The Crown of Thorns is a beautiful multiarmed sea star that holds a notorious reputation around Indo-Pacific waters for being dangerous to shallow coral reefs. They are a corallivores animal, meaning they prey on a primary diet of coral polyps, causing serious damage to unprotected coral. Adults can grow to a diameter of around 12 inches, the majority of which is covered by their large circular bodies of which their short arms protrude from. Their surface is covered by long sharp spines that carry a harmful venom. These spines have no injecting mechanism, however the venom can be carried by tissues if punctured by the spines causing a stinging pain, inflammation, and persistent bleeding as the toxins prevent blood clotting; so watch out! Fortunately Crown of Thorns are relatively easy to spot while diving due to their size and exotic colours which are often displayed as warning colours for would-be predators.
As mentioned, the Crown of Thorns are a danger to coral reefs due to their predatory habits. To begin feeding, first the sea star must station itself on a coral surface and excretes its stomach out underneath its body to spread to the full reach of its diameter. It then secretes a powerful enzyme that begins to break down the coral exoskeleton in order to filter and absorb the nutrients from the liquefied coral tissue. This does not kill the coral, but leaves the bare skeleton very vulnerable to infestation by several algae which prevents rejuvenation.
This is a natural part of the ecosystem in most cases, and prevents fast growing coral like Staghorn or Table coral from out competing slow growing coral like Boulder or Massive coral. However, an increasing population of these predatory sea stars in one area can have a harmful effect on the health of a coral reef ecosystem. The Great Barrier Reef has had such an outbreak in recent years where the Crown of Thorns were thought to be consuming coral at a faster rate than its growth. This can indirectly affect marine species that rely on coral reefs for habitats, protection and food. Population ‘booms’ are thought to be caused by an increase in oceanic phytoplankton, which may in turn be caused by land run-off containing nutrient rich fertilisers from increased industrial farming methods. This allows many more Crown of Thorns larvae to reach maturity than naturally; and considering a single female can produce 65 million eggs in a single spawning season, this could become a huge problem of overpopulation if left uncontrolled.
To prevent coral destruction in vulnerable areas, divers are starting to introduce a number of measures to control Crown of Thorns population sizes. Manual dismemberment is sometimes implemented, but ill-advised due to the likelihood of harmful contact in addition to the sea stars potential ability to regenerate. The most effective and humane way to reduce population sizes is through the injection of salts or chemicals that are deadly to sea stars, but pose no threat to other marine life. Coral scientists and conservation organisations are now even inventing autonomous sea star killing drones to help control population outbreaks and protect valuable coral reefs.