The crown-of-thorns starfish (Acanthaster planci) is a marine invertebrate found in the Indo-Pacific’s coral reef ecosystems. This starfish is named for the venomous spines that cover its upper surface, resembling a thorny crown. While a natural component of these vibrant underwater habitats, its feeding habits can significantly influence the health and structure of coral reefs. Understanding this species involves examining its physical traits, feeding behaviors, reproductive capacity, and the strategies employed to manage its populations.
Physical Characteristics and Habitat
The crown-of-thorns starfish possesses a unique morphology, featuring 10 to 21 arms radiating from a central disc. Its size can vary, with adults commonly ranging from 25 to 35 centimeters (10 to 14 inches) in diameter, and some specimens reaching over 70 centimeters (27 inches). The starfish’s coloration is diverse, presenting in shades of red, orange, and purple, which may be influenced by its diet.
A defining characteristic of this starfish is the presence of large, sharp, venomous spines covering its upper body. These spines contain saponin in their underlying tissue. The starfish’s mouth is located on its underside, and it possesses light-sensitive eyespots at the tips of its arms.
The natural habitat of Acanthaster planci is the coral reefs across the Indo-Pacific region. It is particularly prevalent on Australia’s Great Barrier Reef. These starfish forage in shallow, protected areas of the backreef. Within a healthy reef, the crown-of-thorns starfish acts as a natural predator, consuming corals and contributing to the ecological balance.
Diet and Ecological Impact
The crown-of-thorns starfish feeds almost exclusively on hard corals. The starfish employs a unique feeding mechanism: it everts its stomach over the coral colony, releasing digestive enzymes that externally break down the coral tissue. Once the coral polyps are digested, the starfish absorbs the liquefied nutrients, leaving behind the bleached coral skeleton. An individual starfish can consume a significant amount of coral, with estimates suggesting up to 12 square meters of coral tissue per year.
While a natural predator, its feeding activity becomes destructive during population outbreaks, when densities can surge from 0-1 starfish per hectare to over 1000 starfish per hectare. These outbreaks lead to widespread coral mortality and reef degradation, transforming once-thriving reefs into barren landscapes. For instance, outbreaks on the Great Barrier Reef between 1995 and 2009 caused greater declines in coral cover than any other disturbance, including cyclones and coral bleaching.
The rapid consumption of live coral by large aggregations of crown-of-thorns starfish can decimate benthic communities, reducing both the functional and species diversity of coral reef ecosystems. After corals are consumed, the exposed skeletons can quickly become covered in algae, further impeding coral recovery. The severity of this impact has been observed in various locations, with some reefs experiencing up to 90% loss of live coral cover during outbreaks.
Reproduction and Outbreak Formation
The reproductive cycle of the crown-of-thorns starfish involves broadcast spawning, where males and females release their gametes directly into the seawater for external fertilization. This species is highly fecund, with a single female capable of producing tens of millions of eggs annually, sometimes up to 60 million eggs per year. Spawning occurs once a year during the summer months, often peaking around January in regions like the Great Barrier Reef. The aggregation of adult starfish during spawning events can increase fertilization success and promote genetic diversity within the population.
After fertilization, the eggs develop into planktotrophic larvae that drift in the water column for several weeks, 11 to 22 days, before settling onto a suitable hard surface. These larvae feed on phytoplankton and can adjust their morphology to varying food levels, increasing the size of their feeding structures in low-food conditions to enhance food capture. This adaptability allows them to develop even in naturally low-nutrient reef environments. Once settled, the larvae metamorphose into juvenile starfish, which initially have five arms and feed on crustose coralline algae for about six months before transitioning to a coral diet. They reach reproductive maturity around two years of age and can live for seven to eight years.
Several factors contribute to the formation of crown-of-thorns starfish population outbreaks. Environmental triggers, such as increased nutrient runoff from land, can lead to phytoplankton blooms. These blooms provide an abundant food source for the starfish larvae, significantly increasing their survival rates in the water column. Even a small increase in larval survivorship can result in a much larger settlement of juvenile starfish onto a reef, subsequently leading to an outbreak. Additionally, the absence or reduction of natural predators, which consume juvenile or adult starfish, can further exacerbate population growth.
Managing Population Outbreaks
Managing crown-of-thorns starfish outbreaks involves a combination of direct intervention and broader ecosystem management strategies. Direct intervention methods involve divers manually removing starfish from affected reefs, a labor-intensive approach. More targeted methods include lethal injection, where substances like bile salts or acetic acid are administered directly into the starfish. These injections effectively kill the starfish without broadly harming the surrounding coral reef environment.
Broader ecosystem management approaches focus on addressing the underlying factors that contribute to outbreaks. Improving water quality by reducing nutrient runoff from land is a strategy, as excess nutrients can fuel phytoplankton blooms that support starfish larval survival. Protecting and restoring populations of natural predators of the crown-of-thorns starfish, such as the triton snail and certain fish species, helps maintain a more balanced reef ecosystem.
Control efforts on the Great Barrier Reef, for instance, have involved culling programs to reduce starfish densities. These programs aim to prevent secondary outbreaks by limiting the spread of larvae from initial outbreak areas. While direct control measures can mitigate immediate damage, their effectiveness is enhanced when integrated with efforts to improve overall reef health and resilience. The goal is to reduce the frequency and severity of outbreaks, supporting the long-term recovery and stability of coral reef ecosystems.