Australia is no stranger to outbreaks of invasive species, extending all the way out to the expansive Great Barrier Reef. One species of note that has plagued the Great Barrier Reef on more than one occasion with significant consequences is the crown of thorns starfish, perhaps the most formidable of all sea stars. But the magnitude of the latest population explosion could be more than we can manage this time.
In a normally functioning, balanced reef ecosystem, the crown of thorns starfish isn’t necessarily a villain; rather, it is a piece of the puzzle that serves to maintain coral biodiversity by controlling the populations of fast-growing corals so that they don’t choke out slow-growing species. The crown of thorns starfish feeds solely on living coral, spending the entirety of its life essentially mowing corals down right where they stand.
A single crown of thorns starfish can consume up to six square meters of coral per year, which certainly doesn’t seem like much when compared to the massive scope of the Great Barrier Reef. But multiplied by thousands, the impacts of these carnivorous sea stars become impossible to ignore, and require immediate intervention by marine experts to protect reefs from being literally eaten out of house and home.
The crown of thorns is able to take over a reef almost without interference. As its name suggests, the crown of thorns starfish is covered in long spines on its arms and body, resembling a thorny crown, and each spine is rigid and sharp enough to easily pierce soft surfaces, like flesh. Getting poked by a crown of thorns will result in sharp stinging pain that can last for several hours, swelling around the wound that can last for up to a week, and even persistent bleeding due to the toxin that is found not in the spines, but in the tissue that encases each spine and the rest of its body: saponins. The spine’s function is not only to injure, but to embed the venomous tissue in the attacker.
Saponins are also poisonous to many species of fish, so the few predators that manage to get past the spines could be doing so to their own demise. A single brush with this sea star is sure to leave a lasting impression, thereby decreasing the amount of potential predators. The solid defense mechanisms of the crown of thorns is what makes an outbreak of these predators an elevated threat to coral reefs — there’s not much besides humans that can stand in their way! Injecting the sea stars with solutions that are toxic to them but not the surrounding marine life have met with a measure of success, but their widespread placement and limitations of human divers are presenting significant challenges to getting a firm grip on the problem.
The Great Barrier Reef experienced its first documented crown of thorns outbreak in 1962, and two more would occur between 1978-1991 and 1993-2005. Although marine experts were able to get each of those outbreaks under enough control to prevent a complete takeover, over 40 percent of coral cover has been lost to the crown of thorns alone, with typhoons being the only natural disaster capable of causing more extensive damage to reefs.
Ironically, our activities on land are part of what fuels an outbreak of crown of thorns, who feast on the algae that explode from the added nutrients of agricultural and urban runoff into the sea. Overfishing also plays a part in their proliferation, as any would-be predators become increasingly absent on the reefs. The only question now is whether our attempts to quash the latest crown of thorns outbreak are timely enough to make a difference at this stage of the game.
Take a look at this short but fascinating video about the crown of thorns starfish and its delicate place in the marine ecosystem.