We brought home a new kitchen knife from my parents last month. The knife block was full, but Charlie exchanged the new one for what was previously our smallest and dullest. He wasted no time wrapping the old one up in plastic and hiding it from me. My hand naturally gravitates towards whichever tool will fit nicely inside it, even when I’m cutting a monster squash. We have a good arrangement: Charlie keeps the knives sharp, I keep my fingers, and I toss him the odd carrot slice in return.
But could he eventually be replaced by a sea urchin? A new study in the journal Advanced Functional Materials explains how sea urchin teeth never dull or break. In fact, they get sharper with use1.
Most people are probably familiar with sea urchins as the spiny little balls one occasionally encounters on the beach. Evil looking, but mostly harmless, so long as you avoid stepping on them. Sea urchins live in shallow tidal pools, eating algae and other plant material. So why do they need such sharp teeth? Much like their spines, the teeth probably serve a protective function. The urchins use them to chew burrows, often in solid rock, where they can take shelter from predators and waves.
In the current study, a group of physicists and biologists used an arsenal of sophisticated imaging, chemical and nano-scale stress test procedures to investigate the teeth of the California purple sea urchin (Strongylocentrotus purpuratus). Like starfish and sea cucumbers, urchins are members of a group of animals known for their penta-radial, or five-fold, symmetry. They have five teeth arranged in what is known as Aristotle’s lantern.
Aristotle’s lantern, as viewed from below with teeth closed. From Killian et al. 20111.
This chewing apparatus got its name from Aristotle’s early descriptions of the organ, which takes up a substantial amount of space inside the urchin’s little body. He compared it to a “horn lantern” with the transparent panes left out.
Aristotle’s lantern, dissected from an urchin, showing the set of five teeth. From Wang et al. 19972.
Each tooth has a T-shape with a ridge or “keel” running along the inner surface, and a convex curvature to the outer surface. The researchers knew from previous work that the teeth are composed of an arrangement of calcite crystal plates and fibres2. Using X-ray and scanning electron microscope analyses, they have now shown that the plates and fibres are oriented in a particular way, and that the calcite of the hard central part of the tooth (or “stone”) has a different chemical composition from the material in the outer regions. Their latest results also showed that the weaker matrix in between the plates should act like natural “fault lines”, allowing plates to break off sequentially from the outside of the tooth. This keeps the central “stone” sharp. The authors compare the fault lines to perforated paper, but I think an x-acto blade might be a good analogy.
The outer edge of the sea urchin tooth tip, as seen with backscatter scanning electron microscopy. Plates are shed sequentially with wear, keeping the central “stone” of the tooth sharp. From Killian et al. 20111.
The physicists used a technique called nanoindentation – tapping the tooth with a instrument just a few micrometers wide – to produce “gentle dimples” at different points in the surface, and see how the tooth would break as a result. The nano-scale cracks they induced spread in a predictable pattern: they were deflected away from the sturdy plates and fibres, causing the tooth to break along the fault lines. The researchers also watched live sea urchins to see this process in action, and they recorded a lateral tooth-grinding behaviour in addition to open-and-closed biting. Based on the fact that the plates are shed from the outer surface of the tooth, the authors conclude that tooth-grinding plays a critical role in keeping things sharp.
Apparently this is not the only time self-sharpening teeth have evolved. In rodents, as well as rabbits and their relatives, teeth also grow continuously throughout life. In these mammals, a material design strategy is also behind the self-sharpening process. The enamel on the outer surface of the teeth is much harder than the material on the inner side, known as dentin. Regular gnawing wears down the inner dentin first. On top of this, the enamel of the bottom incisors cuts directly into the dentin of the top ones.
But the sea urchin’s self-sharpening teeth might be especially cool for two reasons. First, there other aquatic animals (such as chitons and limpets) that have very similar continuously growing and self-sharpening teeth. These groups have all evolved independently. If their teeth function in a similar way, it would be a nice example of convergent evolution of material design. Second, the urchins are using teeth made of calcite to chew rocks that are also made mostly of calcite3. Imagine a bone-crunching rodent – these urchin teeth have to be pretty incredible. Killian and his coauthors end with an intriguing speculation: perhaps we could borrow the strategy of pre-determined break points to design better synthetic materials. Given the repeated evolution of self-sharpening teeth in the ocean, I think this could make sense.