Identifying the Spikey Starfish
The Crown-of-Thorns Starfish stands out as a prominent example. Scientifically known as Acanthaster planci or Acanthaster solaris, this marine invertebrate is characterized by its numerous, sharp spines covering its entire upper surface. These spines are composed of calcium carbonate, providing a rigid and protective outer layer. This large marine invertebrate is easily recognized by its body covered in numerous sharp, calcified spines, which can measure up to 5 centimeters in length and are responsible for its common name, evoking the image of a thorny crown. Crown-of-Thorns Starfish can grow quite large, typically ranging from 25 to 35 centimeters in diameter, though some individuals have been observed exceeding 80 centimeters across, and even recorded reaching up to a meter.
The coloration of the Crown-of-Thorns Starfish varies widely, often blending with its coral reef surroundings. Common hues include shades of blue, green, grey, and red, sometimes with mottled patterns. Its coloration is diverse, often reflecting its environment, and can include shades of blue, green, grey, purple, reddish-grey, or brown, sometimes with contrasting spine tips. The purpose of its distinctive spines is primarily defensive, deterring potential predators such as triggerfish, pufferfish, and some large snails. The formidable spines serve as a primary defense mechanism against predators such as the giant triton snail, triggerfish, and pufferfish. These spines are sharp and can inflict painful wounds, and some species also possess mild venom at their tips, which contains saponins, adding another layer of protection and further discouraging potential threats.
Life on the Reef
The Crown-of-Thorns Starfish is endemic to the vast Indo-Pacific region, inhabiting the warm, shallow waters of coral reefs stretching from the Red Sea and East Africa across the Indian Ocean to the Pacific Ocean. They are particularly drawn to areas with abundant hard coral cover, which serves as their primary food source. These starfish typically reside on the reef substrate, moving slowly across the coral formations. They are native to the vast Indo-Pacific region, inhabiting tropical and subtropical coral reefs from the Red Sea and East African coast to the Pacific Ocean. They are particularly drawn to areas with abundant hard coral, which constitutes their exclusive diet.
Their feeding mechanism is highly specialized and involves consuming coral polyps. The starfish extends, or everts, its stomach out through its mouth, which is located on its underside. This everted stomach then envelops the coral, releasing digestive enzymes that break down the soft coral tissues externally, effectively turning the coral tissue into a liquid “soup” that the starfish then absorbs. After digestion, the starfish retracts its stomach, leaving behind a white, denuded coral skeleton. This process leaves behind a distinct white scar of dead coral skeleton. A single starfish can consume an area of coral roughly equivalent to its own size daily, depending on the coral species and the starfish’s size. An adult Crown-of-Thorns Starfish can consume a significant amount of coral, with estimates ranging from 300 square centimeters to as much as 478 square centimeters per day, or up to 10 square meters annually.
These starfish are predominantly nocturnal, with most feeding activity occurring during the night. While some feeding can occur during the day, smaller individuals tend to be nocturnal, remaining hidden during daylight hours. They exhibit a relatively slow movement, typically traversing a few meters per hour across the reef. Their movement across the reef is relatively slow, typically around 2.8 meters per day, although this can increase in areas with low coral availability. Their movement patterns are often dictated by the availability of preferred coral species, allowing them to systematically clear patches of coral as they feed.
Ecological Impact and Management
The Crown-of-Thorns Starfish plays a complex role within coral reef ecosystems. In small, natural populations, they can contribute to reef health by feeding on fast-growing coral species, thereby creating space for slower-growing or less competitive corals. This grazing action can help maintain coral diversity and prevent a single coral species from dominating the reef. The Crown-of-Thorns Starfish occupies a dual role within coral reef ecosystems. In healthy, balanced populations, their grazing on fast-growing coral species can contribute to biodiversity by creating space for other, slower-growing corals to establish themselves. This natural predation helps maintain a diverse coral community. However, their impact becomes detrimental when their populations experience sudden and dramatic increases, leading to what are known as outbreaks. When their populations undergo rapid and significant increases, termed “outbreaks,” their impact becomes severely destructive.
An outbreak is characterized by a significant surge in the number of starfish, typically defined as more than 15 adults per hectare, which can lead to widespread coral mortality. An outbreak is generally defined by a density of 15 or more starfish per hectare, though some definitions suggest over 30 starfish per hectare. During these events, the starfish consume coral at an unsustainable rate, leading to widespread coral mortality and the stripping of up to 90% of living coral tissue on affected reefs, leaving behind vast stretches of dead coral skeletons. These large-scale depredations severely compromise the reef’s structure and function, reducing habitat for fish and other marine life, and hindering the reef’s ability to recover from other disturbances like storms or bleaching events. Such outbreaks are considered a major factor in coral cover decline, particularly on the Great Barrier Reef.
Various management and conservation efforts are implemented to control Crown-of-Thorns Starfish populations during outbreaks and protect vulnerable coral reefs. One common method involves manual removal, where divers collect the starfish directly from the reef. Another strategy is injecting the starfish with substances like ox bile or vinegar, which are lethal to the starfish but generally harmless to the surrounding marine environment. These control programs, such as those on the Great Barrier Reef, involve trained divers and dedicated vessels to survey reefs and cull starfish. Researchers are also exploring biological control methods and innovative approaches, including understanding the chemical cues that influence starfish behavior and the role of natural predators like the giant triton snail in controlling populations. Addressing broader environmental factors, such as improved water quality and reduced nutrient runoff, is also considered important, as these can contribute to the frequency and severity of outbreaks. These interventions are crucial for mitigating the destructive impact of large starfish populations and supporting the resilience of coral reef ecosystems.