Bio-metals: Ancient sea worms hold the secret to a strange new class of materials
Many animals have teeth for cutting, holding, and grinding food before it is swallowed. Humans and fish, for example, have teeth made of the same base material (dentine), but they are covered with slightly different coatings of enamel or enameloid. Rodents like beavers and squirrels have strong front teeth whose enamel contains pockets of iron for strengthy and self-sharpening. Turtles and birds have beaks made of the protein keratin. Crustaceans have a gastric mill apparatus inside the stomach where its teeth and levers (ossicles) do the work. These body parts are made of chitin, much like its exoskeleton, which is a tough carbohydrate material.
Sea worms are a diverse group of worms with body segments like earthworms but which live in sea or fresh water environments. They have many shapes, sizes, and environmental homes.
The Nereis species are generally 5-30 centimeters (2-12 inches) long and about 1 cm (0.4 inches) wide. They are scavengers feeding mostly on small invertebrates or algae. Some of them like the Nereis species burrow into the underwater sand or mud especially near coasts and are classified as bristle worms, clam worms, ragworms, or sandworms.
Mouth parts of the Nereis species of sea worms are held inside the body as it searches for food. Several types of appendages stick out for detection of chemicals and vibrations.
- Tentacles (or antennae) probe ahead of the worm with taste and smell receptors on the ends.
- Cirri are whiskers that sense changes in water pressure, sound, or mechanical pressure in 360 around the head. These are mostly used to detect predators.
- Muscular projections called palps have taste receptors but also help to hold food.
- Slits called nuchal organs have concentrated cilia for long-range scent detection.
When they locate food, the inner part of the mouth comes out like the jaws on the extraterrestrial in the movie Alien. There are 2-4 curved jaws that act like pincers to grab and cut food as they draw it back into the mouth opening.
In 2006, researchers at the University of California Santa Barbera made a cross-section near the tip of one of these jaws. They measured the zinc there and found that it was uniformly distributed through the section. When they removed it with chemical treatment, they noticed the jaw had become soft and more flexible.
- The researchers put the worm's jaw under the instrument.
- They positioned the diamond tip at one of the 11 circles.
- The machine pressed the diamond into the jaw with a very small force.
- It measured how far the diamond penetrated.
- From the force and the resulting indentation, they calculated the material's hardness.
- They made indentations 100–1,000 nanometers deep. A nanometer is one-millionth of a millimeter.
They weren't testing just once at each circle. They performed hundreds of indentations at each location, at different depths—100, 200, 400, 600, 800, and 1,000 nm. They found that the tip was harder than the middle part of the jaw. The two colors of circle just indicates two ranges of hardness. It actually got harder along the jaw up to the tip where it was hardest. Some of that variation is related to the microscopic structure of the jaw—it's made of fibers, and a tiny indentation can land on a fiber or at the interface between fibers, and that affects the measurement.
That fibrous matrix look uneven in composition, not smoothly containing the protein. The same researchers found that metal ions were part of the jaw, connected to the protein fibers and lined up in a pattern called the dislocation line. That line affected the strength of the jaw material to allow it to bend more in certain places. Metal bends, but crystals don't. This arrangements of metals in the jaw acted mechanically somewhere in between a metal and a crystal and was named "bio-metal".
The researchers also took measurements of which types of metals were in the jaw and where they were. As you can see below, the zinc ions were at the tip, where the hardness was greatest.
In a 2026 paper, Japanese scientists used special elemental analysis to measure how much of certain metal ions were in different locations. Look at the numbers on the y-axis of these graphs, and you can see not just where in the jaw the metals are found, but how much more zinc there is than the other metals. It is concentrated mostly at the tip of the jaw and in a high amount.
