Black coral

Despite its name, living black coral is a deep-dwelling anthozoan that appears dark brown to greenish in the colour-filtered light below 20 metres, only revealing its black skeletal axis when dead or harvested. At Babylon it forms bushy trees up to a metre tall on the shaded faces of the pinnacles, providing shelter for tiny hydroids, brittle stars, and longnose hawkfish. It grows extremely slowly — less than 4 centimetres per year — meaning a modest colony represents decades of accumulation.