Eyes and negative phototaxis in juvenile crown-of-thorns starfish, Acanthaster species complex

Abstract

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.