Star gazing from the sea floor

STRI/DICYT The kiddie pool Lauren Sumner-Rooney unfolds on the dock at the Bocas Del Toro Research Station isn’t for relaxing at sundown after a long day of fieldwork. Under the midday sun, she places brittle stars on the pool’s blue plastic bottom and floats a shadow-casting disc on the water’s surface. Her experiment tests the nocturnal creatures’ poorly understood vision system. Can they “see” the disc and move into its protective shadow?

Sumner-Rooney’s trials suggest they can. During a six-week, short-term STRI fellowship, the Ph.D. candidate from Queen’s University Belfast ran hundreds of light-response experiments on the brittle star Ophiocoma wendtii. With specimens she took back with her to Northern Ireland, she’ll now build the first digital, 3D model of the species’ unique vision system—thousands of light-focusing, crystalline bumps on its five, spindly arms and body. She’ll also map the complex web of the brittle star's photoreceptors and nerve connections.

“They don’t have a centralized brain,” said Sumner-Rooney, who gathers her test subjects as she snorkels around Bocas. “They can regenerate their arms if they’re broken off, and each one is probably self-governing. There is so much information coming in that the integration of it must be really weird!”

Brittle stars, along with starfish and sand dollars, are radially symmetrical, “spiny-skinned” animals known as echinoderms. In 1984, Gordon Hendler, now the echinoderm curator at the Natural History Museum of Los Angeles, discovered O. wendtii’s eye structures as a visiting researcher at STRI. Brittle stars have the most advanced visual system documented in this group of animals. Although many studies since then have considered the optical properties of O. wendtii’s lenses, Sumner-Rooney’s Ph.D. project is one of the first to examine its photoreceptors and their connection to the rest of the nervous system.

Her field studies measure O. wendtii’s sensitivity to different wavelengths of light, including blue light, to which it reacts, and red and UV light, which does not appear to generate a response. Her pool experiments test whether the brittle stars can distinguish not just night from day, but also distinct objects. She hopes to answer whether O. wendtii’s vision is similar to that of compound-eyed insects, or whether it perceives the world in an entirely different way.

How to see is an evolutionary problem that has been solved independently numerous times. To better understand O. wendtii’s unique solution, Sumner-Rooney studies the overall evolution of vision in the genus Ophiocoma, which includes three other species of brittle star that are less responsive to light.

“It’s a lot of weird, pioneering and not always applicable science,” she said. “But because their vision is theoretically so straightforward, it could have biotech implications.”