By Orion McCarthy  

CORAL REEFS across the globe are under threat from a predator that looks less like a sea creature and more like an alien life form.

A crown of thorns starfish. Photo credit: Science in Public.

The crown of thorns starfish is a massive, prickly, and venomous inhabitant of the tropical waters surrounding coral reefs. Capable of growing up to a meter in diameter, they can move quite quickly (at least for a starfish) and have an insatiable appetite for coral.

A single crown of thorns starfish, or COTS for short, can eat up to 6 square meters of living coral each year. The starfish clamber on top of branching corals, contorting their body shape to cover as much coral surface area as possible. Then, they extrude their stomach and secrete digestive fluids, dissolving the thin layer of living coral tissue to leave behind a barren white coral skeleton.

An outbreak of crown of thorns starfish, or COTS for short. Photo credit: Science in Public.

Australia has been hit particularly hard by the ravenous starfish. Since 1985, the Great Barrier Reef has lost half of its coral cover, with COTS related damage accounting for 42% of the decrease. After cyclones and mass bleaching events, COTS outbreaks pose the greatest threat to coral reef ecosystems.

Stopping the spread of COTS has emerged as a top priority for coral reef managers and conservationists. Scientists are using every tool in their arsenal to subdue the relentless invertebrate, from high tech automated robots to everyday household products.

The Role of Starfish on the Reef

Surprisingly, a species capable of such destruction is actually a native, naturally occurring component of coral reef ecosystems.

In limited numbers, predatory starfish play a crucial roll on coral reefs by increasing coral diversity. Photo credit: Tourism Queensland.

At densities below 30 starfish per hectare, COTS play a vital roll in coral reef ecosystems. The starfish prefer to eat rapidly growing branching corals, pruning them back like an undersea gardener. This reef maintenance allows slower growing corals to thrive alongside their prolific neighbors, and increases the overall diversity of the reef.

Problems occur when COTS populations rise above manageable thresholds, and hordes of insatiable starfish prune coral faster than they can regenerate.

Sudden increases in COTS concentrations called outbreaks can exceed concentrations of 1000 starfish per hectare. On the Great Barrier Reef, outbreaks can last up to 15 years as the plague of starfish spread along the 2300 km length of the reef. Scientists discovered the first large starfish outbreak in 1962, and the latest COTS outbreak on the Great Barrier Reef commenced in 2010.

The Triton trumpet snail is one of the COTS few predators. Photo credit: Oceanwide Images.

Starfish outbreaks on coral reefs occur when reef ecosystems fall out of balance. Normally, COTS populations are kept in check in two ways. Limited nutrients in the water column keep too many COTS larvae from reaching adulthood, and predation from certain large fish and snails keep adult COTS populations under control.

A large female starfish is capable of releasing 50 million eggs, but with limited nutrients available and lots of predators, only about 5 larvae will reach adulthood. But if added nutrients cause the survival rate to increase in just 0.1%, up to 50,000 coral-consuming starfish will reach adulthood.

For the starfish, human pressures have changed coral reef ecosystems for the better. Overharvesting predatory fish and snails as well as nutrient influxes from sewage and agricultural runoff create ideal conditions for COTS and leave their populations unchecked.

Stopping the Spread

COTSbot, the automated submersible robot that euthanizes killer starfish. Photo credit: QUT.

While policy makers haggle over the best means of limiting agricultural runoff, scientists have been hard at work researching ways to disrupt the current starfish outbreak.

Researchers at the Queensland University of Technology have recently begun testing their very own starfish-killing contraption. Known as COTSbot, the fully automated robot is capable of detecting the prickly predator out in the field, relying on GPS navigation and an enormous database of starfish images taken by scientific divers to positively identify COTS.  Once the robot encounters the starfish, it injects a complex serum of chemicals into the organism, euthanizing it over a period of 24 to 48 hours.

COTSbot in action. The fully automated robot cruises about the water column, scanning for marauding starfish to cull. Photo credit: Australian Broadcasting Corporation.

Thanks to a recent study from James Cook University, COTSbot’s complex serum of chemicals can be replaced with an ecologically harmless, permit free, easily obtained, and inexpensive alternative.

That chemical is dilute acetic acid, also known as vinegar.

By injecting each starfish with 25ml of household vinegar, researchers were able to kill 100% of their starfish test subjects within 48 hours. The acidity of the vinegar essentially disintegrates the starfish’s internal organs, dissolving them from the inside out.

Corals have shown promising signs of recovery on reefs where divers have euthanized COTS.  But euthanizing starfish one by one with automated robots and cooking ingredients isn’t a permanent fix.

With an estimated starfish population of 1.5 million, injecting every COTS on the Great Barrier Reef is an outlandishly enormous task. However, the robot can function as a stopgap for saving individual reefs that make up the Great Barrier Reef. A single dive team can euthanize 1,000 starfish in a 45-minute dive, and COTSbot will be able to stay underwater far longer than a human diver once fully operational.

Still, the best chance for long-term success in controlling COTS populations is concrete action from government officials to control effluent pollution and water quality.

Divers remove COTS from the Great Barrier Reef. Photo credit: NOAA.

What YOU can do

Nutrient runoff. Photo credit: Kim Starr.

Human development and agricultural runoff are responsible for water quality problems across the world. Apart from causing outbreaks of unwanted starfish, polluted waters can harbor disease and spawn algal blooms, which can cause toxic, oxygen-depleted dead zones to form.

Even if you don’t live near a coral reef, you can take action to protect the water quality of costal ecosystems near you.

• Fertilize sparingly – We all want a luscious green lawn, but the fertilizers and pesticides used to keep grass green ultimately end up in the watershed after a rainstorm. If you must fertilize, do so sparingly to reduce nutrient runoff into local waterways. Better yet, opt for native plants that require less fertilizer.
• Reduce runoff – In an urban environment, rainstorms can cause impressive quantities of storm water runoff, leading to flash flooding, erosion, and sediment plumes. Decrease runoff from your property by diverting downspouts into a rain garden to slow drainage, or by storing excess runoff in rain barrels.  You can also limit runoff by constructing pathways with porous materials, such as brick or gravel, instead of solid pavement.
• Buy organic – Large-scale agriculture is by far the largest contributor to nutrient pollution in coastal waters. Encourage the responsible use of fertilizers and pesticides with your wallet by going organic.

LEARN MORE

• VIDEO shows crown of thorns starfish devouring coral
• Research on Lizard Island seeks to control COTS outbreaks
• EASY ways to protect waterways from nutrient pollution
• Master your garden and reduce storm water runoff

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