Do Sea Urchins Have Eyes? Seeing With the Whole Body

How can an animal see without eyes?

Seeing starts with light-sensitive cells called photoreceptors. They contain proteins called opsins that change shape when they absorb light, sending a signal to the nervous system. Our eyes, insect eyes and the simplest eyespots all use opsins.

Photoreceptors on their own only tell an animal whether it is light or dark. To see where light comes from, and to make out shapes, the cells need shading. In an eye, a cup of dark pigment, a lens or a tube blocks light from most directions, so each photoreceptor sees only a small part of the scene.

Sea urchins have no such organs. Yet when the genome of the purple sea urchin was sequenced in 2006, researchers were surprised to find several opsin genes, including types used in the eyes of other animals. Several of them turned out to be active in the tube feet, the small water-filled tubes that urchins use to walk and grip.

Light sensors in the tube feet

In 2011, a team led by Esther Ullrich-Lüter located one of these opsins, called Sp-Opsin4, in the purple sea urchin. It belongs to the same family as the opsins in insect eyes and the melanopsin found in some cells of the human retina.

The opsin sits in photoreceptor cells grouped in two clusters on each tube foot: one around the rim of the sucker-like disc at the tip and one at the base, where the tube foot leaves the shell. Each tube foot carries up to about 140 of these cells, and an urchin has many hundreds of tube feet.

The cells have no dark pigment of their own. The researchers suggested that the opaque calcium carbonate skeleton does the shading instead, blocking light from behind and from the sides, so each cluster receives light from a limited range of directions.

The whole body as a compound eye

An insect’s compound eye is made of many small units, each pointing in a slightly different direction and shielded from its neighbors. Together they build a mosaic image.

A sea urchin may work in a similar way. Thousands of photoreceptors point outward in all directions from the round body, and the spines and test shade each one from light coming from most angles. The urchin would then be one big, all-round compound eye, with the spines setting the sharpness of its vision.

That idea makes a clear prediction: urchins with more closely packed spines should see better. Experiments support it:

For comparison, a person with normal eyesight can resolve detail of about one sixtieth of a degree. Even the purple urchin’s best vision is roughly 600 times coarser, enough to notice a rock, a crevice or a large looming shadow, but nothing like a sharp picture.

What do sea urchins use their vision for?

Low-resolution vision is still useful for an animal that moves slowly over the seabed:

• Finding shelter. Most sea urchins avoid bright light and head for dark crevices, ledges and the undersides of rocks, where they are safer from predators and waves. In arena experiments, urchins steer toward dark targets of the right size.
• Spotting danger. Long-spined Diadema urchins swing their spines toward a sudden shadow, pointing their sharpest defenses at a possible attacker.
• Shading themselves. Many urchins, including the purple sea urchin in tide pools, pick up pebbles, shells and bits of seaweed with their tube feet and hold them over their bodies, especially in strong light. This covering behavior probably protects them from sunlight and ultraviolet rays.

Other senses of a sea urchin

Vision is only one of a sea urchin’s senses, and probably not the most important. Its skin is packed with sensory cells, especially on the tube feet, spines and pincer-like pedicellariae, and around the mouth.

Sea urchins are very sensitive to touch and to chemicals in the water. They can detect the scent of food such as kelp, and of predators: purple urchins wave their spines and move away when a sunflower sea star approaches. Most species also have small balance organs called spheridia, which help them sense gravity and tell which way is up.

All this information is handled without a brain. A nerve ring around the mouth, inside the jaw apparatus described in What is Aristotle’s lantern?, connects to five nerves that run along the body. Each spine and tube foot can respond locally, and the whole animal moves as a loose team. Jellyfish manage in a similar way, as explained in Do jellyfish have brains?

How other animals without a brain see

Sea urchins are not the only echinoderms with unusual vision. Most sea stars have a true, if simple, compound eye at the tip of each arm. In sea stars such as the ochre sea star, it is a tiny cluster of light-sensitive units at the very end of each arm, often colored red or orange. Some tropical brittle stars also detect light with photoreceptors spread across their arms and use it to find dark hiding places.

Box jellyfish go further, with 24 eyes of several kinds, some even with lenses, described in Do box jellyfish have eyes? Animals without brains have evolved many ways of using light, and the sea urchin’s eyeless, whole-body vision is one of the strangest.