How scaring starfish could help to save the Great Barrier Reef

During an outbreak, crown-of-thorns starfish can number in the millions and decimate coral reefs. Australian Institute of Marine Science/AAP

Crown-of-thorns starfish are one of the biggest threats to the Great Barrier Reef. Since 1985, the Great Barrier Reef has lost half its coral cover, with almost half of this coral loss due to the crown-of-thorns starfish.

But researchers are coming up with innovative new ways to manage starfish – including scaring them off the reef.

The crown-of-thorns’ impact on the reef is basically a numbers game: during an outbreak they may number in the many millions. Since a single adult crown-of-thorns can eat up to 10 square metres of living coral a year, this means a significant loss of coral cover. Coral loss can undermine the ecological integrity of the reef, as well as make the reef less attractive to tourists.

With increasing attention on the Great Barrier Reef – including an upcoming decision on whether or not to list this World Heritage Area as in danger – many are looking at ways to improve the reef’s outlook.

Research into the Great Barrier Reef consistently shows the crown-of-thorns starfish as a key driver of reef degradation. Declines in coral cover could be reduced if outbreaks were managed, and it is clear that just leaving the reef alone is no longer a satisfactory option.

Boom and bust

The crown-of-thorns is native to reefs throughout the tropical Indian and Pacific Oceans. While they’re a natural part of reef life, population explosions (outbreaks) cause coral reef degradation, a major threat to the Great Barrier Reef but also to the reefs of Indonesia, Japan and the Red Sea.

Probably more than any other coral reef animal or plant, the crown-of-thorns has traits that lead to booms and busts in their numbers. These include their rapid growth and early age of sexual maturity, as well as some of the highest fertility rates of any known marine invertebrate. They are also specialist coral-eaters – coral forms large pasture-like marinescapes that are an almost unlimited food source for these starfish.

The crown-of-thorns eats in a similar way to this starfish – by extending its stomach out of its mouth.

On the Great Barrier Reef there have been four major outbreaks of the crown-of-thorns: the 1960s; the late 1970s; the early 1990s; with the latest, and ongoing, outbreak starting in 2010.

The crown-of-thorns starfish are considered a classical boom (outbreaks) and bust species – somewhat similar to locust plagues. What drives these booms is complex, and not entirely understood.

Horseshoe Reef after a crown-of-thorns invasion. Since 1985, the crown-of-thorns has destroyed 40% of the coral cover on the Great Barrier Reef. Australian Institute of Marine Science/AAP

One (of several) hypotheses is that apparent increases in outbreaks over the last 30 years on the Great Barrier Reef are related to major flooding events and the increased nutrient flows in the Great Barrier Reef lagoon. This can induce phytoplankton (single cell algae) blooms. Many organisms benefit from phytoplankton blooms, with one of them being increased larvae survival of crown-of-thorns starfish which potentially paves the way for a future outbreak.

The Great Barrier Reef is prime real estate for these starfish. Many of the 3,000 individual reefs are used as stepping stones, so the crown-of-thorns can expand their cover. The interconnected nature of the Great Barrier Reef may make it more vulnerable to the crown-of-thorns than other reefs, as it means they can spread easily.

It is estimated that more than one in three reefs on the Great Barrier Reef have been affected by the crown-of-thorns starfish over the past 60 years.

A constant battle

Although a challenge, crown-of-thorns outbreaks can be controlled so that coral loss can be minimised. Whereas it is impossible to remove each and every individual starfish during severe, on-going population outbreaks, the removal of each adult means one less mouth to feed and hence diminishes the rate of coral cover loss.

Programs that physically remove individual crown-of-thorns have been attempted since the 1950s. These were based on divers collecting the starfish by hand to dispose of individuals, or by injecting them with chemicals that cause death. However, this treatment required divers to inject each starfish multiple times to be effective, which was time-consuming and costly.

Recently, research in Australia found that a single injection of bile salts (sodium cholate and sodium deoxycholate) is highly lethal to the crown-of-thorns. Since only one injection is required, this significantly reduces the time required to eliminate each starfish. This method has increased efficiency by a factor of ten at removing crown-of-thorns compared to other injection methods.

The federal government’s Targeted Crown of Thorns Starfish Control Program uses the single injection method to manage crown-of-thorns numbers at high-value reefs between Cairns and Port Douglas.

However, these direct physical control programs are only applicable at small local scales.

Using scents to fight the crown-of-thorns

The purpose of large scale control programs is to eliminate crown-of-thorns outbreaks which pose an immediate threat to living corals, rather than to kill starfish haphazardly or to eliminate every crown-of-thorns from reefs.

Current research is trying to identify a potential crown-of-thorns “Achilles heel” for controlling these starfish on a large scale.

Efforts are underway to identify possible diseases specific to the crown-of-thorns that could be used as a wide-spread control agent (similar to using Myxomatosis to control feral European rabbit populations).

Another biological control approach includes the search for natural repellents and attractants to control the crown-of-thorns.

The near-sighted and deaf crown-of-thorns has a remarkable sense of smell.

Chemical cues are the primary way marine organisms communicate. Chemical communication – or smell – is the most ancient form of animal communication, but one of the least understood.

One of the predators of crown-of-thorns, the giant triton snail, releases a chemical that induces terror and dispersal in the crown-of-thorns.

The crown-of-thorns starfish has a strong reaction to the scent of a giant triton.

Unfortunately, the giant triton only eats around one crown-of-thorns a week, so they aren’t very effective at controlling crown-of-thorns outbreaks of hundreds of thousands on their own.

If the chemical that causes this fear in the crown-of-thorns can be identified, it may be possible to place alarm-inducing baits on reefs to limit outbreaks.

Baits could also be used to attract crown-of-thorns into a pot-trap fishery. These starfish form large aggregations, which individuals may be attracted to by pheromones. Pot-traps – baited with food, and detected by smell – are an effective and established fishery technology to harvest marine animals.

They could also work for the crown-of-thorns.