The crown-of-thorns starfish is a marine invertebrate found across the Indo-Pacific region. These starfish play a significant role within coral reef ecosystems. They are native to these environments, including the expansive Great Barrier Reef.
Identifying the Crown-of-Thorns Starfish
The crown-of-thorns starfish is recognizable by its multiple arms, typically ranging from 10 to 21, extending from a central disk. Its body is covered in sharp, thorn-like spines. These spines are venomous and can cause pain if touched.
The starfish generally reaches 25 to 35 centimeters (10 to 14 inches) in diameter, with some individuals exceeding 60 centimeters (2 feet). Their coloration varies, often appearing reddish-brown, grey, or greenish, with spine tips that can be red or yellow. Their bodies are flexible, allowing them to bend and twist around corals.
These starfish are commonly found on coral reefs. They often hide within the reef’s structure during the day, making them difficult to locate despite their size and vivid colors. Their underside features rows of tube feet extending to the tip of each arm, aiding in movement and grasping.
Diet and Ecological Impact
The crown-of-thorns starfish primarily feeds on hard coral polyps. Its unique feeding mechanism involves extending its stomach out through its mouth, enveloping the coral, and digesting the polyps externally with digestive enzymes. This process leaves behind a white, bleached coral skeleton that quickly becomes covered by algae.
While a natural component of healthy reefs, where they can consume faster-growing corals and allow slower-growing species to thrive, their populations can experience outbreaks. During such outbreaks, densities can increase significantly, with 15 or more starfish found per hectare. These high densities lead to widespread coral mortality, with outbreaks capable of stripping a reef of up to 90% of its living coral tissue.
The extensive coral loss caused by these outbreaks disrupts the delicate balance of the reef ecosystem. Corals provide habitat for countless marine species, so their decline has a cascading effect throughout the reef, affecting biodiversity and overall ecosystem health. The Great Barrier Reef, stretching over 2,300 kilometers, has experienced multiple outbreaks, with the starfish contributing to a substantial percentage of coral cover decline in past decades.
Research indicates that crown-of-thorns starfish may even thrive in degraded coral habitats. As live coral diminishes, the resulting rubble habitats can serve as nurseries for juvenile starfish, allowing them to accumulate and potentially emerge as coral-eating adults when corals begin to recover. This creates a challenging cycle for reef recovery, as environmental stressors like heatwaves and pollution, which degrade corals, also favor the starfish.
Life Cycle and Reproduction
The crown-of-thorns starfish reproduces sexually through broadcast spawning, typically occurring during warmer months. Females release millions of eggs into the water column, which are then externally fertilized by sperm released by nearby males. A single female can produce up to 100 million eggs in a spawning season.
Fertilized eggs undergo several planktonic larval stages. These larvae are free-swimming and feed on phytoplankton. The bipinnaria stage can clone itself through bisection, with each portion regenerating into a complete larva.
Larval dispersal is influenced by ocean currents, allowing them to spread widely between reefs. After several days to weeks, the brachiolaria larvae settle onto a suitable hard surface, such as the undersides of dead coral rubble, and metamorphose into juvenile starfish. Initially, juveniles have five arms and are very small, feeding on crustose coralline algae. After about six months, they transition to feeding on coral polyps, at which point their growth rate increases significantly.
Outbreaks are influenced by factors that enhance larval survival and settlement. Increased nutrient input, often from terrestrial runoff, can lead to denser phytoplankton blooms, providing more food for larvae and potentially increasing their survival rates. While temperature and salinity have minimal direct effects on larval survival within their normal range, warmer sea temperatures can accelerate larval development.
Managing Outbreaks and Conservation Efforts
Managing crown-of-thorns starfish outbreaks involves direct intervention and broader conservation strategies. One common control method is manual removal by divers, which can be labor-intensive, especially when starfish numbers reach tens of thousands per reef. Another approach involves injecting individual starfish with lethal substances, such as bile salts or common household vinegar.
These injection methods are effective, causing the starfish to die within 24 to 48 hours without apparent immediate flow-on effects on other reef organisms. Vinegar is particularly appealing due to its affordability and wide availability, making it suitable for both large-scale programs and smaller community efforts. Despite these efforts, many starfish remain hidden within the reef structure, posing a challenge to comprehensive control.
Natural predators, such as the giant triton snail, also consume crown-of-thorns starfish. Other marine species, including humphead wrasse, starry pufferfish, and titan triggerfish, are known to prey on adult starfish, while shrimp, crabs, and worms consume younger individuals. Research is ongoing to understand the influence of removing these predators on starfish populations and to explore the use of chemical cues from predators to deter starfish.
Beyond direct culling, broader conservation efforts are aimed at improving the overall resilience of coral reefs. This includes reducing pollution from coastal and agricultural runoff, which can contribute to nutrient spikes that favor starfish larval survival. Addressing climate change to mitigate rising sea temperatures and more intense cyclones is also important, as these stressors weaken corals and limit their ability to recover from outbreaks. Promoting sustainable tourism practices further contributes to the health of reef ecosystems, indirectly helping to manage starfish populations.