Some sea stars channel light like fiber-optic cables

Chocolate‑chip sea stars are famous for the rows of dark, pointy bumps that cover each of their arms. The animals look like they’re dotted with chocolate chips. But what’s inside their arms is even more surprising, a new study finds.

Researchers have discovered tiny structures deep inside the skeleton at the tip of each arm. These channels work like mini fiber‑optic cables, the same technology that carries internet signals around the world. In a sea star, the bony structures collect and channel light deep into the animal’s body, which might help it find its way around if enough light can get through its skin. “The overall effect looks like short fiber‑optic cables made out of their skeleton,” says Sönke Johnsen, a biophysicist at Duke University who studies how animals sense light. He did not take part in this study.

The chocolate‑chip sea star (Protoreaster nodosus) lives in shallow tropical waters. Its five arms grow out from a central disk, and a network of small, interlocking plates make up its skeleton. Each plate is made of a chalky mineral called calcite, a form of calcium carbonate that is also used in some classroom chalk. At the tip of each arm sits a heart‑shaped piece called a terminal plate. Ling Li, a materials scientist at the University of Pennsylvania, was not looking for light‑guiding structures when he began studying these animals. He wanted to know how sea stars build such strong calcite skeletons, a mineral that is weak and brittle.

Li’s team scanned the terminal plates with an X‑ray machine. The scan revealed a surprise: each plate held 90 to 100 cone‑shaped structures that extend deep into the skeleton. All the cones aim toward a hollow space inside the plate. “It seems like something purposely made,” Li says. “Very prominent, very consistent.”

Each cone is built from a single crystal of calcium carbonate. This mineral bends light more than the surrounding seawater because it has a higher refractive index. When light hits the wide part of a cone, it refracts inward. The angle and the high refractive index cause the light to bounce back— or reflect— off the cone’s inner walls. This is called total internal reflection, the same physical phenomenon that makes fiber‑optic cables work. “It’s the same physical phenomenon as fiber optics,” says Pupa Gilbert at the University of Wisconsin–Madison, who studies biomineralization but was not involved in the work.

Each cone transmits about 70 percent of the light that enters it. As light reaches the narrow part of a cone, it’s concentrated and intensified. All the cones in a terminal plate work together, gathering light from one‑third of the field of view around it. They funnel that light into a cavity below, amplifying its intensity as much as eight times. Li’s team tested the cones in the lab to confirm they channel light. When they shined light on a terminal plate, a bright spot appeared in the cavity below. Other bits of the sea‑star skeleton had no such effect; light shined on them simply scattered. Only the terminal plate’s cones focused light this way.

Li and his team shared these findings on 8 June. Their work appears in the Proceedings of the National Academy of Sciences.

Sea stars move toward light, a process known as phototaxis. Li’s team thinks the cone arrays might explain how they do this. A sea star could compare how much light reaches the light‑collecting plates at the tips of each arm. “They can compare the illumination level in different directions,” Li says. “So they can detect which side is darker, which side is brighter.” The brightest light would point them to the light’s source. However, no one has yet found light‑sensitive cells beneath the cones. These would make the light signal useful to the animal. Right now, Gilbert says, “The light detector is the part that’s missing.” Samuel Powell, who has spent years studying vision in marine animals, says the optical stuff is interesting but “doesn’t seem biologically relevant, as far as I can tell.” A simple experiment could help show whether sea stars use their cones to detect light. “Just shine light at the animal. Does it respond?” If it does, that would suggest those cones indeed help the animal sense light. Li’s team plans a study that covers four of the sea star’s five arms to see if one is enough to navigate toward light.

Even if the light‑sensing idea doesn’t pan out, the cones have one confirmed benefit: strength. Computer models show that these structures about triple the stiffness of the terminal plates compared to other parts of the skeleton. “A three‑times improvement in strength without adding that much material is pretty neat,” says Powell. Where this bonus strength shows up may be strategic, Johnsen says. “This structure is in the very tip of the arm, which looks quite pointy and thorny,” he notes. The tips are the animal’s most exposed parts. They are the first things to scrape against rocks or catch a predator’s attention. “Having the tips of your spines be extra strong seems like it makes a lot of sense,” Johnsen says. When engineers design a material, they may build it to be strong, or to transmit light. Rarely would they try for both at once, notes Gilbert. But the sea star may show a way to do just that. “What excites her most is that the cones may do both jobs—channel light and reinforce the skeleton—with a single structure, made of a single material. You don’t have to make those hard choices. You can find the best compromise and have the same structure perform different functions.”

The discovery shows how a simple animal can use the same tiny structures for two very different jobs. Whether sea stars use the light‑guiding cones to find their way or simply to make their arms stronger remains a question for future research.