Crown-of-thorns starfish (COTS) Acanthaster planci and the Great Barrier Reef.

The crown-of-thorns starfish (COTS) (Acanthaster planci) feeds almost exclusively on hard corals and is endemic to coral reef ecosystems throughout the Indo-Pacific (Birkeland 1990). These large starfish are covered in sharp toxic spines and once grown have few natural predators. Over their lifetime they can produce 100’s of millions of eggs that have the highest fertilisation rate recorded for any spawning marine species. The pelagic larvae produced from spawning can float in the water column for weeks before settlement and can easily travel large distances between reefs.

In the 1960’s pioneering coral reef ecologists were horrified when they discovered the propensity of COTS to emerge suddenly in large unsustainable numbers that decimated coral cover on affected reefs (e.g. Barnes 1966). Researchers soon determined that COTS populations oscillated wildly. Periods of low-density, with individuals scarcely distributed among large reef areas were interspersed with briefer episodes of high densities, where unsustainable masses (termed outbreaks) rapidly consumed available hard corals, once thriving reefs were turned to apparent wastelands (Moran 1986).

Crown-of-Thorns Starfish outbreak - Before After on reef

Crown-of-thorns outbreaks have a devastating effect on coral cover. Here is a picture of a reef before a COTS outbreak (left) and after (right). Once the coral has been eaten it rapidly becomes covered in algae and starts to break down.

by AIMS - LTMP

The discovery of outbreaks had a galvanising effect on reef ecologists who feared for the future of the coral reefs that they inhabited (Sapp 1999). On the Great Barrier Reef (GBR) three series of outbreaks (beginning in 1962, 1979, 1994) have had a particularly devastating effect on coral cover over a wide area. Between 1995 and 2009, COTS outbreaks were responsible for greater declines in coral cover on the GBR than any other disturbance including cyclones, disease and coral bleaching (Osborne et al. 2011). Furthermore long term monitoring data showed overall coral cover on the GBR had declined by half from 1985 through to 2012, with 42% of the observed decline attributed to coral predation by COTS (De’ath et. al.2012). This decline has become of critical concern with a new series of outbreaks developing on reefs north of Cairns in recent years that appear to mirror those seen historically. Although, outbreaks have occurred in the GBR for 8000 years (Walbran et. al. 1989) impacts of COTS have become of critical importance in recent times as evidence mounts about the role nutrients in the water column plays generating outbreaks (Mendonca et. al. 2010). There is a concern that the frequency of outbreaks has increased during recent decades and this increase is linked to eutrophication of GBR waters from rivers that flow into the GBR with extensively modified catchments (Fabricius et. al. 2010).

Central to managing the crown of thorns starfish phenomenon is the determination of how outbreaks develop and whether they can be controlled. Hypotheses generally revolve around two basic themes: natural causes due to normal population fluctuations (Vine, 1970) and anthropogenic causes; usually put down to enhanced larval survival due to elevated nutrients from eutrophication due to terrestrial runoff (Birkeland, 1982; Brodie et al., 2005), or increased post settlement survival due to the removal of COTS predators (Randall, 1972; Endean, 1977). In most cases assigning cause(s) for COTS outbreaks tends to be problematic and the explanation may vary among locations with COTS outbreaks likely due to a combination of factors (Pratchett, 2005).

Typically outbreaks start as an abrupt population increase by orders of magnitude from a small parent population termed a ‘primary outbreak’ (Birkeland and Lucas 1990). Once a primary outbreak has occurred more outbreaks are likely to occur if conditions are right on reefs located downstream in prevailing water currents. These are termed secondary outbreaks and are a simple consequence of the large numbers of larvae (in their billions) produced upstream by the primary outbreak. This was first recognised on the Great Barrier Reef where historically the first confirmed COTS outbreak on the GBR was at Green Island - in 1962 (Barnes 1966).

Between 1966 and 1974 surveys were conducted at various times and locations to determine extent of COTS activity on the GBR (Moran 1986). From these surveys it became apparent that outbreaks occurred further south with time (Talbot and Talbot 1971, Pearson 1972, Endean 1974). Under this scenario primary outbreaks produced a cloud of larvae that travelled south with water currents seeding reefs downstream. In turn secondary outbreaks produced larvae that travel further south and outbreaks appeared move down the GBR with time. This pattern had become well established by the third series of outbreaks recorded on the GBR beginning that began in 1993 (Miller 2000).

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AIMS began annual broadscale to document patterns of COTS outbreaks on the GBR in 1985. These surveys addressed methodological differences, common in previous surveys, by taking a systematic approach with well‑defined methodological criteria for (Moran et al. 1988). Results obtained from these surveys are used to describe the pattern and extent of COTS activity on the GBR. By determining the pattern and extent of COTS outbreaks through space and time provides a valuable tool for reef managers to understand possible causes of outbreaks and implement appropriate management strategies to control their impacts.

References

Barnes J. H. (1966). The crown-of-thorns starfish as a destroyer of coral. Australian Natural History 15:257-61

Birkeland, C. (1982). Terrestrial run-off as a cause of outbreaks of Acanthaster planci (Echinodermata: Asteroidea). Marine Biology 69:175–185.

Birkeland, C., and Lucas, J. S. (1990). ‘Acanthaster planci: Major ManagementProblem of Coral Reefs.’ (CRC Press: Boca Raton, LA.)

Brodie, J., Fabricius, K., De'ath, G., and Okaji, K. (2005). Are increased nutrient inputs responsible for more outbreaks of crown-of-thorns starfish? An appraisal of the evidence. Marine Pollution Bulletin 51:266–278.

De'ath, G., Fabricius, K.E., Sweatman, H. and Puotinen, M.J. (2012). The 27–year decline of coral cover on the Great Barrier Reef and its causes. Proceedings of the National Academy of Sciences 109(44):17995–17999.

Endean, R. (1974). Acanthaster planci on the Great Barrier Reef. In Proceedings of the 2nd International Coral Reef Symposium. Vol 1:563-576.

Endean, R. (1977). Acanthaster planci infestations on reefs of the Great Barrier Reef. In Proceedings of the 3rd International Coral Reef Symposium. 1:185-191.

Fabricius K.E., Okaji, K., and De’ath, G. (2010). Three lines of evidence to link outbreaks of the crown-of-thorns seastar Acanthaster planci to the release of larval food limitation. Coral Reefs 29:593–605.

Mendonça, V.A., Al Jabri, M.M., Al Ajmi, I., Al Muharrami, M., Al Areimi, M., Al Aghbari, H.A. (2010). Persistent and expanding population outbreaks of the corallivorous starfish Acanthaster planci in the North western Indian Ocean: are they really a consequence of unsustainable starfish predator removal through overfishing in coral reefs, or a response to a changing environment? Zoological Studies 49:108-123.

Miller, I. (2000). Historical patterns and current trends in the broadscale distribution of crown-of-thorns starfish in the northern and central sections of the Great Barrier Reef Proceedings 9th International Coral Reef Symposium. 2:1273–1279.

Moran, P.J. (1986). The Acanthaster phenomenon. Oceanography and Marine Biology Annual Review 24: 379-48.

Moran, P.J., Bradbury, R.H., and Reichelt, R.E. (1988). Distribution of recent outbreaks of the crown-of-thorns starfish (Acanthaster planci) along the Great Barrier Reef: 1985-1986. Coral Reefs 7:125-137.

Osborne, K., Dolman, A.M., Burgess, S.C. and Johns, K.A. (2011). Disturbance and the Dynamics of Coral Cover on the Great Barrier Reef (1995–2009). PLoS ONE 6(3): e17516. doi:10.1371/journal.pone.0017516, 2011.

Pearson, R.G. (1972). Changes in the distribution of Acanthaster planci populations on the Great Barrier Reef. Nature 237:175.

Pratchett, M.S. (2005). Dynamics of an outbreak population of Acanthaster planci at Lizard Island, northern Great Barrier Reef (1995–1999). Coral Reefs 24:453–462.

Randall, J. E. (1972). Chemical pollution in the sea and the crown-of-thorns starfi sh (Acanthaster planci). Biotropica 4:132–144.

Sapp, J. (1999). What is Natural? Coral Reef Crisis. Oxford University Press, New York.

Talbot, F.H. and Talbot, M.S. (1971). The crown-of-thorns starfish (Acanthaster) and the Great Barrier Reef. Endeavour. 30:38.

Vine, P. J. (1970). Densities of Acanthaster planci in the Pacific Ocean. Nature 228:341–342.

Walbran, P.D., Henderson, R.A., Jull, A.J.T. and Head, J.M. (1989). Evidence from sediments of long-term Acanthaster planci predation on coral of the Great Barrier Reef: Science 245:847–850. doi: 10.1126/ science.245.4920.847.