima as a food source. This of course would assume that there would be ample time to develop such capabilities.
- A Piranha-Proof Fish (news.sciencemag.org)
- Piranha vs. Arapaima: Engineers find inspiration for new materials in piranha-proof armor (eurekalert.org)
- Engineers Find Inspiration for New Materials in Piranha-proof Armor (scienceblog.com)