Starfish, often referred to as sea stars, are common inhabitants of coral reefs. These marine invertebrates, belonging to the class Asteroidea, play diverse roles within these complex environments. Their presence contributes to the overall health and balance of coral reefs, though their impact varies significantly depending on the species and population dynamics.
Where Starfish Are Found on Coral Reefs
Coral reefs globally host a variety of starfish species, each adapted to specific niches. Many, like the blue starfish (Linckia laevigata), are seen in vibrant blue or purple hues on living coral reefs, particularly near the reef edge and slope at depths of 1 to 20 meters. Other types include agile brittle stars, often hiding in crevices, and chubbier cushion stars like the granulated sea star, found on sandy bottoms and coral rubble. The chocolate chip starfish, with its distinctive brown protuberances, is frequently encountered in the Indo-Pacific region.
These creatures occupy various micro-habitats, adapting their behavior to their surroundings. Some starfish cling directly to coral branches, while others burrow into sandy substrates or conceal themselves within reef nooks and crannies. This diversity highlights their integration across different parts of the reef ecosystem.
How Starfish Shape Reef Ecosystems
Starfish perform several ecological functions that influence the structure and health of coral reef ecosystems. Many species act as generalist predators, consuming organisms including microalgae, sponges, bivalves, and other small invertebrates. This predatory activity helps regulate populations, preventing any single species from dominating the reef and contributing to biodiversity.
Some starfish also function as scavengers, consuming dead organic matter and detritus, which helps keep the reef clean and recycles nutrients. Additionally, some starfish are grazers, feeding on algae that can otherwise overgrow and smother corals. This grazing helps control algal growth, ensuring corals have sufficient space and light to thrive.
Key Interactions Between Starfish and Reefs
The interactions between starfish and coral reefs are complex, ranging from beneficial to destructive, with the Crown-of-Thorns Starfish (COTS) being a notable example. COTS (Acanthaster planci), a large, multi-armed starfish covered in venomous spines, primarily preys on hard coral polyps. It feeds by extruding its stomach over the coral, releasing digestive enzymes to liquefy the tissue, leaving behind a white coral skeleton. A single adult COTS can consume up to 10 square meters of coral per year.
While COTS are a natural component of Indo-Pacific coral reef ecosystems, their populations can experience outbreaks where their numbers increase dramatically, leading to widespread coral loss. These outbreaks can decimate entire reef sections, with a single outbreak potentially stripping 90% of living coral tissue in an affected area. Factors contributing to outbreaks include increased nutrient runoff from land, which feeds COTS larvae, and the overfishing of their natural predators like the giant triton snail and certain fish species.