How crown-of-thorns starfish get around

Marine time-lapse photography has given scientists more clues to the movements and habits of crown-of-thorns starfish (Acanthaster spp) that might help efforts to hinder their obliteration of coral reefs in the Indo-Pacific.

The voracious starfish are adept at finding their way home, according to a study published in the journal Proceedings of the Royal Society B, but only when their favourite coral prey, Acropora spp, is around.

This suggests healthy reefs covered in these tasty corals encourage outbreaks. Once the starfish have “eaten themselves out of house and home” they move on, says lead author Scott Ling from Australia’s University of Tasmania.

Like typical teenagers, they also appear to sleep in and surface in the afternoon. “To protect healthy reefs with high cover of Acropora corals,” Ling says, “we’d recommend putting more diving effort into searching and culling starfish from midday until early evening.”

The study adds to a mountain of research striving to save Australia’s Great Barrier Reef from one of its two major threats – the other one being bleaching events from global warming.

It draws from advances in digital photography and underwater housing technology, which Ling says has “revolutionised opportunities to understand the behaviour of key reef species”.

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CoTS feeding on coral. Credit: Morgan Pratchett

“The ability to run multiple cameras on time-lapse mode throughout the day and night, in combination with software designed to track movements, allows us to understand the nocturnal behaviour of reef invertebrates, of which many increase their activity after dark.”

Previously, he had used the technology to understand the behaviour of sea urchins as they gorge on kelp forests.

In the current study, the team – which included researchers from Spain and the UK, as well as from Australia’s ARC Centre of Excellence for Coral Reef Studies – searched out adult starfish at Lizard Island and Swains Reefs, in the north and south of the Great Barrier Reef, and set up tripods with time-lapse cameras above them.

They took photographs every 20 minutes for 20 hours, enabling them to quantify movement patterns and reveal previously unidentified behaviours. Nearly 60 starfish were tracked in more than 1000 hours of observations.

When their preferred coral wasn’t around, the starfish were effectively homeless, roaming up to 20 metres a day. Where their favourite coral was close by, they moved less.

“Ultimately, if they found a shelter nearby to a food source, they demonstrated homing behaviour where they would sequentially forage on coral – typically at night – before returning to a home shelter,” Ling explains.

“This type of homing behaviour is seen amongst other reef echinoderms (starfishes and sea urchins) but was previously undescribed for the crown-of-thorns starfish.”

The individual observations help explain population dynamics, he adds, which are reflected by the mass aggregation of starfish on healthy Acropora dominated coral reefs.

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