Take a Bite: Pink Sea Urchins Have Teeth That Sharpen Themselves

Pink sea urchin. (Mike Frank)

(CN) – Sea urchins’ sharp new teeth are a chip off the old block: Research published Wednesday in the journal Matter confirms how spiny echinoderms sharpen their teeth by breaking them off.

“The material on the outer layer of the tooth exhibits a complex behavior of plasticity and damage that regulates ‘controlled’ chipping of the tooth to maintain its sharpness,” Horacio Espinosa, director of the Institute for Cellular Engineering Technologies at Northwestern University, said in a statement.

Biologists have long hypothesized that sea urchins must carry a mechanism to sharpen their teeth, but until now, no one had ever seen it in action.

Half a dozen researchers led by Espinosa examined the teeth from 15 deep sea dwelling pink sea urchins to see how they wear down and sharpen back up.

Previous experiments scraped substrate against teeth, but these researchers aimed to mimic the movement of sea urchin jaws against ultra-nanocrystalline diamond, and captured 3D images and video with in situ scanning electron microscopy micropillar compression tests.

Researchers observed and identified the “load conditions under which the self-sharpening mechanism is activated,” or how much wear and tear a tooth can take before it begins to break.

Five sea urchin tooth tips stacked atop one another within the jaws. (Horacio Espinosa)

Sea urchin feed through a round skeletal muscular system nicknamed Aristotle’s lantern that holds five curved T-shaped teeth, each on its own jaw. The teeth are made of both soft calcite and hard magnesium calcite, which is often more concentrated toward the tip.

While humans grow two sets of teeth built to last many years, sea urchins grow teeth throughout their lifetime that are built to break as they wear down, keeping the edges sharp.

Like a mechanical pencil, urchin teeth are made of perfectly arranged ceramic plates over strong, supportive calcite fibers. As each brittle plate breaks off, a sharp new plate is ready to take over.

In the wild, sea urchins use their teeth to forage for food and burrow into rocks for safety; they need strong, sharp dentures that scrape, grind and bite.

And sea urchins need to chew on hard coral and rocks in order to maintain their sharp teeth, the researchers explained, “in the absence of stone visco-plastic deformation and wearing, local stresses in the plate region, near the contact zone, would not reach the threshold required for plate delamination and chip formation.”

They added, “The material exhibits a combination of hardness, visco-plasticity, and damage that limits the contact stress and leads to wear, in the form of microscale debris, under certain conditions of combined normal and shear stresses.”

The Air Force Office of Scientific Research funded this study, which, in addition to informing better design for cutting, grinding, and boring equipment, may also help engineer stronger human dentures.

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