Crown-of-Thorns Starfish

Physical Characteristics: The Crown-of-Thorns Starfish is instantly recognisable by its disc-shaped body covered in numerous sharp, venomous spines. These spines, which can grow up to 5 centimetres (2 inches) long, contain a neurotoxin that can cause significant pain, swelling, and nausea if they pierce human skin. The starfish typically has a radial symmetry with multiple arms, differing from the classic five-armed starfish shape, and its colouration can vary widely from red, orange, and purple to green and blue, often blending with its coral environment.

Feeding Habits: COTS are specialist predators of hard corals. They evert their stomach over a coral colony, digesting the coral polyps externally and leaving behind a bleached, white coral skeleton. A single starfish can consume an area of coral roughly its own size daily, making them incredibly efficient predators. Their feeding preferences often target fast-growing, branching corals like Acropora and Pocillopora, but during outbreaks, they will consume almost any hard coral species.

Role in Reef Ecosystems: In moderate numbers, COTS are considered a natural part of coral reef ecosystems, contributing to coral diversity by selectively preying on dominant, fast-growing coral species, thereby allowing slower-growing corals to flourish. This natural grazing can promote healthy competition and a more varied reef structure.

Outbreak Dynamics: However, COTS populations can occasionally surge, leading to what are known as 'outbreaks'. These events, often linked to anthropogenic factors such as increased nutrient runoff from agricultural lands and overfishing of COTS predators (like Triton's trumpet snails and certain fish species), can result in devastating coral mortality over vast areas. An outbreak is generally defined when COTS densities exceed 15 individuals per hectare, leading to coral loss rates greater than coral growth rates. Such severe outbreaks can take decades for affected reefs to recover.

Observing COTS Responsibly: For divers aboard a Blue Rides liveaboard, encountering a Crown-of-Thorns Starfish is a unique opportunity to witness marine biology in action. While fascinating, it's crucial to observe these creatures from a safe distance without touching them due to their venomous spines. Your dive guides are knowledgeable about local reef conditions and COTS management efforts, and can provide further insights into their presence and impact on the specific dive sites you visit.

Conservation Awareness: Liveaboard operators and local conservation groups often participate in COTS control programmes, particularly in areas experiencing outbreaks, to manually remove starfish and protect vulnerable reefs. Divers who witness unusually high numbers of COTS are encouraged to report their observations to their dive guides, contributing to ongoing monitoring efforts. Understanding the challenges faced by coral reefs, including COTS outbreaks, deepens the appreciation for marine conservation and highlights the importance of sustainable diving tourism.

Myth: COTS are always a sign of an unhealthy reef. Fact: In low numbers, COTS are a natural predator that helps maintain coral diversity by selectively eating fast-growing corals, allowing others to thrive. It's only during outbreaks that they become detrimental.

Myth: All COTS outbreaks are caused by pollution. Fact: While nutrient runoff (pollution) and overfishing of COTS predators are significant contributing factors to outbreaks, natural cycles and oceanographic conditions can also play a role. It's often a combination of factors.

Myth: You should try to remove COTS yourself if you see them. Fact: Absolutely not. COTS have venomous spines, and improper removal can lead to serious injury. Furthermore, if not removed correctly (e.g., cut up), parts of the starfish can regenerate, potentially worsening the problem. Leave COTS management to trained professionals.