One of the animal kingdom's most unusual visual systems has to belong to starfish, which see their watery environment using eyes at the tip of each arm.
Starfish and their kin (known as echinoderms) are ancient animals, first appearing in the fossil record around 530 million years ago. This latest finding about starfish eyes, reported by the Society for Experimental Biology, could then mean that such a visual system is among the most primitive. It might help to reveal how eyes in other animals evolved.
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It's been known for a while that starfish have eyes, but researchers weren't sure how they functioned. Anders Garm, of the University of Copenhagen, and his colleagues put starfish with and without eyes on a sand bottom some distance away from a coral reef, which is the source of their food. They then monitored how the starfish headed toward the reef.
The starfish without intact eyes stumbled around, walking randomly. The starfish with eyes made a beeline for the food-rich coral reef.
"The results show 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," Garm said in a press release.
Eyes have photoreceptors, which are cells that sense or receive light. The photoreceptors in starfish eyes are primitive, but have evolved a bit over many years.
Starfish eyes are compound, so they are structured somewhat like those of crabs, spiders and certain insects.
Studying the starfish "can help clarify what the first visual tasks were that drove this important step in eye evolution," Garm said, "namely navigation towards the preferred habitat using large stationary objects (here the reef)."
Our earliest ancestors were likely no different, with their eyes evolving to find the best homes and food.
Credit: Anders Garm, University of Copenhagen