Meeting Abstract
For a long time starfish were thought to be mainly guided by olfaction when navigating their habitat. Still, except for the borrowing species all examined species has a prominent eye at the tip of each arm situated at the base of the distalmost tube foot. They are compound eyes with bright red screening pigment but they lack proper focusing optics. Depending on species and size of the animal there are between 10 and 300 ommatidia in each eyes. All examined eyes have a single opsin with peak absorbance in the deep blue part of the spectrum and in accordance with their slow movements we also find that they have the lowest temporal resolution of any eye examined to date with flicker fusion frequencies of less than 1 Hz. We have shown that for the coral reef inhabiting Blue starfish (Linckia laevigata) and the Crown of thorns starfish (Acanthaster planci) these eyes are necessary and sufficient for homing in on the coral reef. This behaviour utilizes proper image formation with a spatial resolution in the order of 10-15 degrees. The arm tip of A. planci ends in a movable knob holding the terminal tube foot, which allows for actively adjusting the vertical part of the visual field. This is used to stabilize the visual field when the arm bends and possibly to enhance the contrast of horizontal lines through a scanning movement. Despite their many protective spines A. planci has a number of predators including butterfly fish (Chaetodontidae) and light reception is also supporting a protecting behaviour where the exposed distalmost tube feet are withdrawn if a shadow passes. This turns out to be at least partly controlled by extraocular dermal photoreceptors, though, since blinded animals still perform the behaviour.
