Associational refuges among corals mediate impacts of a crown-of-thorns starfish Acanthaster planci outbreak Indirect positive interactions in communities

Interactions among coral populations can moderate the impact of coral predator outbreaks, enhancing community resilience and recovery. This study used predator-exclusion cages and neighbour removals in a field experiment to test how indirect interactions between populations of three coral taxa, Acropora, Pocillopora, and Porites, influenced their survival during an outbreak of the crown-of-thorns starfish, Acanthaster planci, in Moorea, French Polynesia. High densities of corals enhanced survival by generating associational refuges: physical structures that impeded Acanthaster and protected corals, and by simple density-dependent prey dilution that reduced predation rates. Acanthaster showed feeding preferences, resulting in varying intensities of predation on corals, which (1) influenced the type and strength of the associational refuge among corals and (2) resulted in significant loss of the competitive dominants to the benefit of the competitive inferiors. The result was a set of indirect positive interactions (IPIs) that prevented Acanthaster from eradicating Acropora and may have enhanced Porites, a relatively weak competitor among corals. IPIs probably play a key role in many ecosystems, especially in coral reefs in which corals act as engineer species, to reduce impacts of perturbations and enhance community resilience. This study illustrates the importance of IPIs in community regulation with a new conceptual model.     

Predation on crown-of-thorns starfish larvae by damselfishes

Examining the functional response of predators can provide insight into the role of predation in structuring prey populations and ecological communities. This study explored feeding behaviour and functional responses of planktivorous damselfishes when offered captive reared larvae of crown-of-thorns starfish, Acanthaster sp., with the aim of determining whether these predators could ever play a role in moderating outbreaks of Acanthaster sp. We examined predatory behaviour of 11 species of planktivorous damselfish, testing: (1) the relationship between predator size and predation rate, both within and among fish species; (2) consumption rates on larvae of Acanthaster sp. versus larvae of a common, co-occurring coral reef asteroid Linckia laevigata; (3) maximal feeding rates upon both Acanthaster sp. and L. laevigata; and (4) functional responses of planktivorous fishes to increasing densities of Acanthaster sp. Consumption rates of crown-of-thorns larvae by damselfishes were independent of predator size; however, there was a significant negative relationship between predator size and consumption rate of L. laevigata, when pooling across all predatory species. Some damselfishes, including Acanthochromis polyacanthus and Amblyglyphidodon curacao, consumed larval Acanthaster sp. at a greater rate than for L. laevigata. Most predatory species (all except A. curacao and Pomacentrus amboinensis) exhibited a Type II functional response whereby the increasing feeding rate decelerated with increasing prey density. In addition to revealing that a wide range of planktivorous fishes can prey upon larvae of Acanthaster sp., these data suggest that planktivorous damselfishes may have the capacity to buffer against population fluctuations of Acanthaster sp. Importantly, predators with Type II functional responses often contribute to stability of prey populations, though planktivorous fishes may be swamped by an abnormally high influx of larvae, potentially contributing to the characteristic population fluctuations of Acanthaster sp.     

Assessing feeding electivity in Acanthaster planci: a null model analysis

Feeding electivity was investigated in a non-outbreaking population of the crown-of-thorns starfish Acanthaster planci (L.) from North Sulawesi, Indonesia. A null model-based approach was used to assess the feeding pattern of Acanthaster in relation to the availability of coral prey in the field. Of a total of 70 species of corals recorded as prey, massive species, particularly of Faviidae, tended to be more frequently consumed than would be expected under the assumption of random feeding by A. planci. Branched and encrusting/laminar forms of corals that occurred in relatively exposed sites were apparently not preferred, pointing to the importance of non-acroporan massive species of corals in cryptic habitats as prey for A. planci. The null model-based electivity index Z introduced here directly measured the deviation from random feeding, while two common indices (Ivlev’s and Vanderploeg and Scavia’s) only partially reflected such deviations (hence, prey selection cannot be accurately demonstrated by these). Electivity values (Z) for poritid species and Acropora palifera, the most common Acropora species in the study site, were significantly negative, indicating apparent avoidance of them by Acanthaster. Our results indicate that accessibility to different coral species and the choice/avoidance of certain species are the important elements of feeding in non-outbreaking populations of Acanthaster inhabiting spatially variable reef environments. A similar consideration may apply to the feeding patterns of other corallivores that possess superior/inferior mobility to Acanthaster. The present study emphasizes the merit of testing the observed patterns, using null models for a rigorous assessment of feeding preferences.     

No-take reserves protect coral reefs from predatory starfish

The crown-of-thorns starfish, Acanthaster planci, is a predator of corals that is a major management issue on coral reefs [1]. It occurs throughout the Indo–Pacific and shows boom–bust population dynamics with low background densities and intermittent outbreaks. Three waves of population outbreaks have affected Australia's Great Barrier Reef (GBR) since the 1960s. The waves of outbreaks appear to start 15°S [2] and progress southward through the central GBR (Figure 1A), causing major losses of living coral on many reefs across a large area and dwarfing losses from other disturbances such as storms or coral bleaching over the same period [3]. Humans can potentially influence starfish population dynamics by exploiting predators, though evidence to date is circumstantial. Extensive surveys in the GBR Marine Park (GBRMP) show that protection from fishing affects the frequency of outbreaks: the relative frequency of outbreaks on reefs that were open to fishing was 3.75 times higher than that on no-take reefs in the mid-shelf region of the GBR, where most outbreaks occur, and seven times greater on open reefs if all reefs were included. Although exploited fishes are unlikely to prey on starfish directly, trophic cascades could favour invertebrates that prey on juvenile starfish.     

Changes in coral assemblages during an outbreak of Acanthaster planci at Lizard Island, northern Great Barrier Reef (1995–1999)

Population outbreaks of crown-of-thorns starfish (Acanthaster planci L.) represent one of the most significant biological disturbances on tropical coral reefs and have the potential to devastate coral communities, thereby altering the biological and physical structure of reef habitats. This study reports on changes in area cover, species diversity and taxonomic composition of corals during an outbreak of A. planci at Lizard Island, in the northern Great Barrier Reef, Australia. Mean coral cover declined by 28.8% across ten locations studied. However, densities of A. planci, and their effects on local coral assemblages, were very patchy. Declines in coral cover were mostly due to the selective removal of certain coral taxa (mainly Acropora and Pocilloporidae corals); such that the greatest coral loss occurred at locations with highest initial cover of preferred coral prey. Most notably, coral assemblages in back-reef locations were transformed from topographically complex staghorn Acropora-dominated habitats, to relatively depauperate assemblages dominated by alcyonacean soft corals. Although coral loss was greatest among formerly dominant taxa (especially Acropora), effects were sufficiently widespread across different coral taxa, such that overall coral diversity tended to decline. Clearly, moderate outbreaks of A. planci have the potential to greatly alter community structure of coral communities even if they do not devastate live corals. Recovery in this instance is expected to be very rapid given that all coral taxa persisted, and effects were greatest among fast growing corals.     

Spatial resilience of the Great Barrier Reef under cumulative disturbance impacts

In the face of increasing cumulative effects from human and natural disturbances, sustaining coral reefs will require a deeper understanding of the drivers of coral resilience in space and time. Here we develop a high‐resolution, spatially explicit model of coral dynamics on Australia's Great Barrier Reef (GBR). Our model accounts for biological, ecological and environmental processes, as well as spatial variation in water quality and the cumulative effects of coral diseases, bleaching, outbreaks of crown‐of‐thorns starfish (Acanthaster cf. solaris), and tropical cyclones. Our projections reconstruct coral cover trajectories between 1996 and 2017 over a total reef area of 14,780 km², predicting a mean annual coral loss of ?0.67%/year mostly due to the impact of cyclones, followed by starfish outbreaks and coral bleaching. Coral growth rate was the highest for outer shelf coral communities characterized by digitate and tabulate Acropora spp. and exposed to low seasonal variations in salinity and sea surface temperature, and the lowest for inner‐shelf communities exposed to reduced water quality. We show that coral resilience (defined as the net effect of resistance and recovery following disturbance) was negatively related to the frequency of river plume conditions, and to reef accessibility to a lesser extent. Surprisingly, reef resilience was substantially lower within no‐take marine protected areas, however this difference was mostly driven by the effect of water quality. Our model provides a new validated, spatially explicit platform for identifying the reefs that face the greatest risk of biodiversity loss, and those that have the highest chances to persist under increasing disturbance regimes.     

Recovery without resilience: persistent disturbance and long-term shifts in the structure of fish and coral communities at Tiahura Reef, Moorea

Disturbances have a critical effect on the structure of natural communities. In this study long-term changes were examined in the reef community at Tiahura Reef, on the northern coast of Moorea, which had been subject to many and varied disturbances over the last 25 years. Tiahura Reef was subject to an outbreak of crown-of-thorns starfish (Acanthaster planci) in 1980–1981, causing significant declines in the abundance of scleractinian corals and butterflyfishes. By 2003, the abundance of corals and butterflyfishes had returned to former levels, but despite this apparent recovery, the species composition of coral communities and butterflyfish assemblages was very different from those recorded in 1979. Ongoing disturbances (including further outbreaks of crown-of-thorns starfish, cyclones, and coral bleaching events) appear to have prevented recovery of many important coral species (notably, Acropora spp.), which has had subsequent effects on the community structure of coral-feeding butterflyfishes. This study shows that recurrent disturbances may have persistent effects on the structure and dynamics of natural communities.     

Predator Crown-of-Thorns Starfish (Acanthaster planci) Outbreak,Mass Mortality of Corals,and Cascading Effects on Reef Fish and Benthic Communities

Outbreaks of the coral-killing seastar Acanthaster planci are intense disturbances that can decimate coral reefs. These events consist of the emergence of large swarms of the predatory seastar that feed on reef-building corals, often leading to widespread devastation of coral populations. While cyclic occurrences of such outbreaks are reported from many tropical reefs throughout the Indo-Pacific, their causes are hotly debated, and the spatio-temporal dynamics of the outbreaks and impacts to reef communities remain unclear. Based on observations of a recent event around the island of Moorea, French Polynesia, we show that Acanthaster outbreaks are methodic, slow-paced, and diffusive biological disturbances. Acanthaster outbreaks on insular reef systems like Moorea''s appear to originate from restricted areas confined to the ocean-exposed base of reefs. Elevated Acanthaster densities then progressively spread to adjacent and shallower locations by migrations of seastars in aggregative waves that eventually affect the entire reef system. The directional migration across reefs appears to be a search for prey as reef portions affected by dense seastar aggregations are rapidly depleted of living corals and subsequently left behind. Coral decline on impacted reefs occurs by the sequential consumption of species in the order of Acanthaster feeding preferences. Acanthaster outbreaks thus result in predictable alteration of the coral community structure. The outbreak we report here is among the most intense and devastating ever reported. Using a hierarchical, multi-scale approach, we also show how sessile benthic communities and resident coral-feeding fish assemblages were subsequently affected by the decline of corals. By elucidating the processes involved in an Acanthaster outbreak, our study contributes to comprehending this widespread disturbance and should thus benefit targeted management actions for coral reef ecosystems.     

Coral reef environmental science: truth versus the Cassandra syndrome

In 1970, coral reef science was warned that the crown-of-thorns starfish, Acanthaster planci, might cause the extinction of scleractinian corals in the Pacific Ocean. Now, 20 years later we can fortunately say that this alarm was almost certainly too severe. Many reefs were devastated by the starfish, but none are extinct, none have disappeared and many are in various stages of recovery. But now in the 1990's a new alarm is being sounded. This time the concern is over widespread destruction of coral reefs by elevated surface temperatures. Once again a few scientists have issued a dire warning that these events may represent a harbinger of ocean warming caused by the Greenhouse Effect. Has not Acanthaster taught coral reef science a lesson? The debate is far from over but this time the mood in general is not one of over-reaction. This time the Cassandras will be tested by the truth of careful experimentation, long-term monitoring and objective interpretation. Coral reef science appears to have come of age.     

Competitors as accomplices: seaweed competitors hide corals from predatory sea stars

Indirect biotic effects arising from multispecies interactions can alter the structure and function of ecological communities—often in surprising ways that can vary in direction and magnitude. On Pacific coral reefs, predation by the crown-of-thorns sea star, Acanthaster planci, is associated with broad-scale losses of coral cover and increases of macroalgal cover. Macroalgal blooms increase coral–macroalgal competition and can generate further coral decline. However, using a combination of manipulative field experiments and observations, we demonstrate that macroalgae, such as Sargassum polycystum, produce associational refuges for corals and dramatically reduce their consumption by Acanthaster. Thus, as Acanthaster densities increase, macroalgae can become coral mutualists, despite being competitors that significantly suppress coral growth. Field feeding experiments revealed that the protective effects of macroalgae were strong enough to cause Acanthaster to consume low-preference corals instead of high-preference corals surrounded by macroalgae. This highlights the context-dependent nature of coral–algal interactions when consumers are common. Macroalgal creation of associational refuges from Acanthaster predation may have important implications for the structure, function and resilience of reef communities subject to an increasing number of biotic disturbances.     

Dynamics of an outbreak population of Acanthaster planci at Lizard Island, northern Great Barrier Reef (1995–1999)

Despite their significant influence on coral reef ecosystems, causes of population outbreaks of crown-of-thorns starfish (Acanthaster planci L.) are still poorly understood. Essentially, outbreaks of A. planci could arise from either (1) a single mass recruitment event or (2) the progressive accumulation of starfish from multiple cohorts. This study explored fine-scale variation in the size, distribution, and abundance of A. planci, during an outbreak at Lizard Island in the northern Great Barrier Reef, to assess the mechanism by which the outbreak occurred. Densities of A. planci around Lizard Island increased very gradually from October 1994 until December 1996, then remained at around 1.0 starfish per 200 m² until June 1998. The population of A. planci comprised individuals ranging in size from 11-cm to 62-cm diameter, representing individuals from multiple (at least four) different cohorts. These data suggest that the outbreak of A. planci at Lizard Island resulted from a prolonged build-up in starfish numbers through multiple successive recruitment events. This study shows that outbreaks of A. planci may arise independently of any sudden or substantial increase in rates of recruitment, such that any factor(s) responsible for the initial onset of outbreaks are likely to be very subtle and difficult to detect.     

Biochemical investigation of two responses involved in the feeding behaviour of Acanthaster planci (L). I. Assay methods and preliminary results

Material obtained by simple biochemical fractionation of aqueous extracts of coral has been tested for its ability to induce responses normally associated with feeding on live corals in the crown-of-thorns starfish, Acanthaster planci (L.) With the use of a new assay method, it has been shown that both dialysable and non-dialysable components of aqueous coral extract cause cessation of movement, settlement, and eversion of the stomach as if over a whole coral. A second response, apparently of avoidance, seen as a starfish approaches a coral prior to feeding, has also been examined; it is evoked by the dialysable fraction of coral extract.     

Crown-of-thorns starfish predation and physical injuries promote brown band disease on corals

Brown band (BrB) disease manifests on corals as a ciliate-dominated lesion that typically progresses rapidly causing extensive mortality, but it is unclear whether the dominant ciliate Porpostoma guamense is a primary or an opportunistic pathogen, the latter taking advantage of compromised coral tissue or depressed host resistance. In this study, manipulative aquarium-based experiments were used to investigate the role of P. guamense as a pathogen when inoculated onto fragments of the coral Acropora hyacinthus that were either healthy, preyed on by Acanthaster planci (crown-of-thorns starfish; COTS), or experimentally injured. Following ciliate inoculation, BrB lesions developed on all of COTS-predated fragments (n = 9 fragments) and progressed up to 4.6 ± 0.3 cm d^?1, resulting in ~70 % of coral tissue loss after 4 d. Similarly, BrB lesions developed rapidly on experimentally injured corals and ~38 % of coral tissue area was lost 60 h after inoculation. In contrast, no BrB lesions were observed on healthy corals following experimental inoculations. A choice experiment demonstrated that ciliates are strongly attracted to physically injured corals, with over 55 % of inoculated ciliates migrating to injured corals and forming distinct lesions, whereas ciliates did not migrate to healthy corals. Our results indicate that ciliates characteristic of BrB disease are opportunistic pathogens that rapidly migrate to and colonise compromised coral tissue, leading to rapid coral mortality, particularly following predation or injury. Predicted increases in tropical storms, cyclones, and COTS outbreaks are likely to increase the incidence of coral injury in the near future, promoting BrB disease and further contributing to declines in coral cover.     

Distribution of recent outbreaks of the crown-of-thorns starfish (Acanthaster planci) along the Great Barrier Reef: 1985–1986

A large survey program was conducted during 1985/1986 to determine the extent of activity of the crown-of-thorns starfish, Acanthaster planci, and its broad effects on the coral communities of the Great Barrier Reef (GBR). The perimeters of 228 reefs (about 9% of reefs in the GBR system) were surveyed within 1 year using rapid survey, manta tow techniques. These reefs encompassed the broad latitudinal and longitudinal gradients within the GBR. Approximately 27% (62 reefs) of the reefs surveyed had recently experienced (18%), or were experiencing (9%), an outbreak of the crown-of-thorns starfish. These outbreaks were mainly confined to reefs in the central third of the GBR (between Lizard Island and Townsville) and had affected, to varying degrees, approximately 65% of the reefs surveyed within this region. A greater proportion of mid-shelf reefs had experienced outbreaks than outer-shelf reefs, although this difference was not statistically significant. Of the small number of inner-shelf reefs surveyed, none had been recently affected by an outbreak. Large active outbreaks of starfish were reported on many of the reefs located off Townsville while much smaller outbreaks were found on several reefs at the southern end of the GBR, in the Swain Reef complex. Almost 86% of reefs currently experiencing an outbreak had moderate to high coral mortality over at least a third of their perimeters. Only 10% of reefs with active outbreaks had high coral mortality over most of their windward and leeward margins. A similar proportion of reefs had low to moderate coral mortality over less than a third of their perimeters.     

Influence of Resource Availability on the Foraging Strategies of the Triangle Butterflyfish Chaetodon triangulum in the Maldives

Obligate coral feeders such as many members of the Chaetodontidae family (also known as butterflyfish) often show strong preferences for particular coral species. This is thought to have evolved through natural selection as an energy-maximising strategy. Although some species remain as highly specialised feeders throughout their lifetime, many corallivores show a degree of dietary versatility when food abundance is limited; a strategy described by the optimal foraging theory. This study aimed to examine if, within-reef differences in the feeding regime and territory size of the Triangle Butterflyfish Chaetodon triangulum occurred, as a function of resource availability. Results showed that the dietary specialisation of C. triangulum was significant in both areas of low and high coral cover (χL2² = 2.52 x 102, P<0.001 and χL2² = 3.78 x 102, P<0.001 respectively). Resource selection functions (RSFs), calculated for the two main sites of contrasting coral assemblage, showed that in the resource-rich environments, only two Genera (Acropora and Pocillopora) were preferentially selected for, with the majority of other corals being actively ‘avoided’. Conversely, in territories of lower coral coverage, C. triangulum was being less selective in its prey choice and consuming corals in a more even distribution with respect to their availability. Interestingly, coral cover appeared to show no significant effect on feeding rate, however it was a primary determinant of territory size. The findings of the study agree with the predictions of the optimal foraging theory, in that where food supply is scarce, dietary specialisation is minimised and territory size increased. This results in maximising energy intake. This study represents the first scientific evidence that C. triangulum is an obligate corallivore and, as with many other butterflyfish, is therefore dependent on healthy scleractinian corals for survival.     

Microhabitat use and prey selection of the coral-feeding snail <Emphasis Type="Italic">Drupella cornus</Emphasis> in the northern Red Sea

Corallivorous gastropods of the genus Drupella are known for population outbreaks throughout the Indo-Pacific region. Despite their potential to destroy wide areas of coral reef, prey preferences have never been analyzed with respect to prey availability, and juvenile ecology and food selectivity remain largely unknown. Here, the influence of water depth, coral abundance, colony shape, prey species, and intraspecific attraction among snails on distribution patterns, prey selection, and microhabitat use of D. cornus was studied in the northern Red Sea. Special emphasis was put on ontogenetic differences. The snails were most abundant in the shallowest reef zone (1 m depth). Adults were associated with several substrates and coral growth forms, whereas juveniles were highly cryptic and restricted to live branching corals. The genus Acropora was significantly preferred over other acroporid and pocilloporid corals. As revealed by resource selection ratios, Acropora acuminata was preferred by juveniles, A. selago by adults. In aquarium experiments, intraspecific attraction was high among both life stages. Overall, significant differences in juvenile and adult microhabitat and prey use suggest that juveniles have more specific habitat requirements, and indicate ecological impacts on coral communities different from that of adults. Prey preferences seem to depend on both coral genus and colony shape. Acropora corals provide the best combination of food and shelter and therefore determine distribution patterns of D. cornus.     

The Importance of Coral Larval Recruitment for the Recovery of Reefs Impacted by Cyclone Yasi in the Central Great Barrier Reef

Cyclone Yasi, one of the most severe tropical storms on record, crossed the central Great Barrier Reef (GBR) in February 2011, bringing wind speeds of up to 285 km hr⁻¹ and wave heights of at least 10 m, and causing massive destruction to exposed reefs in the Palm Island Group. Following the cyclone, mean (± S.E.) hard coral cover ranged from just 2.1 (0.2) % to 5.3 (0.4) % on exposed reefs and no reproductively mature colonies of any species of Acropora remained. Although no fragments of Acropora were found at impacted exposed sites following the cyclone, small juvenile colonies of Acropora (<10 cm diameter) were present, suggesting that their small size and compact morphologies enabled them to survive the cyclone. By contrast, sheltered reefs appeared to be unaffected by the cyclone. Mean (± S.E.) hard coral cover ranged from 18.2 (2.4) % to 30.0 (1.0) % and a large proportion of colonies of Acropora were reproductively mature. Macroalgae accounted for 8 to 16% of benthic cover at exposed sites impacted by cyclone Yasi but were absent at sheltered sites. Mean (± S.E.) recruitment of acroporids to settlement tiles declined from 25.3 (4.8) recruits tile⁻¹ in the pre-cyclone spawning event (2010) to 15.4 (2.2) recruits tile⁻¹ in the first post-cyclone spawning event (2011). Yet, post-cyclone recruitment did not differ between exposed (15.2±2.1 S.E.) and sheltered sites (15.6±2.2 S.E.), despite the loss of reproductive colonies at the exposed sites, indicating larval input from external sources. Spatial variation in impacts, the survival of small colonies, and larval replenishment to impacted reefs suggest that populations of Acropora have the potential to recover from this severe disturbance, provided that the Palm Islands are not impacted by acute disturbances or suffer additional chronic stressors in the near future.     

Acanthaster planci is a major cause of coral mortality in Indonesia

The corallivorous crown-of-thorns starfish (COTS), Acanthaster planci, is recognised as a major cause of coral reef degradation throughout much of the Pacific Ocean. However, the effects of COTS on the high diversity reefs in Indonesia have been largely overlooked. In 2007, high densities of COTS were observed in two regions of Indonesia: Aceh and Halmahera. Densities of COTS ranged from 0 to 52 starfish 2,000 m² across 24 sites in Aceh and from 0 to 18 starfish 2,000 m² at 10 sites in Halmahera. Mortality rates of Acropora spp. were very high at affected sites: over 50 % of colonies had been killed at seven of the 16 affected sites. A review of historical sources going back to 1969 suggests that COTS have damaged many reefs throughout Indonesia, including much activity within the Indonesian section of the Coral Triangle. Furthermore, the data suggest that COTS activity has increased rapidly since 2000. Very little of this activity has been reported in the primary literature, and there is a general lack of awareness in Indonesia of COTS as a potential cause of reef degradation. This lack of awareness, combined with limited monitoring efforts, means that damage caused by COTS is often attributed to other causes, such as destructive fishing, bleaching or tsunami. COTS are clearly a major source of coral mortality in Indonesia of which scientists and government need to be more cognizant.     

Biochemical investigation of two responses involved in the feeding behaviour of Acanthaster planci (L.). II. Isolation and characterization of chemical stimuli

Feeding behaviour is induced in the crown-of-thorns starfish, Acanthaster planet (L.), by two kinds of chemical stimulus originating in coral. One is macromolecular, and the other of low molecular weight, largely accounted for by small peptides and amino acids.It is found that, under experimental conditions, starfish rapidly habituate to a chemical stimulus which at first strongly induces feeding behaviour.The pronounced withdrawal response which occurs when a crude extract of coral is applied to the starfish (a response apparently identical with that evoked by contact with live coral) is caused by a component which corresponds closely in chromatographie behaviour with proline. Proline itself produces the response, but is present at too low a concentration in coral to account for the observed activity.     

Assessment of crown-of-thorns skeletal elements in surface sediment of the Great Barrier Reef

A total of 1655 crown-of-thorns starfish skeletal elements were recovered from 237 surface sediment samples from Davies, Centipede, Myrmidon, Hope, Holbourne Island, 22–110, Gannet Cay and Lady Musgrave Island Reefs of the central and southern sectors of the Great Barrier Reef. Three categories of reef may be recognised on the incidence of Acanthaster planci skeletal elements in surface sediment from these and previously studied reefs: category A (abundant, >12 elements kg^1-), category C (common, 3–8 elements kg^-1) and category C (rare, 0–0.1 elements kg^-1). These categories parallel estimates of crown-of-thorns populations in the period 1986–1990. A reefs have generally experienced high intensity outbreaks, C reefs less intense or perhaps less frequent outbreaks and R reefs have had little or no crown-of-thorns presence. The incidence of crown-of-thorns skeletal elements in surface sediment potentially provides an indication of population densities and outbreaks over a time scale of several decades. A perspective of contemporary crown-of-thorns incidence on the many reefs of the GBR lacking direct observational records may thereby be obtained. For Holbourne Island a comparison was made of element incidence in an area of known mass mortality induced by poisoning with a control area that was undisturbed. The incidence of A. planci skeletal elements is comparable in the two areas and similar to the incidence established for other reefs such as Green Island and John Brewer where high intensity outbreaks are known to have occurred. A direct relationship between high incidence of elements in surface sediment and mass mortality following outbreak events is indicated.