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What a Brittle Star “Sees”

How these echinoderms make sense of their underwater world

A brittle star in the Echinoderm Collections at NHMLA.

 

Brittle stars aren’t afraid of the dark. If you drop one of these echinoderms (a group that includes sea stars, sea cucumbers, and sea urchins, among others) into a half-shaded tank, it will quickly scuttle into the shadows. This preference for darkness serves them well in the ocean, as they hide from predators in crevices.

this animated gif shows a brittle star dropped into a half-shaded tank quickly moving into the shadow
This brittle star quickly moves toward the shadowy side of the tank.

But how do they know where it’s light or dark? It’s not like they have eyes. Or do they… sort of?

Some species of starfish have “eyes” at the ends of their arms, but similar structures hadn’t been found in brittle stars, so it was unclear how — and how well — they could “see.”

In the early 1980s, Gordon Hendler, NHMLA’s curator of echinoderms, first suspected that brittle stars might be uniquely equipped to sense light. He had collected specimens of a Caribbean brittle star and noticed they were different colors at different times. During the day they were dark brown, but each night their arms had grey and black bands.

“Years before this, I thought these might be two different species of brittle star,” said Hendler. “But then I noticed they were slowly changing color at sunset and dawn.”

a photo of two brittle stars in a box. one is all brown, but the other has grey and black stripes on its arms
The brittle star Ophiocoma wendtii on the left was preserved during the day, so it’s brown, but the one on the right was preserved after sunset, so it has grey and black stripes.

As Hendler examined the structure of the brittle stars’ arms, he made some amazing discoveries. For one, it had amoeba-like pigment cells in the animals’ skin that could move and change their shape, and in doing so, affect the color of the brittle star. And as he looked closer, there was more: these amoeboid pigment cells were shape-shifting around never-before-seen structures on the surface of the skeleton. These structures interested Hendler because they were both rounded and completely transparent — “as clear as glass,” said Hendler. They looked an awful lot like lenses.

He wondered if these pigment cells were regulating how much light passed through the structures that looked so much like lenses — similarly to how the pupils of our eyes shrink if it’s bright outside, but open wider and admit more light when it’s dark. He further speculated that underneath these lenses there might be photosensitive tissues — cells that could sense light, much like the cells of our retina do. For the first time, it seemed possible these brittle stars had all the elements of an eye — with the same functions as our lenses, pupils, and retinas.

Indeed, in the following years, Hendler and research colleagues found evidence to support his hypothesis. There are, in fact, light-sensitive nerves beneath the transparent lens-shaped structures, and these nerves responded differently to light depending on the coverage of pigmented cells.

And finally, a team of scientists including Hendler discovered that these lens-like structures are indeed optically sophisticated double lenses. They don’t just look like lenses; they are lenses. Just like Hendler had long suspected, these oh-so-very-tiny lenses precisely focus light onto special nerve cells just below themselves within the brittle star’s arm, which enables the animal to tell the difference between light and dark.

This means nearly the whole body of the brittle star might function as a compound eye (something like a fly’s eye). And in case you’re wondering, it’s possible that brittle stars can put all this information together, rendering an image of the surroundings. Whether or not they can truly “see,” though, is a whole other story.

a brittle star sits on the stage of a microscope, with emphasis on its arms
The lenses dot the brittle star's shield-shaped arm plates, which run down the middle of each arm like roof shingles.
a view of a brittle star under a microscope shows dots on its arm plates
Under a microscope, you can just barely make out bumps on each of the arm plates. In comparison, the arm spines are quite smooth.
a black and white photo of the brittle star lenses up close shows rounded structures and spaces in between
This high-magnification scanning electron microscopy clearly shows rounded lenses with spaces in between. These spaces enclose amoeba-like pigmented cells that extend upward and cover the lenses or move downward into the holes, causing the color changes Hendler first noticed.

This new paper, published in the journal Science, takes a much closer look at the structure of these lenses, which are made mostly of calcium carbonate, or calcite — a notoriously brittle material. But in these echinoderms, the calcite skeleton is surprisingly strong. These little underwater engineers, over millions of years of evolution, found a way to make a material that offers them strength, yet flexibility, with light detection as an added bonus. That’s some serious material multitasking.

According to the study, the lenses appear to be strengthened by the presence of magnesium atoms locked within the skeleton’s calcite crystals. It seems that layers of magnesium-rich particles give the calcite a tempered-glass-like quality that resists cracking.

Most impressively, these brittle stars manufacture this wonder material at roughly “room” temperature (even though there aren’t really rooms in the ocean) using only a few elements, an impressive feat that human engineers would be all too happy to mimic — that is, when we more fully understand how these clever echinoderms do it.        

 

 

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