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They hide during the day, eat algae at night, but eat coral if there is no algae. What animal is this? They are Diadema, a type of sea urchin that graze on algae growing on coral reefs. There are six different species of Diadema and all of them play an important role in keeping coral reefs healthy. Moderate populations of Diadema are critical to maintaining the natural balance between algae and coral in a healthy reef ecosystem. This balance can be disturbed by the removal of predators, such as triggerfish and large wrasses that would normally keep Diadema populations from getting too big. An outbreak occurs when the population levels of Diadema gets too big, causing them to scrape away coral polyps from the reef surface. On the flip side, too few Diadema along with the removal of other algae eating organisms, results in algae taking over coral surfaces and threatening the health of reef ecosystems.
Sea urchins graze on algae which compete with corals for space and light.
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