Petie R et al. (2016), Crown-of-thorns starfish have true image formin...

ECB-ART-44909

Front Zool 2016 Sep 06;131:41. doi: 10.1186/s12983-016-0174-9.

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Crown-of-thorns starfish have true image forming vision.

Petie R , Garm A , Hall MR .

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BACKGROUND: Photoreceptors have evolved numerous times giving organisms the ability to detect light and respond to specific visual stimuli. Studies into the visual abilities of the Asteroidea (Echinodermata) have recently shown that species within this class have a more developed visual sense than previously thought and it has been demonstrated that starfish use visual information for orientation within their habitat. Whereas image forming eyes have been suggested for starfish, direct experimental proof of true spatial vision has not yet been obtained. RESULTS: The behavioural response of the coral reef inhabiting crown-of-thorns starfish (Acanthaster planci) was tested in controlled aquarium experiments using an array of stimuli to examine their visual performance. We presented starfish with various black-and-white shapes against a mid-intensity grey background, designed such that the animals would need to possess true spatial vision to detect these shapes. Starfish responded to black-and-white rectangles, but no directional response was found to black-and-white circles, despite equal areas of black and white. Additionally, we confirmed that starfish were attracted to black circles on a white background when the visual angle is larger than 14°. When changing the grey tone of the largest circle from black to white, we found responses to contrasts of 0.5 and up. The starfish were attracted to the dark area''s of the visual stimuli and were found to be both attracted and repelled by the visual targets. CONCLUSIONS: For crown-of-thorns starfish, visual cues are essential for close range orientation towards objects, such as coral boulders, in the wild. These visually guided behaviours can be replicated in aquarium conditions. Our observation that crown-of-thorns starfish respond to black-and-white shapes on a mid-intensity grey background is the first direct proof of true spatial vision in starfish and in the phylum Echinodermata.

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Genes referenced: LOC100893746 LOC100893907

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	Fig. 1. Behavioural arena. a Schematic representation of the behavioural arena. Visual stimuli were attached to a Plexiglas sheet (indicated by hatching) using Velcro, with the bottom of the stimulus on the floor of the arena. The sheet was lowered into the arena and fastened by clamps. The wall of the arena was white. For the experiments with black-and-white patterns, a mid-intensity grey cloth was attached to the inside wall of the arena. b Five different stimuli were presented to the animals: three black-and-white stimuli, a black circular stimulus and a grey circular stimulus. For each black-and-white stimulus the area of black was equal to the area of white. For all similar sized circular, or rectangular, stimuli the area of black was the equal. c The arena during an experiment. Recordings were made with a camera floating on the surface of the water. Abbreviations: c, camera; cl, clamps for attaching the Plexiglas sheet; m, middle of the arena; s, stimulus; sf, starfish. Example tracks for: d black circles (angular height 37Â°) on a white background and e the control experiment with only the Plexiglas sheet on a white background. The stimulus is located at 0Â°
	Fig. 2. Behavioural results: paired black and white rectangles presented on the mid-intensity grey background. a-e Animals were attracted to stimulus when the angular height of the stimuli was 32Â° or larger. The stimuli were always presented with the black rectangle to the left. The mean vector of the significant responses is pointing left of 0Â°, indicating that on average the animals were headed to the black half of the stimulus. In the circular plots, the small, black, filled circles represent the final angular positions of the animals. The direction of the arrow indicates the mean direction and the length of the arrow represents the length of the mean vector (rho). The radius of the circle represents a vector length of 1. The 95Â % confidence intervals for the response are indicated by dashed lines, only when the p-value for the Rayleigh test is smaller than 0.05. A summary of the circular statistics is given in TableÂ 1
	Fig. 3. Behavioural results: black rectangle centred on a white square, presented on the mid-intensity grey background. a-e Starfish were either attracted to this stimulus or repelled, as seen by the axial nature of the directional responses. Animals responded to rectangles with an angular height of 5Â°, 32Â° and 43Â°. For more details on the circular plots see caption of Fig.Â 2. A summary of the circular statistics is given in TableÂ 1
	Fig. 4. Behavioural results: black-and-white circles on a mid-intensity grey background. a-f Animals did not show directional responses to the circles or the control experiment, where the Plexiglas sheet was presented against the grey background. The angular height of the black centre of the stimulus is given first, followed by the angular height of the entire stimulus. For more details on the circular plots see caption of Fig.Â 2. A summary of the circular statistics is given in TableÂ 1
	Fig. 5. Crown-of-thorns starfish perception of the black-and-white circles at varying distances. a Visual representation of the stimulus and the field of view of a single eye at varying distance between the starfish and the stimulus. The numbers indicate the distance of the eye to the stimulus in centimetres and the red lines mark the outline of the visual field. The stimulus circle shown here has an angular height of 37Â° seen from the centre of the arena. b Relation between the proportion of white in the field of view and the distance to the stimulus. Note that at close distance to the stimulus, the white area sampled by the eye was twice the size of the sampled black area
	Fig. 6. Behavioural results: black circle on a white background. a-e Starfish were attracted to circles with initial angular sizes of 14Â° or larger. All stimuli were attached to a Plexiglas sheet using Velcro, and the entire sheet was placed in the behavioural arena. f Starfish where not attracted to the control stimulus, the Plexiglas sheet alone, without stimulus patterns. For more details on the circular plots see caption of Fig.Â 2. A summary of the circular statistics is given in TableÂ 1
	Fig. 7. Contrasts between corals and the surrounding water within the coral reef environment. Contrasts were measured on images taken through a blue filter with a transmittance curve similar to the spectral sensitivity curve of the starfish eye. The contrast at 1Â m measured 0.43 for the coral boulder and 0.18 for the blue staghorn coral. Two measurements were done on the same small reef, the first with the sun in the back and the second facing the sun
	Fig. 8. Behavioural results: grey circle on a white background. a-e Animals responded to circles with an initial angular size of 37Â° when the contrast was 0.5 or higher. For more details on the circular plots see caption of Fig.Â 2. A summary of the circular statistics is given in TableÂ 1

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