Among the broad range of large-scale disturbances that affect Indo-Pacific coral reefs, the coral-eating starfish Acanthaster planci (“crown‑of‑thorns”, COTs hereafter) is a major cause of coral reef destruction, whose impact is quantitatively comparable to cyclones.
Kayal et al. 2012
Crown-of-thorns, a coral predator
Acanthaster planci (“crown-of-thorns”, COTS) is a very large starfish species observed throughout the Indo-Pacific region on coral reefs. Specimens reach up to 70 cm in diameter and 3 kg in size; colour varies according to region. This species has a large number of arms (usually 16 to 18) and carries on its upper face long spines coated with a highly toxic venom. COTS have a tremendous reproductive capacity: a single adult female can release up to 60 million eggs, with high fertilization rates. While larval stages mostly feed on plankton, adults are exclusively corallivores: they feed on coral polyps by pushing their stomachs out of their mouth before releasing their digestive enzymes (extracorporeal digestion). Once the process is complete, only the calcareous skeleton of the coral remains. These food scars are easily recognized by their white color, and are often the first sign of a COT infestations.
Acanthaster planci (COT) is a natural component of coral reef ecosystems. In a “healthy” reef, population density is usually low, ~1 to 15 individuals per hectare, and has no detrimental effect on the abundance and diversity of coral assemblages. In fact, COTS contribute to maintaining a high coral diversity through its marked food preferences: fast growing, branching or sub-massive species (such as Acropora or Pocillopora) are preferred over slower-growing, massive corals (such as Porites). Adult specimens are reported to consume up to 12 m² of coral per year. They have few regular predators except the Triton's trumpet (Charonia tritonis); however recent studies increasingly report several fish or invertebrate species occasionally preying on young and, to a lesser extent, adults COTS.
The Acanthaster Phenomenon
While Acanthaster planci generally occurs at very low densities, populations can dramatically increase during certain periods, eventually reaching extremely high values (e.g. several thousand individuals per hectare). These outbreaks represent one of the most significant biotic disturbances on coral reefs, causing massive and widespread coral mortality. Over a third of Indo-Pacific reefs were recently affected by severe episodes of COTs outbreaks, leading to growing concerns that they are becoming more frequent and more prevalent. While there is historical evidence that coral reefs can recover from COTs outbreaks, they drive even more pressure on already weakened systems. The cascading effects from coral loss can severely harm the entire coral reef community, which raises serious concerns in areas where coastal resources (fish, invertebrates) form the basis of traditional, subsistence fishing. Despite sustained research efforts by the scientific community since the early 70s, the causes and dynamics of these outbreaks are still poorly understood.
Managing COTS outbreaks: what works?
A variety of methods have been developed in an attempt to control COTS outbreaks, generally with limited success: direct removal using sticks, hooks or similar tools; cutting up or crushing up in situ; leaving on starve in bags; construction of underwater fences; and more recently injections with a variety of chemicals or pathogens. In the Indo-Pacific countries, manual collection followed by disposal ashore is the most commonly used for small-scale cleanup efforts. Yet, the efficiency of these measures is still controversial, in particular to protect entire reef systems: “success stories” tend to be small in scales, providing only short-term solutions to a complex phenomenon whose ultimate causes are not fully understood. Removal initiatives may be in particular challenged by the great ability of A. planci to recolonize cleaned areas, which is regulated by biological and environmental factors interacting over a range of scales (e.g. reef extent and topography, local/regional connectivity, distance/structure of source populations, available human & financial resources etc.).
Acidic injections: a new “cheap and natural” alternative?
Injection approaches – in which A. planci is injected with a variety of noxious solutions – are increasingly used as an alternative to manual methods, as they are highly cost-effective and fairly safe when handled correctly. However, most solutions injected over recent decades were not only noxious for COTS but for the coral reef community as well. For example, injections with copper sulphate were performed in the Great Barrier Reef until it was judged highly toxic to fish and many invertebrates. Similarly, injections with sodium bisulphate are required at such high concentrations that they entail the risk of lowering oxygen levels in seawater.
In 2014, a new alternative based upon acidic injections of widely-available, 100% natural products was successfully developed in Vanuatu. Fresh lime juice and white spirit vinegar were found to induce high mortality in COTS at small volumes, with 100% mortality reached within 12–24h. This method offers great advantages when compared to current best practices and is a cheap and natural option for all countries affected by COTS.
See : Moutardier et al. 2015