Crown-of-Thorns Starfish: Coral Reef Predator
The crown-of-thorns starfish is a sea star easily recognized by the sharp spines covering its body and arms. These spines are flexible and serve two purposes: aiding in movement and deterring predators. While fascinating, this species is infamous as a coral reef predator. Outbreaks involving tens of thousands of starfish have devastated reefs across the Indo-Pacific, stripping corals of their living tissue.
Causes of Outbreaks
Scientists suggest multiple reasons for these sudden population booms. One theory points to the overfishing of their main predator, the giant triton. Others see outbreaks as natural cycles. However, reefs already under stress from human activities or coral bleaching are especially vulnerable. During these times, the starfish are more efficient at destroying coral colonies.
Feeding Behavior
The crown-of-thorns starfish uses a unique and destructive feeding method. It pushes its stomach out through its mouth, spreads it directly on a coral’s surface, digests the thin layer of tissue, and absorbs the nutrients. This process leaves only the coral’s skeleton behind, causing widespread damage when populations are high.
Control Efforts
Historically, fishers and conservationists attempted to control outbreaks by cutting starfish into pieces. Unfortunately, this backfired. The species can regenerate arms, and in some cases even regrow from just half an individual. Modern management now focuses on two methods:
- Physical removal: Collecting and taking starfish away from the reef.
- Targeted poisoning: Injecting substances that kill starfish without harming corals or other marine life.
Natural Defenses Against Starfish
Some coral-associated animals also help defend reefs. Guard crabs (Trapezia spp.), which live in branching corals such as cauliflower corals, attack crown-of-thorns starfish when they attempt to feed. The crabs pinch the starfish’s tube feet or even its stomach lining. This relationship is a form of symbiosis: the coral gains protection while the crabs gain shelter from predators.
Reproduction and Population Dynamics
The crown-of-thorns starfish reproduces through broadcast spawning. Males and females release sperm and eggs simultaneously into the water column, increasing the odds of fertilization while reducing the risk of eggs being eaten by reef predators. Populations naturally fluctuate between low numbers and outbreak-level densities.
Ecological Impact
Humans do not harvest this species, but scientists monitor it closely because of its impact on coral reefs. While low populations pose little threat, large outbreaks are destructive enough to threaten reef health and biodiversity. As reefs already face climate change and pollution, managing crown-of-thorns starfish remains a critical aspect of conservation efforts.
