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Biol Open
2019 Jun 10;86:. doi: 10.1242/bio.041814.
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Eyes and negative phototaxis in juvenile crown-of-thorns starfish, Acanthaster species complex.
Korsvig-Nielsen C
,
Hall M
,
Motti C
,
Garm A
.
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As a corallivore, the crown-of-thorns starfish (COTS; Acanthaster species complex), has significant impacts on coral mortality and community structure on tropical reefs throughout its Indo-Pacific range. COTS form aggregations which systematically move through and across reefs causing significant loss in hard coral cover. Previous work has shown that their behaviours on the reef are influenced by rheotaxis, olfaction and vision, with vision guiding adult animals to their coral habitat at short distances. As the compound eye of starfish grows throughout life the visual capacity of juvenile eyes is putatively less than for adult animals. Here we show this to be the case. Juvenile eyes have approximately the same visual field as adult eyes but significantly lower spatial resolution. They display negative phototaxis, as observed in adults, but we found no direct proof for the use of spatial resolution in this behaviour. Our results show that juveniles are able to use their eyes to locate their habitat: the coral reef. However, their putatively lower spatial resolution would make this visual task more difficult than for the adults.This article has an associated First Person interview with the first author of the paper.
Fig. 1. Compound eyes of COTS. (A) Two juvenile COTS (white arrows) sitting close to a Fungia coral. Picture was taken during daytime. Copyright AIMS/Credit: LTMP. (B) Compound eye from a juvenile with a diameter of 3.3 cm. Average eye size was approximately 260 µm in width and 350 µm in length. Spatial resolution is estimated from the inter ommatidial angle (IOA) which is the angle between the optical axes of two neighbouring ommatidia (broken lines). (C) The compound eye of an adult COTS possesses approximately 250 ommatidia. Note that the compound eye grows continuously, with the length of the eye increasing more so than the width as the number of ommatidia multiplies.
Fig. 2. Examples of trajectories from behavioural experiments. (A) In the control experiment with no visual stimuli, the juvenile COTS walked randomly in the arena. (B) When presented with a black circle with an initial angular height of 14° on a white background, the animals still did not show significant attraction to the stimuli (see Table 2 for statistics). (C) Animals presented with a larger black circle (initial angular height 27°) were attracted to the stimuli (P=0.018, see Table 2 for details).
Fig. 3. Circular statistics from the control experiment using an evenly grey background without visual stimuli. The blue dots represent the angular position where individual animals contacted the arena wall. The mean heading (°) of the animals is indicated by the mean vector (central arrow). In these experiments the animals displayed no directionality and walked randomly in the arena (P=0.954). A summary of the circular statistics is given in Table 2. r, length of mean vector; N, number of test animals; PR, result of Rayleigh test.
Fig. 4. Circular statistics of behavioural experiments using black circles on a white background. Black circles of different sizes were placed on the bottom at a randomly chosen position along the wall of the behavioural arena. The position of the stimulus was changed between each trial (always set to position 0 in the plot). (A,B) Animals were not attracted to stimuli with an angular height of 7° and 14°. (C–E) When presented stimuli of 27° or larger, the animals were attracted and walked towards the stimulus. See also Fig. 2. A summary of the circular statistics is given in Table 2. r, length of mean vector; N, number of test animals; Pr, result of Rayleigh test. Dashed lines indicate 95% confidence intervals.
Fig. 5. Circular statistics of behavioural experiments using paired white and black rectangles (black always to the right) on grey background. The average intensity of the rectangles matched the grey background and they can thus only be detected using spatial vision. Stimuli of different sizes were placed on the bottom at a randomly chosen position along the wall of the behavioural arena. The position of the stimulus was changed between each trial (always set to position 0 in the plot). (A–E) Animals were not attracted to any of the five differently sized stimuli when the testing with unidirectional circular statistics. Using an axial test, there was a strong tendency for a directional response to the two largest stimuli (grey double arrows). For a summary of the circular statistics, see Table 2. r, length of mean vector; N, number of test animals; Pr, result of Rayleigh test; Pa, result of axial Rayleigh test.
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