B is for Bioerosion

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As a part of the A-Z Photographic Glossary of Biological Terms challenge, today it is B for Bioerosion, as illustrated by this Green Rock-boring Urchin (Echinometra viridis), Sanctuary Caye, Belize, depth 15’.

Bioerosion is the removal of the calcium carbonate substrate by biological agents. Corals accrete (grow by gradual accumulation) calcium carbonate, or limestone, to form their skeleton; this skeletal framework forms the structure that provides food and shelter to the majority of reef organisms. While corals are regularly accreting calcium carbonate, bioeroders are naturally breaking it down to make rubble and sand. The balance between accretion and erosion is delicate – if bioerosion is excessive then coral destruction will be faster than coral growth.

The bioeroder community is made of many different types of animals, plants, and even bacteria. They erode coral skeleton, mostly dead coral, mechanically by removing calcium carbonate with teeth-like structures or chemically by excreting acidic compounds and dissolving the coral skeleton. There are three main functional groups of bioeroders: microborers (tiny endolithic algae, cyanobacteria, and fungi); macroborers (small marine worms, sponges, bivalves, and barnacles); and grazers- mainly fish such as the all-important parrotfish, and urchins such as this Green Rock-boring Urchin. This shallow reef organism emerges at night to feed by grazing on algae with its five teeth, part of what is called the Aristotle’s lantern organ that surrounds its mouth; although it is not believed to actually bore holes, its grazing still causes bioerosion in reefs.

So, is bioerosion bad? No. Bioerosion is a natural process which breaks down old dead coral making room for new coral growth and when in balance, the bioeroding communities are major players in coral reef resilience and in the ecological services that we receive; however, if the process of bioerosion is altered by extreme natural events or by anthropogenic human activities then the delicate accretion-erosion balance can be thrown off and it could become destructive.

(parts of this post were “grazed” from Nyssa Silbiger’s blog “It’s Not Boring”. Nyssa is a PhD candidate in Zoology from the Hawai‘i Institute of Marine Biology, University of Hawai‘i at Mānoa)

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