Mustard hill coral

When it grows in fast-flowing, shallow water, Porites astreoides is encrusting but in calmer water at medium depths it is a massive coral with a smooth, mounded, semi-spherical form and can grow to 60 centimetres (24 in) in diameter. At greater depths it is usually plate-like and in caves and under overhangs the plates are angled to receive the maximum amount of light. It is the only species within the genus Porites not to have a finger-like form. The corallites are small and tightly-packed and give the coral a porous appearance. The polyps each have six tentacles and are generally retracted during the day. This coral is yellow, yellowish-green, pale grey or pale brown, a colour given to it by the dinoflagellate zooxanthellae, microscopic algae that live symbiotically within the coral's tissue.