Visual navigation in starfish: first evidence for the use of vision and eyes in starfish

Abstract

Most known starfish species possess a compound eye at the tip of each arm, which, except for the lack of true optics, resembles an arthropod compound eye. Although these compound eyes have been known for about two centuries, no visually guided behaviour has ever been directly associated with their presence. There are indications that they are involved in negative phototaxis but this may also be governed by extraocular photoreceptors. Here, we show that the eyes of the coral-reef-associated starfish Linckia laevigata are slow and colour blind. The eyes are capable of true image formation although with low spatial resolution. Further, our behavioural experiments reveal that only specimens with intact eyes can navigate back to their reef habitat when displaced, demonstrating that this is a visually guided behaviour. This is, to our knowledge, the first report of a function of starfish compound eyes. We also show that the spectral sensitivity optimizes the contrast between the reef and the open ocean. Our results provide an example of an eye supporting only low-resolution vision, which is believed to be an essential stage in eye evolution, preceding the high-resolution vision required for detecting prey, predators and conspecifics.

Visual orientation by the crown-of-thorns starfish (Acanthaster planci)

“…In addition, we found that visual information was heavily filtered by the eye, enabling the animals to see large, dark, inanimate objects, like coral boulders, against a blue background best. The methods used in this study and the results obtained closely resemble those of a previous study (Garm and Nilsson 2014) on L. laevigata.…”

“…8) and would be easiest to detect visually by starfish. As found in L. laevigata, the measured spectral sensitivity curve was narrower than the opsin template (Garm and Nilsson 2014). It could be that the damaging ultraviolet rays are filtered out by the eye, thereby narrowing the spectral sensitivity curve at the blue end of the spectrum (O'Connor et al 2010b).…”

“…Likely, visual orientation can be accomplished faster and more accurately, without the necessity for scanning movements. The average interommatidial angle of 8°puts the resolution of the crown-ofthorns starfish in the range of the 4.6°found in Drosophila (Borst 2009) while it is better than the 16°found in the blue starfish L. laevigata (Garm and Nilsson 2014).…”

“…Orientation to visual stimuli in echinoderms occurs in one species of starfish, L. laevigata (Garm and Nilsson 2014) and two species of sea urchins (Blevins and Johnsen 2004;Yerramilli and Johnsen 2010). In addition, physiological studies have reported phototactic movements in the northern Pacific seastar, Asterias amurensis Ohtsuki 1966, 1968).…”

“…The eye consists of a thickening at the base of the unpaired tube foot at the very tip of the arm and resembles a simple compound eye. The function of the starfish eye has only recently been proven for one species of starfish, the blue starfish (Linckia laevigata), which uses its eyes for orientation towards coral reefs (Garm and Nilsson 2014). This starfish has simple compound eyes with about 150 ommatidia.…”