Crown-of-Thorns

The Crown-of-Thorns starfish (Acanthaster planci) feeds on coral. Low numbers of this starfish increase reef diversity, but large numbers can destroy reefs. Avoiding human activities that increase starfish numbers is more effective than trying to control Crown-of-Thorns outbreaks once they happen.

Crown-of-Thorns starfish are good?

The Crown-of-Thorns starfish is a predator of coral. In low densities, this starfish promotes diversity in coral reef ecosystems, because it prefers to feed on fast growing Acropoid coral. This gives slower growing coral space to establish and grow.

Crown-of-Thorns starfish are bad?

Populations of the Crown-of-Thorns starfish periodically boom and when this happens thousands of starfishes swarm a reef devouring up to 90% of the coral there. This alters the coral reef ecosystem in 3 ways by:

1. lowering coral diversity and abundance
2. switching the reef ecosystem to an algae-dominated system
3. changing species composition on the reef

Crown-of-Thorns starfish outbreaks reduce diversity because many of the species that rely on coral to provide food and shelter are lost when coral is removed. Diversity loss decreases reef productivity and resilience against further disturbance.

How we influence Crown-of-Thorns outbreaks

We can make Crown-of-Thorns outbreaks worse by:

• increasing nutrient levels on the reef
• removing starfish predators

Increasing nutrient levels on the reef
Crown-of-Thorns starfish larvae float in the water eating plankton. Excess nutrients, from sources like fertilizer runoff and sediment, provide extra food for these immature starfish and allow more to survive to become adults.

Removing starfish predators
Only a few species and reef fish can feed on the adult Crown-of-Thorns starfish because its protection of venomous spines. These predators, such as Trigger fish and the Pacific Triton, are also gathered for food or for their shells. Removing predators which help to control the Crown-of-thorns population allows starfish numbers to boom. Crown-of-thorns outbreaks happen nearly 4 times more often on reefs where fishing is allowed compared to reefs that aren’t fished.

Biological control of the Crown-of-Thorns starfish

Predators of the Crown-of-Thorns starfish change throughout its lifecycle because its defense of venomous spines becomes more formidable as it gets older. Effective biological control of the Crown-of-thorns relies on coral reef diversity because these predators change throughout the starfish’s life cycle.

Predators of Crown-of-thorns starfish larvae

• sponges
• bivalves
• sea squirts
• coral

Predators of juvenile Crown-of-thorns starfish

• Fireworms
• Harlequin shrimp
• some marine snails

Predators of adult Crown-of-thorns starfish

• Pufferfish
• Triggerfish
• Pacific Triton

Some species of crab and shrimp species defend their coral colony homes from Crown-of-Thorns attacks. These animals drive the starfish away from the coral, but do not kill them.

Human control of the Crown-of-Thorns starfish

Our attempts to control Crown-of-Thorns outbreaks are usually unsuccessful because they are labor-intensive, ineffective over wide areas, expensive, and/or potentially dangerous. Outbreaks typically continue until disease or starvation causes starfish populations to crash.

Attempts to control the Crown-of-Thorns have included:

• Cutting them up
• Removing from the sea and burying them
• Underwater fences
• Injecting starfish with poison

Crown-of-Thorns and Sustainability

Like many types of disturbance, the Crown- of-Thorns starfish can be good or bad for the coral reef ecosystem. Low numbers of this starfish increases diversity, but large numbers lower it. Our activities can add to the disturbance rate by increasing Crown-of-Thorns numbers. This erodes the sustainability of coral reefs.

References

AIMS. (2007, November 7). Crown- of-Thorns starfish. Australian Institute of Marine Science. Retrieved 18 August 2008 from http://www.aims.gov.au/docs/research/biodiversity-ecology/threats/cots.html

CRC Reef Research Centre. (2003, November). Crown- of-Thorns starfish on the Great Barrier Reef. CRC Reef Research Centre. PDF retrieved 18 August 2008 from http://www.reef.crc.org.au/publications/brochures/COTS_web_Nov2003.pdf

Great Barrier Reef Marine Park Authority. (1995, April). Controlling Crown- of-Thorns starfish. CRC Reef Research Centre. Crown- of-Thorns starfish. Retrieved 18 August 2008 from http://www.reef.crc.org.au/publications/explore/feat45.html

Kosarek, N. (2000). Acanthaster planci. Animal Diversity Web. Retrieved 18 August 2008 from http://animaldiversity.ummz.umich.edu/site/accounts/information/Acanthaster_planci.html

Madl, P. (2002, December). Acanthaster planci (An overview of the Crown- of-Thorns starfish). University of Salzburg. Retrieved 18 August 2008 from http://www.sbg.ac.at/ipk/avstudio/pierofun/planci/planci.htm

Sweatman, H. (2008, 22 July). No-take reserves protect coral reefs from predatory starfish. Current Biology 18(14): 598-599. PDF retrieved 28 August 2008 from http://www.aims.gov.au/source/media/news2008/pdf/sweatman-2008-cots-mpas-biology-paper.pdf

Wikipedia. (2008, August 8). Crown- of-Thorns starfish. Wikipedia. Retrieved 18 August 2008 from http://en.wikipedia.org/wiki/Crown-of-thorns_starfish