Some tropical fish survive by eating the scales of their neighbors

Science · Published 11 October 2026 Swipe more like this →

In short: While most fish focus on hunting smaller prey, a specialized group has mastered a much stranger niche. These carnivorous grazers use everything from blunt-force impact to massive, wide-opening jaws to pry nutritious scales directly off the bodies of other fish.

In the tropics, a small group of fishes has evolved to occupy a highly specialized ecological niche: scale-eating. Rather than hunting for meat, these fish act as extra-large parasites, targeting the protective plating of their peers. Research published on January 17 in the journal Royal Society Open Science suggests that this diet leads to a surprising diversity of biological strategies rather than a single evolutionary blueprint. This unique dietary habit influences both their physical evolution and their general behaviour. Scientists are currently working to understand how this specific food source shapes their development.

The methods used to harvest these scales vary significantly between species. For instance, the piranha species Catoprion mento spends its entire life eating scales. It hunts by swimming behind its prey and opening its massive, Jay Leno-like jaw to 120 degrees, prying scales loose with its teeth. These fish are remarkably efficient; a single Catoprion mento can tolerate nearly a dozen large scales in its stomach at once. This particular piranha species is known for its tendency to live a solitary existence. It is capable of swallowing a vast amount of scales in one single gulp.

Other species, such as the characin fish known as tetras, employ different tactics. The blunt-faced Roeboides affinis uses a more forceful approach, ramming its nose—which is equipped with teeth—into the sides of other fish to knock scales loose. By using CT scanning and iodine-contrast staining at Friday Harbor Laboratories, biologists have discovered that this niche is a mosaic of different body shapes and behaviors, highlighting the immense complexity of life's evolutionary paths. Researchers conducted their investigations on San Juan Island. They utilised iodine-contrast staining to inspect the internal anatomy of the specimens.

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