Crown-of-thorns Seastar (Acanthaster planci)

Three COTS off Oahu. NOAA photo by Jean Kenyon.
Three COTS off Oahu. NOAA photo by Jean Kenyon.

The crown-of-thorns seastar (COTS) is a unique and interesting starfish found throughout the Indo-Pacific region, from the Red Sea to the western coast of Panama. Reaching up to a half meter in diameter, it is the largest tropical starfish. The following characteristics distinguish COTS from other seastars found in coral reefs:

  • Disc-shaped morphology
  • Multiple arms (13–16)
  • Venomous spines (thorns)
  • Pliable and prehensible nature

The large disc-shaped morphology of COTS supports a considerable stomach that is capable of gorging great quantities of its favorite prey, corals. When consuming a coral, it extrudes its stomach and excretes emulsifying enzymes that digest the coral polyps thereby enabling it to ingest the resulting nutrients. When it has finished its meal, the consumed area of the coral becomes white.

A partially exposed stomach of a COTS. NOAA photo by Molly Timmers.
A partially exposed stomach of a COTS. NOAA photo by Molly Timmers.
A COTS with its meal. NOAA photo by Molly Timmers.
A COTS with its meal. NOAA photo by Molly Timmers.

Because it can take four to six hours to feed, COTS have evolved with venomous thorns that protect them during this vulnerable state. If the thorns or spines of a COTS puncture your skin, the venom can cause a reaction.

Thorns on a COTS. NOAA photo by Molly Timmers.
Thorns on a COTS. NOAA photo by Molly Timmers.
A reaction to COTS venom from a puncture on a diver's pointer finger. NOAA photo by Molly Timmers.
A reaction to COTS venom from a puncture on a diver's pointer finger. NOAA photo by Molly Timmers.
Tube feet on the underside of a COTS arm. NOAA photo by Molly Timmers.
Tube feet on the underside of a COTS arm. NOAA photo by Molly Timmers.

Interestingly, because this seastar is pliable and has many arms containing hundreds of prehensible tube feet, it does not need a flat or round surface to feed. It can target corals of any shape and size. Tube feet not only enable a COTS to climb, grasp, and hang onto its prey but also provide movement. COTS can travel at speeds up to 20 m per hour, allowing for easy movement from one prey to the next.

All of these traits produce a distinguished and, at times, devastating coral predator. In low densities, COTS have little effect on coral reefs. However, when COTS aggregate and form large densities called outbreaks, they can decimate coral habitats.

Outbreaks

One of the greatest biological disturbances to coral reefs is an outbreak population of COTS. Outbreak populations can devour up to 6 km2 of living coral tissue in a single year, thereby altering coral community structure, promoting algal colonization, and affecting fish population dynamics. Outbreaks on the Great Barrier Reef, Guam, American Samoa, and Japan have resulted in up to 90% coral mortality in localized areas.

An outbreak of COTS at Kingman Reef.  NOAA photo by Molly Timmers. An outbreak of COTS at Kingman Reef.  NOAA photo by Molly Timmers.
An outbreak of COTS at Kingman Reef. NOAA photo by Molly Timmers.

How are outbreaks formed?

What specifically triggers outbreaks is still under debate. The following are some of the explanations that have been postulated:

  • Hurricanes
  • El Niño-Southern Oscillation events
  • Larval retention from eddy formation
  • Rising sea-water temperatures
  • Predator removal
  • Nutrient discharge
  • Transition zone chlorophyll front
  • Mass dispersal events

The most-accepted driver of outbreak formation is enhanced phytoplankton food biomass for larvae as a result of environmental and anthropogenic events. COTS are highly fecund broadcast spawners. Adult females are capable of producing and releasing up to 50 million eggs into the water column. Subsequent larval stages are planktonic, and larvae feast on phytoplankton until they settle on the benthos where they then metamorphose into starfish. If the larval food source is readily available, survivorship increases and, in turn, so does the rate of settlement.

What types of research are we conducting on COTS?

For more information contact Molly Timmers.