What Is Aristotle's Lantern? How Sea Urchins Eat

What does Aristotle’s lantern look like?

A sea urchin’s mouth is in the middle of its underside, facing the rock. From outside you can see only the tips of five white teeth poking through a soft, leathery membrane. The rest of the lantern sits inside the shell, or test, like a five-sided pyramid pointing downward.

The lantern is built from five jaws, often called pyramids, each made of two halves fused around one long, curved tooth. Smaller skeletal parts connect the jaws at the top, and the whole frame hangs from a ring of supports on the inside of the test. Most counts give about 40 separate pieces:

Dozens of muscles work the system. Some push the whole lantern out of the mouth and pull it back in, others open the teeth outward and close them together. The muscles attached to the compasses help pump fluid through the urchin’s small gills, so the lantern also helps the animal breathe.

Why is it called Aristotle’s lantern?

The Greek philosopher Aristotle described sea urchins in his History of Animals, written in the 4th century BC. He noted the five hollow teeth with a fleshy, tongue-like structure between them. In D’Arcy Thompson’s 1910 English translation, the mouth apparatus looks like “a horn lantern with the panes of horn left out”.

Lanterns of the time had a frame with thin panes of translucent horn instead of glass. Naturalists later took up Aristotle’s phrase as the name for the urchin’s jaw. Some scholars have since argued that Aristotle was actually comparing the whole urchin shell, with its plates and rows of holes, to a lantern, rather than the jaw alone. Either way, the name stuck.

How do sea urchins eat with it?

Most sea urchins are grazers of algae. A purple sea urchin in a kelp forest catches drifting pieces of kelp with its tube feet and spines and passes them down to the mouth. The lantern then pushes forward, the five teeth spread open and snap together, and the urchin bites, tears and pulls the food inside.

On bare rock, the urchin uses its teeth like a scraper, rasping off thin films of microscopic algae and even the hard pink crusts of coralline algae. Grazing urchins leave tiny five-rayed scratch marks on rock and shells, and paleontologists find similar star-shaped marks in fossils, a sign that urchins were feeding this way millions of years ago.

From the mouth, food passes up through the middle of the lantern into the esophagus and then into a long intestine that loops around the inside of the test. Waste leaves through the anus on top of the body.

When too many urchins graze at once, this efficient jaw becomes destructive. Herds of hungry urchins chew through the stems and holdfasts of kelp and can turn a forest into bare rock, as described in What are urchin barrens?

Not every echinoderm eats like this. Sea stars have no teeth at all and digest prey by pushing their stomach out of the body, explained in How do starfish eat?

Self-sharpening teeth

Each sea urchin tooth is a long, curved blade that grows continuously from its upper end, inside the body, while the tip wears down against rock. This is a little like the ever-growing front teeth of rodents, and quite different from the ribbon of tiny teeth that snails use, covered in Do snails have teeth?

The tooth is built from calcite, the mineral of limestone, arranged as thin plates and fibers glued together by a mineral cement. Its hardest region, the stone, runs down the center and is rich in magnesium.

In 2011, a team led by physicist Pupa Gilbert at the University of Wisconsin–Madison studied the teeth of purple sea urchins and showed how they stay sharp. As the tooth grinds against rock, tiny pieces break off along weak organic layers between the plates and fibers, exposing a fresh, sharp edge of the stone. The tooth sharpens itself as it wears. The researchers suggested the same design could inspire tools that never need honing.

Boring into rock

Some sea urchins use their lanterns as drills. On many Pacific shores, purple sea urchins sit in cup-shaped pits that they have scraped into rock with their teeth and spines over years. A pit gives a firm hold against waves and keeps the urchin wet at low tide.

A young urchin can enlarge the inside of its pit faster than the opening, until it is too big to leave and spends the rest of its life there, eating drift seaweed that washes in. The Monterey Bay Aquarium notes that urchins can even wear into steel pilings by flaking away the rust. Other species, such as the Atlantic and Mediterranean rock urchin Paracentrotus lividus and tropical Echinometra urchins, also bore into rock.

Not all urchins have a lantern

The lantern belongs mainly to regular sea urchins, the familiar round, spiny kinds. Sand dollars, flattened relatives that live on sandy seabeds, keep a modified, flattened lantern for crushing tiny food particles. Heart urchins, which burrow through sand and swallow sediment, lose the lantern altogether and gather food particles in strings of mucus moved by tiny beating hairs.

Engineering inspired by the lantern

The lantern’s design has attracted engineers. In 2016, a team at the University of California, San Diego, led by Michael Frank and Joanna McKittrick, studied the jaws of pink sea urchins and built a claw-like sampler with five curved teeth that close together. Mounted on a small remote-controlled rover, the device scooped up sand and simulated Martian soil. The team suggested that a tool like this could one day collect samples on other planets, although it remains a prototype.