Spotlight on the Crown of Thorns starfish

It just so happened that two of the three quadrats studied in the Adopt-a-Coral event contained a thriving example of an adult Crown of Thorns Starfish (Acanthaster planci). Often abbreviated COTS, these animals don’t look like your everyday starfish. They usually have between 14-18 arms and are covered by long, sinister-looking spines of up to five centimeters long. The spines are quite sharp, and can inflict a nasty wound!

COTS are coral predators, and prefer to feed on live corals. An adult starfish can consume around five to six square meters of coral per year. Low numbers of individuals commonly occur on reefs, but occasionally their numbers increase dramatically, with thousands of starfish occurring on a single reef. These outbreaks have caused massive destruction on coral reefs around the Pacific, as well as in the Red Sea and the Indian Ocean.

Here in the CNMI the numbers have not been as catastrophic, but we have experienced several different instances of higher-than-average populations. In 2001, MMT scientists witnessed increased numbers of individuals in Lau Lau Bay and at Tinian’s Unai Babui site. More recently, COTS have become increasingly abundant at Obyan, Boy Scout Reef, and at other sites around Naftan Point.

There are two main theories as to what causes COTS outbreaks. One explanation is that they occur naturally, and their effects on coral reefs are similar to that of typhoons. Another theory is that human activities contribute to COTS outbreaks. Increased runoff and land-based pollution from coastal development encourage the growth of phytoplankton, a major food source for COTS larvae. Thus larvae are better able to survive and grow into adult populations. In addition, humans may increase adult numbers by removing natural predators such as tangison (humphead wrasse) and the Triton Trumpet snail.

Whatever the cause, the immediate effects are well documented on coral reefs around the world. Locally, between 2003, when increased numbers of COTS were first seen at Obyan and Boy Scout, and 2004, the starfish devoured many of the fast-growing, branching species of coral. Before COTS appeared on the reef, Boy Scout had a mixture of different types of coral, all fulfilling important functions in the reef community. A year later, most of those corals eaten by the COTS have been replaced by a single genus of coral, Porites. Although the overall coverage of live coral has not changed significantly, the diversity has dropped.

The question is, does it matter in the long run? According to DFW biologist Mike Trianni, it’s way too soon to tell. “COTS outbreaks are a naturally occurring, cyclical phenomenon, and their long term effects are not very well understood,” Trianni says. The Coral Reef Science Committee has recently agreed to implement a long term study on the effects of removing COTS, to address some of the unanswered questions surrounding these organisms.

There are many factors other than COTS that impact reef habitats and community structure. Perhaps the biggest problem is the combined effect of multiple environmental stresses on coral reefs. A healthy reef damaged by a COTS outbreak can often recover in three or four years, once the COTS have moved on. This illustrates the concept of “resilience,” or ability to recover from a destructive event.

In areas that have high levels of runoff, sedimentation, fresh water input, or sewage outfalls, reefs do not have that resilience, and cannot recover as quickly. Like us humans, when reefs are stressed, they aren’t able to heal as well. In these cases, dead skeletons left by COTS, disease, or disturbances may be colonized not by new corals, but by various types of algae, causing what scientists term a “phase shift.” It is similar to the shifting baselines that I wrote about two weeks ago. A once thriving coral community shifts to a mainly algal habitat, unable to support a diversity of fish and other species, ultimately affecting our use and enjoyment of the area.

Scientists hope that the corals at Boy Scout will be able to quickly recover their natural diversity because, unlike in LauLau Bay, terrestrial runoff is still relatively low. Only time will tell, however. Efforts such as the Youth Alliance’s coral monitoring program will help the CNMI’s natural resource agencies to keep track of the changes occurring on our reefs, and help us learn to improve management strategies. (Qamar Schuyler)

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Thanks to Peter Houk, Mike Trianni, and the Australian Institute of Marine Science website for information on COTS.