by TFH Magazine on July 8, 2013 at 7:26 am
The starfish compound eye (red cups) is seen at the tip of the arm. Each red cup corresponds to a single optical unit, ommatidium, in an arthropod compound eye. Photograph courtesy of Dan-Erik Nilsson, Lund University.
A study has shown for the first time that starfish use primitive eyes at the tip of their arms to visually navigate their environment. Research headed by Dr. Anders Garm at the Marine Biological Section of the University of Copenhagen in Denmark, showed that starfish eyes are image-forming and could be an essential stage in eye evolution.
The researchers removed starfish with and without eyes from their food rich habitat, the coral reef, and placed them on the sand bottom one meter away, where they would starve. They monitored the starfishes’ behavior from the surface and found that while starfish with intact eyes head towards the direction of the reef, starfish without eyes walk randomly.
Dr Garm said: “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.”
Analyzing the morphology of the photoreceptors in the starfish eyes the researchers further confirmed that they constitute an intermediate state between the two large known groups of rhabdomeric and ciliary photoreceptors, in that they have both microvilli and a modified cilium.
Most known starfish species possess a compound eye at the tip of each arm, which, except for the lack of true optics, resembles arthropod compound eye. Despite being known for about two centuries, no visually guided behavior has ever been documented before.