A palaeobiological examination of the geological evidence for recurring outbreaks of the crown-of-thorns starfish,Acanthaster planci (L.)

Geological investigations of the crown-of-thorns starfish (COTS) have concluded that outbreaks are not only recent but also have occurred in the past. The evidence lies in the abundance of COTS skeletal elements found both on the sea floor and within the underlying sedimentary record. These studies are flawed in three respects. First, the processes of fossil preservation from the living population to the fossil assemblage have been ignored. Second, it has not been demonstrated that the fossil skeletal elements representing alleged outbreak populations of starfish are of the same age. Third, the existence of a relationship between the number of COTS skeletal elements sampled from the sedimentary record and the relative abundance of COTS in the once living population has not been substantiated. The limitations introduced when studying the fossil record need to be established through taphonomic analyses of the COTS. Techniques which will allow greater temporal resolution of skeletal element age include amino-acid geochronology, analysis of sedimentation mode and rate, and correlation among sub-surface cores. In order to establish a relationship between the number of fossil COTS elements and the original population size, methods must be developed which will relate the number of fossil skeletal elements to the relative abundance of starfish in both the fossil and death assemblages and then to relate the latter to the relative size of the original population. When these approaches are used together it may be possible to make some estimate of relative COTS abundances based on data contained in the fossil record.     

A gonad-stimulating peptide of the crown-of-thorns starfish,Acanthaster planci

The crown-of thorns starfish, Acanthaster planci, has been blamed for coral mortality in a large number of coral reef systems in the Indo-Pacific region. Because population outbreaks of A. planci are closely related to reproduction, it is important to examine the mechanism of reproductive control in this starfish. Previously, a relaxin-like gonad-stimulating peptide (RGP) in starfish Asterina pectinifera has been identified as the gonadotropin responsible for final gamete maturation. On the basis of homology research on RGP cDNAs from several species, this study was carried out to identify gonadotropin in A. planci. The cDNA sequence of RGP was determined using a RACE product of mRNA from the radial nerves of A. planci. The coding DNA sequence consisted of 351 base pairs with an open reading frame encoding a peptide of 116 amino acids (aa), including a signal peptide (29 aa), B-chain (19 aa), C-peptide (44 aa), and A-chain (24 aa). The chemical structure of A. planci RGP was exactly the same as that of A. pectinifera RGP. Furthermore, synthetic RGP could induce gamete spawning and oocyte maturation in the ovarian fragments of A. planci. This strongly suggested that the RGP is a gonadotropin in A. planci.     

Modelling the larval dispersal of Acanthaster planci

The desire to understand the observed patterns of Acanthaster population spread throughout the Great Barrier Reef (GBR) has necessitated the development of numerical models which are capable of simulating the hydrodynamics associated with large assemblages of reefs over the time scale of larval dispersal. Models are presented which provide an overview of the physical processes controlling the advection of larvae within the Cairns Section of the GBR Marine Park, on the scale of tens to hundreds of kilometres. The relevant scales and relative importance of the distinct processes are discussed. The models are based on a previously tested numerical hydrodynamic model and are validated by comparison with observations documented in the physical oceanographic literature. Overall, model outputs correspond well to our understanding of the large-scale features and dynamics of water circulation over the continental shelf. Regional variation in circulation predicted by the models may result in significant differences in the movement of larvae.     

Preliminary observations of the decomposition of crown-of-thorns starfish,Acanthaster planci (L.)

The rate of decomposition of 67 crown-of-thorns starfish was monitored over 7–9 days on 2 reefs of the Great Barrier Reef in 7 permanent quadrats in different depths and habitats. The pattern of decomposition was similar in all quadrats irrespective of the site or depth. Many different organisms, including crown-of-thorns starfish, fed on or scavenged the remains. Despite this activity the remains could be recognised as crown-of-thorns starfish for up to 5 days. The reason for the sudden disappearance of starfish at the ends of outbreaks is briefly discussed.     

Selective feeding by larvae of the crown-of-thorns starfish, Acanthaster planci (L.)

 The effect of phytoplankton size on feeding rates of planktonic larvae of the crown-of-thorns starfish Acanthaster planci (L.) was evaluated by examining their gut contents under an epifluorescence microscope. Concentrations of coccoid cyanobacteria in natural seawater ranged between 1.73 and 5.33×10⁵ cells ml^-1 and were three to four orders of magnitude greater than that of eukaryotes. Under these conditions, A. planci larvae ingested similar or smaller numbers of cyanobacteria than eukaryotes. Consequently, clearance rates of A. planci larvae on cyanobacteria were approximately three orders of magnitude lower than those on eukaryotes. Cyanobacteria and eukaryotes in the gut of A. planci larvae had mean equivalent spherical diameters (ESD) of 1–2 μm and 3.6–4.6 μm, respectively. Thus, the volume of cyanobacteria ingested was less than 10% of the volume of eukaryotes ingested. Acanthaster planci larvae were fed cultured phytoplankton Dunaliella tertiolecta and suspensions of three different sizes of plastic beads with fluorescence labelling. There was no significant difference in clearance rates on 6 and 20 μm plastic beads. Clearance rates on 1 μm plastic beads were, however, much lower than those on 6 and 20 μm plastic beads. Clearance rates of A. planci larvae on D. tertiolecta (ca. 5 μm ESD) were significantly higher than those on 6 and 20 μm plastic beads. Apart from particle size, this result shows that feeding of A. planci larvae is influenced by other properties of potential food particles. Accepted: 24 May 1996     

Molecular cloning of two toxic phospholipases A2 from the crown-of-thorns starfish Acanthaster planci venom

The full-length cDNAs encoding two toxic phospholipases A2 (AP-PLA2-I and -II) from the crown-of-thorns starfish Acanthaster planci venom were individually cloned by RT-PCR, 3'RACE and 5'RACE. In common with both AP-PLA2s, the precursor protein is composed of a signal peptide, a propeptide and a mature protein (136 and 135 residues for AP-PLA2-I and -II, respectively). The four motifs (Ca2+-binding loop, Ca2+-binding site, active site and catalytic network) characteristic of groups I and II PLA2s are well conserved in both AP-PLA2s. In addition to this, the presence of the elapid and pancreatic loops and the involvement of a propeptide in the precursors suggested that AP-PLA2s are highly analogous to the group IB PLA2s. However, when compared to the amino acid sequence of bovine pancreatic PLA2, the representative group IB PLA2, AP-PLA2s require some amino acid insertions and deletions in the region 76-100, as previously observed for the starfish Asterina pectinifera PLA2s. Furthermore, the phylogenetic tree made clearly demonstrated that AP-PLA2s and A. pectinifera PLA2s are distinguishable from the group IB PLA2s as well as other PLA2s, being classified into a new group.     

Distribution of Acanthaster planci outbreaks on the Great Barrier Reef between 1966 and 1989

Analysis of data from 1966 to 1989 indicates 2 periods of abundant starfish outbreaks on the Great Barrier Reef (GBR). While the data for the first peak of activity (1966–1975) are relatively limited, the data for the most recent peak of activity (1981–1989) support the hypothesis of southward moving waves of outbreaks. The southward drift of outbreak activity is consistent with speed and direction of average summer currents on the GBR but the concept of a discrete seed area to initiate the wave is not substantiated, nor testable, with presently available data. As the present wave of outbreaks appears to be declining in the central section of the GBR (17–19°S) small residual populations may remain. If the outbreaks are coupled to coral recovery patterns then the next period of high starfish activity in the central section would be expected in the late-1990's.     

Morphology of spines and spine joint in the crown-of-thorns starfish Acanthaster planci (Echinodermata,Asteroida)

Summary Morphology and movement of the spines of Acanthaster planci were studied. All surfaces of the animal are covered with spines. The spines on the aboral surface are cylindrical with sharp tips. The spines on the oral surface are flat; they bend over to cover the mouth and the ambulacral grooves when these soft parts are stimulated. Those on the side of the animal make a barrier of crossed spines. Thus the structure and movement of the spines are well-adapted for defense.The junction between the primary aboral spine and its pedicel makes a movable joint. The ultrastructure of the connective tissue at the joint was studied. The connective tissue is mainly composed of collagen fibers. Presumed neurosecretory cells with processes which are filled with electron-dense granules of 0.2 m diameter were found between collagen fibers. Muscle fibers are mainly found in the connective tissue at the central holes. These observations support the view that the joint connective tissue has catch properties.     

History and dynamics of the crown-of-thorns starfish Acanthaster planci (L.) in the Suva area,Fiji

Annual recruitment of Acanthaster planci over a 13 year period in a known juvenile refuge substratum on Suva Reef was very patchy in time and space. Macroscale recruitment events resulted in outbreaks over many thousands of hectares over 100 km of coastline whereas localised but intensive mesoscale events resulted in concenrated aggregations over areas of several tens of hectares. A very low level of recruitment occurred in most years but three intensive, macroscale recruitment events occurred in 1977, 1984 and 1987, and two localised but intensive recruitment events occurred in 1982 and 1983. Annual recruitment of A. planci between 1975–89 had no overt relationship with rainfall events, as proposed in the terrestrial run-off hypothesis on the cause of primary out-breaks. Growth rates within monitored cohorts were highly variable, but mean growth rates of two cohorts 1984 and 1987 were relatively similar. Mortality rates of three monitored cohorts were variable. Longevity of the 1977 cohort was 7–8 years, while that of the 1984 cohort was 2–3 years. Mortality of the 1987 cohort has been low to date (i.e. 4 years of age). Disease, attributed to an undescribed sporozoan pathogen, was responsible for two mass mortalities in the 1984 cohort, in 1984 and in 1986, and eventually resulted in the extinction of this cohort. Anecdotal reports indicate that outbreaks of A. planci are increasing in frequency and severity. According to Fijian reef fishermen, the starfish were low to moderate in abundance in the Suva area from the 1920s to the 1960s. A progressive build up occurred on some reefs in 1963–67 leading to a major outbreak episode from 1967–1971. The outbreaks were in a chronic phase between 1977–1989. Our findings demonstrate that there is no invariable pattern or process involved in the population dynamics of A. planci and that a single explanation of the A. planci phenomenon is therefore unlikely.     

Population genetic studies of the crown-of-thorns starfish,Acanthaster planci (L.), in the Great Barrier Reef region

Seven populations of the crown-of-thorns starfish, Acanthaster planci, were compared genetically using starch gel electrophoresis in order to investigate the extent of genetic exchange throughout the Great Barrier Reef (GBR) region. These populations extended from Lizard Island in the north to One Tree Island in the south, a distance of approximately 1300 km. Thirteen of 36 enzymes assayed were genetically interpretable, and 10 (77%) of these were polymorphic. Mean heterozygosity over all loci was 0.225. An analysis of the gene frequency heterogeneity between the populations using Wright's F _ST statistic gave an overall F _ST of 0.019. The mean unbiased value of Nei's genetic distance between the populations was 0.009. These values indicate a homogeneous genetic composition throughout the range, and are consistent with the hypothesis that gene flow between these populations is high, and that A. planci throughout the GBR region are members of a single, effectively panmictic population. Within this group, the Green Island population was most distinct genetically because of differences in allele frequencies at the MDH-1 locus. Although there is no rigorous method for determining the selective basis for such differences, it is argued that the differences observed in the Green Island population were the result of selection. The basis for selective differences was possibly food availability since, at the time of sampling, the Green Island A. planci were the remnants of a large, high-density population that caused extensive coral mortality, and suffered severe population decline as food became scarce. These findings are consistent with observations of a relatively ordered sequence of outbreaks from north to south along the GBR, suggesting that all outbreaks but the first are secondary. Control measures, both on the GBR and elsewhere, have been unsuccessful except on a very small scale. Unless a vulnerable part of the A. planci life cycle can be identified, it would seem that the greatest chance for successful control would be to identify and control the causes of the primary population outbreak.     

An assessment of the geological evidence for previous Acanthaster outbreaks

Much debate has surrounded the notion that outbreaks of the crown-of-thorns starfish (Acanthaster planci) have occurred in the geological past and hence are natural phenomena. As this debate has recently been renewed, we have reassessed statistically data presented by Frankel (1977, 1978) as evidence for the occurrence of past outbreaks. This was done using Frankel's data as well as those from extensive starfish surveys conducted prior to the commencement of his research. Our analysis of these data indicates that the occurrence of A. planci remains in recent sediments is independent of whether or not the reef from which the sample was collected had experienced a recent outbreak. Based on this premise, it is not possible to infer from Frankel's data the occurrence of past outbreaks from similar material in much older sediments. Thus while the data presented by Frankel (1977, 1978) may show that A. planci has existed within the Great Barrier Reef for at least several thousand years it does not demonstrate that outbreaks of this starfish have occurred in the geological past.     

Lipolytic enzymes of the digestive organs of the crown-of-thorns starfish (Acanthaster planci): comparison of the stomach and pyloric caeca

1. Stomach and pyloric caeca homogenates from the crown-of-thorns starfish hydrolysed p-nitrophenyl esters, alpha-naphthyl esters, cholesteryl oleate and tributyrin. The pyloric caeca contained the highest activities. 2. The p-nitrophenyl acetate hydrolytic activity eluted at 0.23 M NaCl on ion exchange chromatography while the p-nitrophenyl palmitate hydrolytic activity eluted between 0.2 and 1.0 M NaCl. 3. Polyacrylamide gel zymograms for alpha-naphthyl acetate hydrolytic activity revealed one major band and several minor bands of activity for both tissues. 4. Isoelectric focusing zymograms revealed one major band with a pI = 4.2 for both tissues, with an additional band at pI = 3.5 for pyloric caeca. 5. The pyloric caeca contained twice as much lipid as the stomach. Lipid extracts contained mixtures of steroids and steroid-esters; a cholesterol-like sterol was tentatively identified.     

Evaluation of a crown-of-thorns starfish (Acanthaster planci) control program at Grub Reef (central Great Barrier Reef)

A crown-of-thorns starfish control program was conducted at Grub Reef (central Great Barrier Reef) in an area (0.64 km²) which encompassed 53 individual patch reefs. During a two week period, 15 divers injected 3175 starfish with copper sulphate. The program was considered unsuccessful. Although starfish abundance had declined significantly after the control efforts, biological surveys indicated that a relatively large number of starfish remained. The surveys also indicated a general decline in the number of starfish along the reef perimeter, outside the control area. The total cost of the control program was $35 per starfish. These results have important implications for the implementation of future control programs and highlight the need to undertake before and after biological surveys to assess the effectiveness of the control efforts.     

Aspects of the ecology of the coral-eating starfish Acanthaster planci

In the central region of the Great Barrier Reef, Acanthaster planci eats its own disk area of coral each day. At the southern end of the reef lagoon populations of A. planci eat substantially less than this amount of coral per day. Branching and plate corals are preferred food species and massive and encrusting forms are rejected while the preferred food species are available. Only when branching and plate forms on a reef have been consumed will A. planci attack massive and encrusting species. On Australian reefs preferred food species form between 70–99% of the coral cover.
On the Great Barrier Reef A. planci spawns in January and juveniles settle in the top 3 m of water on the windward edge of reefs or on isolated patch reefs behind the main reef. Intolerance of wave attack forces the growing starfish to migrate into deeper water. Lateral movements, probably induced by shortage of living coral in deep water, bring the starfish around the ends of the reef to the leeward side. Here they destroy most of the living coral.
It is suggested that the visual impact of A. planci on reefs of the Indo-Pacific region is related to the composition of the coral fauna. Reefs with a high proportion of preferred food species will be severely damaged while those with faunas composed mainly of massive and encrusting forms will not be altered greatly by starfish predation.
Work on larval development of A. planci carried out by Henderson & Lucas, 1971 showed that metamorphosis took place only at water temperatures of 28^o -29^o C. This suggests that the A. planci plague on the Great Barrier Reef will not spread south of latitude 20^o S (29^o C isotherm in January).     

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 (<20 cm) strongly favouring nocturnal feeding. For larger 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.     

The thermal tolerance of crown-of-thorns (Acanthaster planci) embryos and bipinnaria larvae: implications for spatial and temporal variation in adult populations

To understand the role of sea temperature on the population biology of the crown-of-thorns sea star Acanthaster planci, the thermal window for embryonic and larval development was investigated. In two experiments, the response of embryos and larvae across 12 temperatures from 19.4 to 36.5 °C was quantified as the percentage of individuals reaching cleavage stage embryos, blastula, gastrula, early-bipinnaria, late-bipinnaria larvae or abnormal. Measurements were made at 7 times up to 72 h post-fertilisation, with the morphometrics of larvae measured in the 72-h sample. Acanthaster planci developed at temperatures between 19.4 and 33.2 °C, with a thermal window for development to the late-bipinnaria stage between 25.6 and 31.6 °C. Development rate, normal development and larval size were optimal at 28.7 °C, with development rates remaining relatively constant up to 31.6 °C. Rates of abnormality increased steadily (early embryonic stages) above 28.7 °C and was 100 % at temperatures approaching 33 °C. These experiments provide a more detailed insight into the response of A. planci developmental stages to temperature. The present day distribution of the species in eastern Australia overlap with the optimal thermal window for development to the late-bipinnaria stage (≈25–32 °C), implying a role of temperature in controlling population distributions and abundances. Despite this, short- or long-term temperature increases may not be a major modulator of the crown-of-thorns recruitment success, population dynamics and distribution in the future as no significant change in development rates, larval survival and growth occurred within this thermal window. Therefore, moderate (1–2 °C) increases in sea temperatures caused by El Niño or near-future ocean warming may not drive an increase in developmental and settlement success. Indeed, without any acclimation to warmer temperatures expected under near-future warming (+2 to 4 °C), climate change could ultimately reduce larval survival due to elevated mortality above the optimal development temperature.     

eDNA detection of corallivorous seastar (Acanthaster cf. solaris) outbreaks on the Great Barrier Reef using digital droplet PCR

Coral Reefs - Coral loss through consumption by corallivorous crown-of-thorns seastars (CoTS, Acanthaster spp.) is a major contributor to the coral reef crisis in the Indo-Pacific region. The...