Biogeographical variation in diurnal behaviour of Acanthaster planci versus Acanthaster cf. solaris

Crown-of-thorns seastar (Acanthaster spp.) feeding ecology across species and regions

The coral predators, crown-of-thorns starfish (COTS, Acanthaster spp.) remain a major cause of extensive and widespread coral loss in Indo-Pacific coral reefs. With increased phylogenetic understanding of these seastars, at least five species appear to be present across different regions. We compare the feeding ecology of these species. Where acroporid corals are prevalent, Acanthaster spp. often exhibit a preference for these corals, with Porites being least preferred, as seen in most species including Acanthaster planci in the northern Indian Ocean and Acanthaster cf. solaris in the west Pacific. In the eastern Pacific, where Acropora is largely absent, Acanthaster cf. ellisii prey on a range of coral species, including Porites. Coral predation by COTS is influenced by several factors including food availability, coral nutritional value, protective crustaceans and coral defenses, with differences in feeding ecology and behaviour emerging across the different COTS species. Feeding behaviour of COTS can act to increase coral species richness by reducing the dominance of fast-growing species. In outbreaking populations, COTS impacts reef systems by reducing live coral cover, eroding reef complexity and causing shifts in reef trophic structure. Where data are available, we synthesise and contrast the feeding preferences and foraging behaviour of Acanthaster species, and their impact on coral assemblages across the different species and regions. For areas where focal predation on Acropora occurs, also the fastest growing coral with the greatest recovery potential following mass mortality events, the combination of climate change and COTS outbreaks presents an imminent threat to coral reefs. This is exacerbated by the dietary flexibility of Acanthaster species. The impacts of heatwaves, COTS and other stressors are creating a negative feedback loop accelerating coral reef decline.

Increasing densities of Pacific crown-of-thorns starfish (Acanthaster cf. solaris) at Lizard Island, northern Great Barrier Reef, resolved using a novel survey method

Recurrent population irruptions of Pacific crown-of-thorns starfish (CoTS, Acanthaster cf. solaris) are among the foremost causes of coral mortality on Australia's Great Barrier Reef (GBR). Early intervention during the initiation of new population irruptions represents the best opportunity to effectively manage this threat. However, current survey methods are not sufficiently sensitive to detect changes in CoTS densities during the early onset of population irruptions. Using scooter-assisted large area diver-based (SALAD) surveys, this study revealed increasing densities of CoTS at Lizard Island from 2019 to 2022. Inferred densities of adult CoTS (which account for distinct sets of observed feeding scars where starfish were not detected) increased from 4.90 ha-1 (± 0.85 SE) in 2019 to 17.71 ha-1 (± 2.3 SE) in 2022. A wide range of size classes were recorded suggesting that recruitment over several years is contributing to increasing densities. Importantly, the sustained density increases reported here denote that renewed CoTS population irruptions may soon become fully established at Lizard Island and more broadly in the northern GBR, especially without early intervention through effective population management.

Culling corallivores improves short-term coral recovery under bleaching scenarios

Management of coral predators, corallivores, is recommended to improve coral cover on tropical coral reefs under projected increasing levels of accumulated thermal stress, but whether corallivore management can improve coral cover, which is necessary for large-scale operationalisation, remains equivocal. Here, using a multispecies ecosystem model, we investigate intensive management of an invertebrate corallivore, the Crown-of-Thorns Starfish (Acanthaster cf. solaris), and show that culling could improve coral cover at sub-reef spatial scales, but efficacy varied substantially within and among reefs. Simulated thermal stress events attenuated management-derived coral cover improvements and was dependent on the level of accumulated thermal stress, the thermal sensitivity of coral communities and the rate of corallivore recruitment at fine spatial scales. Corallivore management was most effective when accumulated thermal stress was low, coral communities were less sensitive to heat stress and in areas of high corallivore recruitment success. Our analysis informs how to manage a pest species to promote coral cover under future thermal stress events.

This study uses multispecies modelling to show that the management of a coral predator, the crown-of-thorns starfish, could help corals recover following bleaching events. They show that management was most effective when heat stress severity for corals was low to moderate, when corals had lower heat sensitivity and when the recruitment rate of starfish was high.

Homing behaviour by destructive crown-of-thorns starfish is triggered by local availability of coral prey

Corallivorous crown-of-thorns starfishes (Acanthaster spp.) can decimate coral assemblages on Indo-Pacific coral reefs during population outbreaks. While initial drivers of population irruptions leading to outbreaks remain largely unknown, subsequent dispersal of outbreaks appears coincident with depletion of coral prey. Here, we used in situ time-lapse photography to characterize movement of the Pacific crown-of-thorns starfish (Acanthaster cf. solaris) in the northern and southern Great Barrier Reef in 2015, during the fourth recorded population outbreak of the starfish, but prior to widespread coral bleaching. Daily tracking of 58 individuals over a total of 1117 h revealed all starfish to move a minimum of 0.52 m, with around half of all tracked starfish showing negligible daily displacement (less than 1 m day^-1), ranging up to a maximum of 19 m day^-1. Movement was primarily nocturnal and daily displacement varied spatially with variation in local availability of Acropora spp., which is the preferred coral prey. Two distinct behavioural modes emerged: (i) homing movement, whereby tracked paths (as tested against a random-walk-model) involved short displacement distances following distinct 'outward' movement to Acropora prey (typically displaying 'feeding scars') and 'homebound' movement to nearby shelter; versus (ii) roaming movement, whereby individuals showed directional movement beyond initial tracking positions without return. Logistic modelling revealed more than half of all tracked starfish demonstrated homing when local abundance (percentage cover) of preferred Acropora coral prey was greater than 33%. Our results reveal facultative homing by Acanthaster with the prey-dependent behavioural switch to roaming forays providing a mechanism explaining localized aggregations and diffusion of these population irruptions as prey is locally depleted.

Relative efficacy of three approaches to mitigate Crown-of-Thorns Starfish outbreaks on Australia's Great Barrier Reef

Population outbreaks of Crown-of-Thorns Starfish (COTS; Acanthaster spp.) are a major contributor to loss of hard coral throughout the Indo-Pacific. On Australia’s Great Barrier Reef (GBR), management interventions have evolved over four COTS outbreaks to include: (1) manual COTS control, (2) Marine Protected Area (MPA) zoning, and, (3) water quality improvement. Here we evaluate the contribution of these three approaches to managing population outbreaks of COTS to minimize coral loss. Strategic manual control at sites reduced COTS numbers, including larger, more fecund and damaging individuals. Sustained reduction in COTS densities and improvements in hard coral cover at a site were achieved through repeated control visits. MPAs influenced initial COTS densities but only marginally influenced final hard coral cover following COTS control. Water quality improvement programs have achieved only marginal reductions in river nutrient loads delivered to the GBR and the study region. This, a subsequent COTS outbreak, and declining coral cover across the region suggest their contributions are negligible. These findings support manual control as the most direct, and only effective, means of reducing COTS densities and improving hard coral cover currently available at a site. We provide recommendations for improving control program effectiveness with application to supporting reef resilience across the Indo-Pacific.