Factors affecting the behaviour of crown-of-thorns starfish (Acanthaster planci L.) on the Great Barrier Reef:: 1: Patterns of activity

The feeding, moving and cryptic behaviours of 5749 crown-of-thorns starfish, Acanthaster planci (L.), were studied on 15 reefs of the Great Barrier Reef between June 1986 and December 1987. Log-linear analyses of field data showed that these behaviours were influenced by complex interactions between a number of important physical and biological factors including the size of starfish, their depth and physical condition, time of day, and substratum type. For most of these factors, the patterns of behaviour were shown to be consistent across reefs. Irrespective of their size, starfish spent on average about 45% of their time feeding. However, there were strong diurnal effects with smaller starfish ( 40 cm) this pattern was reversed, though the diurnal effects were not as strong. Diurnal variation was greater at shallower depths (<3 m). Larger starfish spent more time moving (∼20%), particularly during the day, compared to smaller starfish (∼5%), with the latter moving mainly at night. Motion increased with depth for all starfish. The density and degree of aggregation of starfish had no effect on behaviour. The cryptic behaviour of starfish decreased with size, from ∼60% for small starfish to less than 10% for large starfish. For starfish <30 cm in size, there were strong diurnal effects on cryptic behaviour, with smaller starfish being almost exclusively cryptic during the day. Cryptic behaviour was also influenced by the condition of starfish. Those in poor condition were 5 times more likely to be moving than those in good condition. Cryptic starfish were 10 times more likely to be in good condition than non-cryptic starfish. This may partly explain the relatively rapid breakdown in starfish aggregations which has been observed at the ends of outbreaks.