There's no denying we regard sea stars as beautiful creatures, but, until recently, scientists haven't been very clear about how they see us. Overturning a long-held scientific view, new research has just revealed the secrets of how brittle stars really look at the world around them.
An international team of researchers led by Lauren Sumner-Rooney, a research fellow at Oxford University Museum of Natural History, has shown for the first time how brittle stars (Ophiocoma wendtii), also know as red ophiocoma, find their way around their habitats on the coral reefs of the Caribbean.
And it hasn't been as easy as you might imagine. After all, at first glance, sea stars don't appear to have anything even resembling eyes.
Despite this obvious disadvantage, Ophiocoma have a strong dislike of bright lights, they exhibit dynamic changes in color depending on the time of day or night, and they don't hesitate to seek shelter when a predator's shadow falls across them. So what's going on?
For a long time, biologists thought the brittle star might have a compound eye, like those found in many species of crustaceans and insects. A compound eye is made up of many small visual receivers (unlike human eyes that consist of just one), from which the brain collects and processes information in order to form an image.
The sea star has hundreds of tiny microscopic bumps on its arms, and it was thought each of these probably contained a tiny "micro-lens," each one part of the creature's eye. However, no actual evidence for this theory was ever found.
The research team — which included experts from Oxford University, the Smithsonian Tropical Research Institute in Panama, the Museum für Naturkunde in Berlin, and Queen's University Belfast — used specially designed fluorescent antibodies to examine the sea star at a molecular level.
They were able to prove that, rather than lenses, Ophiocoma wendtii is covered in light-sensitive cells — and not just on its arms. They cover the creature's entire body.
"We were very surprised to find these photo-receptors in such huge numbers and all over the animals," said Sumner-Rooney, whose report was published in the Proceedings of the Royal Society. "They really do cover every inch, top and bottom, and they're very much dotted around at the surface in between what we previously thought were lenses, not underneath them. They certainly wouldn't receive any focused light."
This means that, even if the sea star does form an image, it's likely to be a very rough one — probably not much more than light and shade. The sea star seems to have no difficulty spotting trouble, though. A looming shadow can send it hurtling for the safety of a cave or hollow.
However, while they do have a much clearer picture of how the brittle star operates, even now the scientists are still not completely sure how it processes all the information from the myriad cells.
"It's amazing that we've had this lens-based hypothesis for several decades," Sumner-Rooney said, "yet it seems that the actual mechanism behind Ophiocoma wendtii's incredible behavior is even stranger than we thought."