At night, we'd be able to see their tentacles. Tubastreas, a form of hard coral, expand for nighttime feeding. Astrolabe Reef, Kandavu, Fiji - Depth 85 feet Let's look at an individual coral polyp. It has a ring of tentacles around a central mouth opening, and the polyp itself sits on a limestone skeletal case. These polyps are linked together by living tissues, which enable them to share the nutrients that are captured.
Hermatypic corals have brownish coloured, unicellular algae living within the tissues of the polyps.
These corals use sunlight and carbon dioxide to produce carbon enriched organic compounds, which in turn, provide the nutrients that the polyps require. They may also absorb dissolved nutrients from the water, or feed on bacteria and organic debris. Ahermatypic corals do not have any algae, or zooxanthellae living within their tissues, so they have to use their stinging cells to capture prey, which includes mainly of microorganisms, but sometimes include worms, urchins and even fishes. Injuries involving Hard Corals Hard Coral Many hard corals have sharp serrated edges, which can inflict nasty cuts and abrasions upon contact and a light brush against them. Secondary infection may result from the deposits and mucus of the hard corals.