During our dives on Hawaiian reefs, we are surprised by the amount and diversity of endemic species of fish, coral, sea stars, sea urchins, and other invertebrates. All these creatures play different roles in balancing these delicate ecosystems. Herbivores could be one of the most interesting groups in coral reefs because they are able to remove algae from dead coral structures along hard substratum, creating new space for coral recruits favouring the reef growth.
One of the most curious grazers on Hawaiian reefs is sea urchins, spiny creatures who are active day and night. They are faster than you think, able to move around the hard substrate reaching up to eight centimetres per minute, using special locomotion structures and spines. Their feeding activities not only involve the ingestion of algae, but by scraping the surface with their powerful teeth, they remove the carbonate from the reef matrix, thereby sculpting the reef.
This process is known as bioerosion, and it is important in structuring and forming coral reefs, playing a large role in the carbonate dynamics on Hawaiian reefs. Other powerful bioeroders and grazers are the parrotfish. On some Maui reefs, we were delighted by these colourful fishes and the sounds that they produced when scraping the hard substrate.
Although bioerosion is often viewed with negative connotations due to the loss of reef substrate, these important organisms contribute to a complex dynamic. When sea urchins and parrotfishes feed, they sculpt the reef, enhancing reef heterogeneity and thus creating new habitats that support a large diversity of species. At the same time, they modify particle size, produce new sediments, forming amazing sandy beaches.
Like everything in life, it is important to maintain a balance. However, there are many factors that can alter the balance between growth and erosion, and weakening of the reef structure, such as ocean acidification, coral bleaching and diseases. If the rate of erosion is higher than the growth rate, the amazing reefs will become pieces of rubble in the future.