Blue tangs eat algae off of the coral. This prevents algae from overgrowing and suffocating the coral. Blue tangs are often found swimming in large schools cruising over the reef top, grazing on algae. These schools are usually composed of multiple species within the surgeonfish and tang families.
If you saw the movie Finding Nemo, Dory was a blue tang. She is the one who went along with Marlin on his journey to find little Nemo.
In a ten-year study conducted in the waters adjacent to the Virgin Islands, blue tangs were found to be more plentiful than any other fish species, accounting for an observed 15% of the region's overall fish population. These Blue Tangs are featured in the Diving St. Thomas, Volume 1 DVD. http://island-showcase.com/blue-tangs-supermarket-reef.html
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