How can a hungry school of fish help to create a thriving reef island? In this clip from the BBC’s The Blue Planet, neon-colored Humphead Parrotfish crunch rocks and hard corals alike as they forage for coralline algae. Some details via wikipedia:
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The teeth grow continuously, replacing material worn away by feeding. Their pharyngeal teeth grind up the coral and coralline algae the fish ingest during feeding. After they digest the edible portions from the rock, they excrete it as sand, helping to create small islands and the sandy beaches of the Caribbean. The green humphead parrotfish can produce 90 kg (200 lb) of sand each year. Or, very averagely (as there are so many variables i.e. size/species/location/depth etc.), about 275 g per parrotfish per day.