Acanthaster planci

Acanthaster planci is found throughout the Indo-Pacific region, ranging from the Indian ocean (Red Sea and East Africa) to the Pacific (from mainland Japan south to Lord Howe Island, and from the west coast of Panama to the Gulf of California). This species is particularly common on the Great Barrier Reef of Australia. (Moran, 1988a; Moran, 1988b)

Acanthaster planci is commonly found on coral reefs, foraging over coral colonies in shallow, protected areas of the backreef. (Moran, 1988b)

Acanthaster planci bears between 8 and 21 arms that radiate from a central disc. Adults normally range from 250 to 350 mm in diameter, with some individuals over 700 mm in diameter. The mouth is located on the underside of the central disc (the aboral surface), and light-sensitive eyespots are present at the tips of the arms. Individual coloration varies from red and orange to purple, and is thought to be the result of differences in diet. The interior of the body contains the internal organs (stomach, digestive gland, and gonads). The skeletal structure is composed of tiny structures called ossicles, made of magnesium calcite. Acanthaster planci possesses large, venomous spines in contrast to the short, blunt spines usually present on starfish. The venomous quality of these spines is not fully understood; saponin has been discovered in the spines’ underlying tissue, though the quantity is not sufficient to trigger the painful reactions seen in humans who have come into contact with the spines. There is no evidence that A. planci injects toxins through the spines. (Moran, 1988a; Moran, 1988b)

Like most echinoderms, A. planci reproduces sexually through broadcast spawning. The female releases millions of eggs into the water column that are fertilized by a male's sperm. Fertilized eggs develop into planktonic larvae, which depend on phytoplankton for nutrition while they pass through several developmental stages, from gastrula to bipinnaria to brachiolaria. Near the end of the brachiolaria stage, the larva settles onto a suitable hard surface and metamorphoses into a juvenile starfish. Its arms will begin to develop as it matures. The juvenile starfish begins with 5 arms, which will increase to as many as 21 arms by adulthood.

Researchers note three age classes for A. planci: juvenile, sub-adult, and adult. Growth rates are age-specific: growth is rapid for juveniles (up to 16.7 mm per month) while the rate slows as they transition from sub-adult to adult (4.5 mm per month). (Engelhardt, et al., 1999; Moran, 1988b; Stump, 1996)

Crown-of-thorns starfish reproduce by spawning, in which males and females release their gametes into the seawater, where fertilization occurs. Unlike some other starfish, which can reproduce through somatic fission or arm autonomy, A. planci is not known to reproduce asexually. There is evidence that A. planci releases chemicals that induces spawning in nearby individuals. However, not all individuals in a given population spawn at the same time.

When spawning, A. planci will climb to a high place on a coral outcrop, then arch its body. Gametes are released through five pores on the aboral surface of the body, as the animal waves its arms and moves its tubefeet vigorously. (Birkelanci and Lucas, 1990; Moran, 1988a; Moran, 1988b)

Acanthaster planci spawns seasonally during summer months, according to each population’s location. Populations in the northern hemisphere generally spawn between May and August, while populations in the southern hemisphere spawn between November and February. These seasons have been roughly correlated with periods of warmer water temperature in the respective habitats. Gravid females may contain anywhere from 12 to 24 million eggs, and may produce as many as 60 million eggs throughout a season. (Birkelanci and Lucas, 1990; Moran, 1988a; Moran, 1988b)

As this asteroid is a broadcast spawner with a planktonic larval stage, there is no parental investment in offspring. (Birkelanci and Lucas, 1990)

Acanthaster planci is expected to live to about 15-17 years barring predators or limiting resources; however, the actual lifespan of this organism in the wild is unknown. (Engelhardt, et al., 1999; Stump, 1996)

Juveniles and sub-adults are the most geographically widespread asteroids recorded within the Great Barrier Reef region. One year after settlement onto the reef, newly formed adults migrate great distances over reef habitats. Locomotory behaviors observed in A. planci are typical of predatory starfish. Individuals crawl at the rate of up to 35 cm per minute over coral reefs and rubble, encountering and consuming stony corals by everting the stomach onto the coral substratum and digesting the polyps. Juveniles feed at night on exposed front reef zones, where apparently they are less likely to be noticed by predators, while adults are more commonly seen in protected back reef zones. (Engelhardt, et al., 1999; Engelhardt, et al., 2001; Stump, 1996)

This species does not maintain a home range or territory.

Like other asteroids, A. planci uses a combination of chemical detection and tactile senses via its tube feet to locate mates, detect its prey, and perceive its environment. (Clark and Downey, 1992)

While developing as larvae in the water column, individuals of this species consume smaller planktonic organisms. As an adult, this asteroid is an opportunistic carnivore, consuming sclerectinian corals, encrusting sessile invertebrates, and dead animals. It feeds by everting its stomach through its mouth onto its prey and digesting the tissues, absorbing the nutrients through the stomach wall. Acanthaster planci consumes most types of Indo-Pacific stony corals, such as Pocillopora, Acropora, Pavona, and Porites. (Keesing and Lucas, 1992; Moran, 1988a; Pratchett, 2007)

The crown-of-thorns starfish is protected from many types of predators by its long, venomous spines, though many adults (up to 60% within a population) may have missing arms, indicating that predation does occur. Juveniles assume more cryptic behaviors, inhabiting crevices and the undersides of ledges. Predators of A. planci include the giant triton shell Charonia tritonis and various fishes in the families Balistidae and Tetraodontidae, which have horny plate-like scales and strong sharp teeth that allow them to remove chunks of tissue from A. planci. (Moran, 1988b)

This asteroid is a corallivore, almost exclusively consuming live sclerectinian corals. An average sized adult (40 cm) can kill up to 478 square cm of live coral per day through its grazing activities. The crown-of-thorns starfish can be seen as an ongoing disturbance factor on the reef, removing swaths of clonal corals in its path, and opening up bare areas of coral rock for settlement and recruitment of other species of sessile invertebrates. Thus, A. planci can be seen to have a role in diversifying the habitat. However, if coral cover is drastically reduced, populations of coral reef specialists (animals that depend exclusively on coral cover for shelter and food) may decrease. Thus the impact of A. planci in their environment depends on how abundant they become. (Glynn, 1976; Keesing and Lucas, 1992; Wilson, et al., 2008)

Acanthaster planci harbors several genera of ectoparasitic copepod crustaceans on its dermal surface. (Mah, 2010)

There are no known economic benefits for humans.

Much research has been conducted on the grazing effects of A. planci on coral reef cover and survival. Large populations of these starfish can devastate a reef, which has occurred on the Great Barrier Reef. Furthermore, after live coral cover has been reduced, both juvenile and sub-adult starfish preferentially choose to feed on newly-formed hard coral, which significantly impacts the coral recovery process. Surveys conducted since the early 1990’s have illustrated the decline in live hard coral cover coincident with crown-of-thorns outbreaks along the reef systems between Lizard Island and Townsville (coastal Queensland, Australia). Researchers have emphasized the importance of raising public awareness of these continually increasing outbreaks, since starfish predation on coral can seriously damage the reefs to the point where sustainability of the lucrative reef tourism industry could be impacted. To protect these reefs as well as the people who depend on them for their economic livelihood, researchers need to determine how human activities affect the cycle of starfish outbreaks. Specifically, more research needs to be conducted on the effects of overfishing known predators of A. planci, and on how increased nutrient runoff from land affects survival, recruitment, and growth of larval A. planci. (Engelhardt, et al., 1999; Engelhardt, et al., 2001; Stump, 1996)

This species is not listed under any conservation program.