Crown-of-thorns Starfish

The Crown-of-thorns starfish (often abbreviated to COTS), Acanthaster planci, is a large starfish that preys on hard, or stony, coral polyps (Scleractinia). It gets its name from venomous thorn-like spines covering its upper surface, resembling the biblical crown of thorns. It is one of the largest starfish in the world.

Crown-of-thorns starfish has a wide Indo-Pacific distribution. It is most commonly found around 🇦🇺 Australia, but can also be found at tropical and subtropical latitudes from the 🌊 Red Sea and the East African coast across the 🌊 Indian Ocean, and across the 🌊 Pacific Ocean to the west coast of Central America. It is found in regions where coral reefs or hard coral communities exist.

The body form of the crown-of-thorns starfish is similar to that of a typical starfish, with a central disk and radiating arms. However, it has unique traits such as being disc-shaped, multiple-armed, flexible, prehensile, heavily spined, and having a large ratio of stomach surface to body mass. Its prehensile ability comes from two rows of numerous tube feet that extend to the tip of each arm. While it begins its lifecycle with five-fold symmetry (pentamerism), it loses this symmetry as it grows. The starfish has true image-forming vision. Adult crown-of-thorns starfish typically range in size from 25 to 35 cm (10 to 14 in) and can have up to 21 arms. Despite its stiff appearance, the starfish is able to bend and twist to fit around the contours of the corals on which it feeds. The underside of each arm has closely fitting plates that form a groove and extend in rows to the mouth. The color of the starfish can vary depending on diet or geographic region, ranging from purple, purple-blue, reddish grey, or brown with red spine tips, to green with yellow spine tips. The long, sharp spines on the sides of the starfish's arms and upper surface resemble thorns, giving it its name. The spines can range from 4 to 5 cm long and are stiff, very sharp, and capable of piercing through soft surfaces. Despite the sharp spines, the starfish's general body surface is membranous and soft. When removed from the water, the body surface ruptures and the body fluid leaks out, causing the body to collapse and flatten. The spines also bend over and flatten, but they recover their shape when reimmersed if the starfish is still alive.

The family Acanthasteridae is monogeneric, and its position within the Asteroides is unsettled. It is generally recognized as a distinctly isolated taxon. Recent studies suggest that Crown-of-thorns starfish has strong similarities with various members of the Oreasteridae, leading to its transfer from the Spinulosida to the Valvatida. Acanthaster morphology may have evolved in association with its locomotion over irregular coral surfaces in higher energy environments. However, there is a complication as Acanthaster is not a monospecific genus, and the consideration of the genus must also take into account another species, Acanthaster brevispinus, which lives in a completely different environment. A. brevispinus lives on soft substrates, at moderate depths, where the surface is regular and little wave action occurs.

Starfish contain saponins known as asterosaponins in their tissues. These saponins have detergent-like properties. Crown-of-thorns starfish has no mechanism for injecting the toxin, but if the spines pierce a predator or unwary person, the tissue containing the saponins is lost into the wound. In humans, this causes sharp, stinging pain, persistent bleeding due to the haemolytic effect of saponins, and nausea and tissue swelling that may persist for a week or more. The spines, which are brittle, may also break off and become embedded in the tissue, requiring surgical removal. Saponins occur throughout the lifecycle of the crown-of-thorns starfish, including in the eggs and larvae. Predators of juvenile starfish exhibit mouthing behavior and rejection, suggesting the presence of saponins in the juveniles.

The adult crown-of-thorns is a corallivorous predator that typically preys on reef coral polyps. It climbs onto a section of living coral colony using its numerous tube feet, which lie in distinct ambulacral grooves on the oral surface. It fits closely to the surface of the coral, even on complex surfaces of branching corals. It then extrudes its stomach out through its mouth over the surface to virtually its own diameter. The stomach surface secretes digestive enzymes that allow the starfish to absorb nutrients from the liquefied coral tissue. This process leaves a white scar of coral skeleton that quickly becomes infested with filamentous algae. An individual starfish can consume up to 6 square meters (65 sq ft) of living coral reef per year. Feeding rates vary depending on the size of the starfish and the season. In a study conducted in the central Great Barrier Reef region, large starfish (40 cm and greater diameter) killed about 61 cm2 (9 in2) per day in winter and 357 to 478 cm2 (55 to 74 in2) per day in summer. Smaller starfish (20–39 cm) killed 155 to 234 cm2 (24 to 36 in2) per day in the equivalent seasons. Feeding and locomotion rates also vary with the temperature of the surrounding water. The starfish show a preference for feeding on branching corals and table-like corals, such as Acropora species. They tend to avoid rounded corals with less exposed surface area, such as Porites species. The starfish may also avoid corals that have resident bivalve molluscs, polychaete worms, or symbionts living within them, as these can discourage the starfish. In areas with low densities of hard coral, the starfish may feed on soft corals (Alcyonacea). The starfish are cryptic in behavior during their first two years, emerging at night to feed. They usually remain solitary as adults, leaving white feeding scars on adjacent coral as the only evidence of their presence. However, their behavior changes during the breeding season, when they aggregate high on a reef and synchronously release gametes. They also become more active when at high densities, moving day and night and competing for living coral.

The elongated, sharp spines covering nearly the entire upper surface of the crown-of-thorns serve as a mechanical defense against large predators. The starfish also has a chemical defense, as the saponins in its spines serve as an irritant when they pierce a predator. These defenses make the starfish unattractive to coral community predators. However, Acanthaster populations typically consist of a proportion of individuals with regenerating arms.

While there are several species that occasionally prey on healthy adults of Crown-of-thorns starfish, they do not specifically prefer the starfish as a food source. These species include certain pufferfish and triggerfish found in the 🌊 Red Sea and the Indo-Pacific waters. Triton's trumpet, a large gastropod mollusk, is also known to prey on Acanthaster in some areas. The small painted shrimp Hymenocera picta preys on Crown-of-thorns starfish at certain locations, along with a polychaete worm called Pherecardia striata. When Crown-of-thorns starfish is damaged or dead, it attracts scavengers such as polychaete worms, hermit crabs, sea urchins, and small reef fish. Another predator of the crown-of-thorns starfish is Pseudocorynactis, a large polyp-like creature of the cnidarian genus, which can ingest starfish specimens up to 34 cm (13 in) in diameter.

Large populations of crown-of-thorns starfish have been observed at various locations of coral reefs from the 1960s to the 1980s. Outbreaks of high densities of starfish have occurred at these locations, causing damage to the coral reefs.

Attention! If you spot Crown-of-thorns starfish in the 🌊 Red Sea, kindly report it promptly to HEPCA or contact us, providing the most precise location and, if possible, a photo! Please refrain from touching it and avoid attempting any actions on your own!