Crown of thorns starfish. Photo @ David Burdick

Diadema in a Montastrea annularis patch. Photo © Adán-Guillermo Jordán-Garza 2008/Marine Photobank

Sea urchins are important herbivores on coral reefs, and in some ecosystems they play a critical role in maintaining the balance between coral and algae. Their role can be especially important on reefs where other herbivores (such as parrotfishes and rabbitfishes) have been depleted. Urchins contribute to reef resilience by grazing algae and providing settlement space for corals, thereby helping to maintain conditions necessary for coral communities to recover after acute disturbances (such as storms or bleaching events).

Urchins can also be susceptible to disease. In places where urchins are key herbivores, managers may wish to consider helping to re-establish urchin populations where densities have been depleted.

Urchins can also have negative impacts on coral reefs. In some situations, where the processes of reef calcification may be depressed and/or urchin populations reach outbreak densities, the scraping effects of urchin feeding can remove coral recruits, reduce cover of important coralline algae ref and lead to unsustainable bio-erosion. While there are likely to be more serious, systemic issues causing urchins to be a problem, managers may need to consider urchin control (removal) as a short-term measure to minimize reef damage or assist recovery.

What Causes Sea Urchin Outbreaks?

There are a number of possible reasons for sea urchin outbreaks. They may be the result of depletion of natural predators, such as triggerfishes and wrasses, ref or reduced food competition that might result from over-harvesting of herbivorous fishes. Nutrient pollution on coral reefs may also contribute to urchin outbreaks by increasing algal growth.

See urchin management for information on controlling and enhancing urchin populations.

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