Missing Link to Eye Evolution Discovered in Starfish

Eyes are complex organs in some species. They can allow humans to see color and shapes, allow falcons to see tiny movements from far away and allow chameleons to view different angles of their environment. While eyes have certainly evolved to be highly developed in some animals, though, they've lagged behind in others. Now, scientists may have gained a bit of insight into the evolution of vision by examining primitive eyes--the ones found on the arms of a starfish.

In order to examine how eyes worked in starfish, researchers removed starfish with and without eyes from their food rich habitat, the coral reef. They then placed the starfish on the sandy bottom a few feet away--a place where they would starve if they didn't move. The scientists then watched the starfish, observing their behavior from the surface.

It turns out that the starfish with intact eyes headed toward the direction of the reef. Starfish without eyes, in contrast, walked randomly across the sandy bottom. The findings showed that the starfish actually were using some sort of "sight" in order to navigate.

"The results show us that the starfish nervous system must be able to process visual information, which points to a clear underestimation of the capacity found in the circular and somewhat dispersed central nervous system of echinoderms," said Anders Garm, one of the researchers, in a news release.

The researchers didn't stop there, though. They then examined the eyes more closely, analyzing the photoreceptors within them. They found that the eyes actually constituted an intermediate state between the two large known groups of rhabdomeric and ciliary photoreceptors; they had both a microvilli and a modified cilium. In other words, starfish possess eyes that seem to an evolutionary link.

"From an evolutionary point of view it is interesting because the morphology of the starfish eyes along with their optical quality is close to the theoretical eye early in eye evolution when image formation first appeared," said Garm. "In this way it can help clarify what the first visual tasks were that drove this important step in eye evolution, namely navigation towards the preferred habitat using large stationary objects."

The findings reveal that starfish eyes are a type of intermediate stage when it comes to eye development. Not only does this study show a bit more about starfish, but also reveals a little bit more about eyes in general.