According to a new study conducted by marine biologists Dr Anders Garm from the University of Copenhagen and Dr Dan-Erik Nilsson from Lund University, starfish use primitive eyes at the tip of their arms to visually navigate environment.
Linckia laevigata (Kevin Mitchell / Hobart and William Smith Colleges)
“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 that these compound eyes have been known for about two centuries, no visually guided behavior has ever be directly associated with their presence,” Dr Garm and Dr Nilsson explained.
They removed starfish with and without eyes from their food rich habitat, the coral reef, and placed them on the sand bottom 1 m away, where they would starve. They then 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.
“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,” Dr Garm said.
Analyzing the morphology of the photoreceptors in the eyes of the coral-reef-associated starfish Linckia laevigata, the team 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.
The starfish compound eye, red cups, is seen at the tip of the arm. Each red cup corresponds to a single optical unit, ommatidium (Dan-Erik Nilsson / Lund University)
“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. 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,” Dr Garm said.
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Bibliographic information: Anders Garm, Dan-E Nilsson. Starfish vision and visually guided behaviours. SEB Valencia 2013. Abstract # A11.112
