Crown-of-Thorns Starfish

Crown-of-thorns starfish, also called COTS, is a heavily spined species of marine invertebrates that eat coral. It is the second-largest starfish in the world that gets its name from the poisonous thorn-like spines covering its body, which resemble the crown of thorns worn by Jesus Christ. Like other starfish species, the COTS has a disk-shaped, flexible, and prehensile body with multiple radiating arms. It is mostly found in the regions where stony or hard corals occur in the Indo-Pacific.

Physical Description and Appearance

Size: Adult crown-of-thorns starfish vary in size, ranging from 9.8-13.8 in (25-35 cm). The larger specimens can grow over 100 cm in diameter.

Weight: This spiny sea star species can weigh anywhere between 200 g and 3 kg.

Color: It typically has soft and light colors, including gray-green, pale brown, purplish-blue, and reddish-gray. Some specimens can have a bright red, yellow, and bluish appearance.

Arms: Adult COTS can have 12-23 arms that extend from the central disk. Underneath each of the arms, there are a series of closely fitted plates that create a groove.

Spines: The spines are stiff, long, and very sharp, occurring on the sides of its arms and upper surface; these spines form a crown-like shape.

Body: Its body may appear stiff, but the COTS can easily bend and twist its body to fit around the curves and outlines of the coral that it eats.

Distribution

The crown-of-thorns starfish is widely distributed across the Indo-Pacific region. Although they are commonly found in Australia, large populations can exist in tropical and subtropical environments, extending from the Red Sea through the Indian Ocean off the east African coast, across the Pacific to the coastal waters off western parts of Central America.

What Kind of Habitats do Crown-of-Thorns Starfish live in

They prefer living in relatively undisturbed waters, such as lagoons, deep seawater, and protected, shallow parts of the backreef where they feed on coral colonies.

Lifespan

Although the actual life expectancy of crown-of-thorns starfish in the wild is not known, they grow old, reaching the senile phase after 3-4 years. Most specimens that have been studied died between 5 and 7.5 years. In captivity, they can survive for about 8-15 years.

Diet

The crown-of-thorns starfish is a corallivore, which means it preys on coral. As a voracious predator, it is estimated that this starfish species can eat up to 65 sq. ft (6 sq. m) of living reef coral per year.

Behavioral Characteristics

• These starfish climb onto the coral colony and attach themselves tightly to the coral’s surface. Then, they stick their stomach out through the mouth to release digestive enzymes, allowing them to liquefy the coral tissue and absorb nutrients.
• In the Great Barrier Reef, it was found that they consume 60-155 sq. cm of coral per day in the winter, but 234-478 sq. cm per day in the summer. They have higher locomotion and feeding rates in the summer than those in winter because their metabolic rate and body temperature are affected by the ocean temperature.
• The COTS prefer feeding on table-like and branching hard corals like the Acropora species. They tend to avoid rounded corals, like the Porites species, which have a lesser exposed surface area.
• Crown-of-thorns starfish are cryptic by nature during the first two years of their life, as they emerge only during the night to feed. Adult starfish may show such behavior when solitary. High densities of starfish, however, may move during both day and night to search for living coral communities.

Adaptations

• The long, sharp spines cover almost the whole upper surface of the COTS, serving as a defense against predators. These spines are so sharp and stiff that they easily pierce through the body of their predators.
• They have saponins, which are a type of venom in their tissues. When the spines pierce the predator’s body or the human skin, the saponins are released into the wound, resulting in sharp, stinging pain.
• Their numerous tube feet, extending to the tip of their arms, give them the prehensile ability so that they can easily climb onto the living coral colony.

Mating and Reproduction

Crown-of-thorns starfish, like most echinoderms, reproduce by spawning. The male and female starfish release or deposit their gametes into the water so that fertilization can occur. This starfish species release chemicals, which trigger spawning in other individuals. Not all individual starfish in a specific population, however, spawn simultaneously.

They seasonally spawn during the summer months, depending on the location of each population. In the northern hemisphere, spawning occurs between May and August, but in the southern hemisphere, it takes place between November and February.

The spawning process involves a crown-of-thorns starfish to climb high up on a coral reef and arch its body. It vigorously moves its arms and tube feet so that it can release the gametes through the five holes located on the aboral surface of its body. Female starfish may produce around 60 million eggs all through a breeding season.

What do the Baby Crown-of-Thorns Starfish look like

Eggs that have been fertilized turn into planktonic larvae, passing through different developmental stages. The embryo that hatches from the egg is known as a gastrula, after which the gut forms, and the larva develops into a bipinnaria. By day 5, it turns into a brachiolaria larva. When the brachiolaria stage ends, the larva sits on a hard surface and undergoes metamorphosis into a juvenile COTS. The juvenile starts with five arms, which gradually increases to around 23 by adulthood.

Conservation

Crown-of-thorns starfish have not been listed under IUCN Red List, US Federal List, or any other conservation programs.

Interesting Facts

• The crown-of-thorns starfish have the ability to regrow missing or damaged arms. Some specimens may regenerate even after being cut in half.
• About 11 predator species occasionally feed on healthy and uninjured crown-of-thorns starfish. Some of those predators include white-spotted puffers, Titan triggerfish, Triton’s trumpet, harlequin shrimp, a sea urchin, and a hermit crab species.
• Crown-of-thorns starfish population outbreaks have had occurred at 21 different locations since the 1960s. High-density populations were seen in the Red Sea, tropical Indo-Pacific region, French Polynesia, Central Great Barrier Reef region, and about Green Island near Cairns.
• Over-collecting of Triton’s snail, agricultural land runoff that causes phytoplankton bloom giving enough food source for larvae, overfishing of triggerfish and pufferfish, and warmer sea temperatures all contribute toward COTS population outbreak.
• An autonomous robot named COTSBot has been developed in Australia for killing crown-of-thorns starfish and controlling population outbreaks.
• The Toxapex Pokémon, featured in the role-playing video games Pokémon Sun and Pokémon Moon, is based on the crown-of-thorns starfish.