Keppel Island Bleaching: Understanding the Cause

Keppel Island, located within the Great Barrier Reef World Heritage Area, has experienced significant coral bleaching. Coral bleaching is a process where corals expel the symbiotic algae (zooxanthellae) living in their tissues, causing them to turn white. This makes the coral more vulnerable to disease and death if the stress is prolonged.

While rising sea temperatures are a major driver of mass coral bleaching events globally, coral reefs can also suffer from other stressors. The question specifically points to the "expansion of" a marine animal population as the primary cause of bleaching on Keppel Island.

Analyzing the Options

Let's look at the options provided:

• Starfish
• Blue whale
• Octopus
• Sea horse

We need to identify which of these animals, when its population expands, is known to cause significant damage leading to coral bleaching.

The Impact of Starfish Expansion

Certain species of starfish are known predators of corals. The most notorious is the Crown-of-Thorns Starfish (Acanthaster planci). These starfish feed by extruding their stomach over the coral and digesting the polyps. A single starfish can consume large amounts of coral tissue, leaving behind a bleached, dead coral skeleton.

When the population of Crown-of-Thorns Starfish experiences an "expansion" or outbreak, they can devastate vast areas of coral reef. These outbreaks are considered a significant biological stressor on coral ecosystems, contributing to coral loss and what appears as widespread bleaching (due to tissue removal leaving the white skeleton visible).

The Great Barrier Reef, including areas like Keppel Island, has historically been affected by outbreaks of Crown-of-Thorns Starfish. The expansion of these starfish populations is a well-documented cause of coral mortality and damage, often resulting in areas appearing extensively bleached.

Eliminating Other Options

Let's consider why the other options are unlikely causes of widespread bleaching due to population expansion:

• Blue whale: Blue whales are marine mammals that feed primarily on krill. They do not interact with coral reefs in a way that would cause bleaching. Their expansion would not lead to reef damage.
• Octopus: Octopuses are cephalopod molluscs. While some species live on or near reefs, they are generally predators of crustaceans and fish, not coral polyps. Their population expansion is not linked to widespread coral bleaching.
• Sea horse: Seahorses are fish that typically live in seagrass beds or among soft corals and seaweeds. They feed on small crustaceans. They do not prey on hard corals, and their population expansion would not cause coral bleaching.

Conclusion

Based on scientific understanding of coral reef ecosystems and the animals listed, the expansion of Starfish populations, specifically the predatory Crown-of-Thorns Starfish, is a known cause of significant coral damage and mortality, which can lead to extensive areas appearing bleached. Therefore, the bleaching on Keppel Island is mainly due to the expansion of Starfish.

Revision Table: Keppel Island Bleaching Factors

Additional Information: Crown-of-Thorns Starfish

The Crown-of-Thorns Starfish (CoTS) is a large starfish native to coral reefs in the Indo-Pacific region. It is easily recognizable by its many arms (up to 21) and venomous spines covering its upper surface.

• Feeding: CoTS feeds by climbing onto a coral colony, everting its stomach out through its mouth, and secreting digestive enzymes to liquefy the coral tissue. The liquefied tissue is then absorbed.
• Reproduction: CoTS can reproduce in large numbers, especially when conditions are favorable (e.g., increased nutrient runoff from land can boost the survival of their larvae).
• Impact: During outbreaks, CoTS populations can become extremely dense, leading to rapid consumption of large areas of coral reef. This predation is one of the most significant biological disturbances affecting the Great Barrier Reef.
• Management: Managing CoTS outbreaks is a key part of reef management efforts. Methods include manual removal and injection of toxic substances.