Starfish use the light-sensitive organs at the tips of their arms to form images, helping the animals find their way home if they stray from the reef.
We have known about the sensors that starfish have at the ends of their arms for 200 years, but no one knew whether they are real eyes that form images or simply structures that detect changes in light intensity.
We finally have an answer: they appear to act as real eyes. The discovery is another blow to creationist arguments that something as complex as a human eye could never evolve from simpler structures.
The blue sea star (Linckia laevigata), which is widely sold as dried souvenirs, lives on shallow rock reefs in the Indian and Pacific oceans. It can detect light, preferring to come out at night to graze on algae.
The light sensitivity has recently been found to be due to pigments called opsins, expressed in cells close to the animal's nerve net.
What has not been clear, says Anders Garm at the University of Copenhagen in Denmark, is whether these cells simply tell the starfish about ambient light levels, as happens in more primitive light-sensitive animals, or whether they actually form spatial images.
To find out, Garm collected healthy starfish and removed the arm-tip photoreceptors from a third of them. He made similar incisions on another third of the starfish but left the eyes intact, for a control "sham" operation. The remaining starfish were left untouched. He then took the starfish off their rocks, and put them on the sandy bottom – where they would starve if they didn't get back to the reef.
He told the Society for Experimental Biology meeting in Valencia, Spain, this week that intact starfish promptly scuttled back to the rocks. Eyeless starfish scuttled just as fast, but in random directions – demonstrating that the starfish needed the photoreceptors to recognise and move towards the reef. To do this, Garm says, they had to be able to form an image of the reef, meaning that their simple nerve net must be able to process visual information.
"Amazingly, image vision in starfish has not been investigated before," saysDan-Eric Nilsson at Lund University in Sweden, who collaborated with Garm on the study.
In evolutionary terms, says Garm, it is interesting because starfish eyes are structurally close in form to the hypothesised first image-forming eyes.
For instance, light receptors in more advanced eyes are built either out of modified cytoplasmic projections called microvilli, or out of filament-shaped cell organelles called cilia. Starfish eyes contain both structures, so "have features that look a bit ancestral", says Nilsson.
"This shows what visual task drove this important step in eye evolution," says Garm. "Navigation towards large stationary objects – here the reef – that were preferred habitats." In other words, he thinks our eyes may have first evolved so we could find our way home.