Crown of Thorns starfish

Crown Of ThornsStarfish

Outline• COTS are one of the main threats tocorals that affect the entire reef• History of outbreaks• COTS biology• What causes outbreaks?

THE CROWN-OF-THORNS STARFISHAcanthaster planci (Linnaeus, 1758)

The crown-of-thornsstarfish• Native to reefs throughout the Indianand Pacific oceans (not in Atlantic)• Feeds predominantly onscleractinian corals (each starfishconsumes 100-250cm 2 .day -1 )• Found mostly at very low densities,and have little impact on coralcommunities

Outbreaks ofcrown-of-thornsstarfish

Major disturbances to coral reefsA. planci outbreaksCoral BleachingCyclones

History of Outbreaks on theGBR• First recorded outbreak at Green Island in1962 (the first published account of starfishoutbreaks, in english)Large numbers ofstarfish formed afront whichtraversed the reef,decimating allcorals in their path

History of Outbreaks on theGBR• First recorded outbreak at Green Island in 1962• Subsequent surveys (conducted 1965-1972)revealed outbreaks along the entire GBR1967-691962-6519671969-72

History of Outbreaks on theGBR• First recorded outbreak at Green Island in 1962• Subsequent surveys (conducted 1965-1972)revealed outbreaks along the entire GBR• Initial outbreaks (around Cairns) regarded asprimary outbreaks, sparking secondaryoutbreaks on reefs to the north and south• During this period (1962-1972) outbreaks werealso recorded from reefs elsewhere in the Pacific(Hawaii, Micronesia and southern Japan).

Geographical distribution of A. plancioutbreaks(After Moran 1986)

1970’s: The heyday of COTS research• Worldwide outbreaks of A. planci generatedconsiderable public concern• Scientist lobbied for considerable researchfunding, often expressing highlysensationalist views of starfish outbreaks• AUS$3.5 million spent on COTS research,directed mostly at futile control effortsBy the late 1970’s A. planci had all butdisappeared and interest quickly faded

History of Outbreaks on theGBR• The recurrence of oubreaks in the early 1980’sgenerated renewed interest in A. planci• Reef-wide monitoring was initiated

History of Outbreaks on theGBR• The recurrence of oubreaks in the early 1980’sgenerated renewed interest in A. planci• Reef-wide monitoring was initiated• Outbreaks spread southward from their center oforigin somewhere around Lizard Is.• Only about 15% of the 500 reefs surveyed wereaffected (mostly mid-shelf reefs)• The second outbreak reported to have had amuch greater impact than the first (but verylimited data)• Outbreaks regarded as cyclical with a third seriesof outbreaks expected in the mid 1990’s

Starfish densitiesHistory of Outbreaks on theGBR• In all, there have been at least three separateepisodes of starfish outbreaks on the GBR1968 1981 1995

History of Outbreaks on theGBR• In all, there have been at least threeseparate episodes of starfish outbreakson the GBR• Outbreaks originate on northern reefs andthen spread southwards• THE CAUSE OF OUTBREAKS ISUNKNOWN!

What is an outbreak?• Biological Definition - Rapid and dramaticincreases in the abundance of an organismindependent of changes in the resource base• Ecological Definition - Increases in thedensities of organisms to a level above whichthe available resources can sustain.• Operational Definition for Crown-of-ThornsStarfish Outbreak - A population whichexceeds 200 starfish per hectare.

Starfish densitiesWhy 200 starfish per hectare?• Historically - This represents the mean densitiesof starfish on Green Island in 1962.1968 1981 1995200 starfish.ha -1• Ecologically - Estimated to be the maximumsustainable density of starfish on reefs in theGBR with reasonable coral cover (Moran &DeAth 1992).

What causes COTSOutbreaks?

Predisposing factors for OutbreaksKey FactorsHigh fecundityShort generationtimesHigh juvenilemortalityDietary flexibilityHabitat generalistBiology of crown-of-thorns starfishFemales produce up to 60million eggs each yearStarfish are capable ofreproducing within 2 yearsVery high larval (>90%) andjuvenile (ca 30%) mortalityA. planci can eat a widevariety of different preyA. planci live in a wide rangeof different environments

General Biology of A. planciREPRODUCTIONFertilization success (%)804000 5 30 60Distance apart (metres)

General Biology of A. planciClearly, crown-of-thorns starfish arepredisposed to major populationfluctuations (through their uniquelife-history characteristics), but thisdoes not explain when and whyoutbreaks actually occur.

Numerous hypotheses have been put forward toexplain the occurrence of COTS outbreaksNaturalPre-SettlementNatural causeshypothesis (Vine 1973)Post-settlementAdult aggregationhypothesisAdult aggregation(Dana 1972)hypothesis Prey-threshold (Dana et al.hypothesis (Antonelli1972)1984)AnthropogenicTerrestrial-runoffhypothesis (Birkeland1982)Predator-removalhypothesis (Endean1969)

Coral associated reef fishButterflyfish(Family Chaetodontidae)Damselfish(Family Pomacentridae)

Summary• COTS are one of the main threatsto corals that affect the entire reef• Three major outbreaks haveoccurred, causing major damage• Causes unknown

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