Golden cup coral

This is the coral that makes Maltese cave diving look the way it does. Colonies form low, rounded crusts of stony cups packed tightly together, each cup a few millimetres across, the whole patch usually 10 to 30 cm wide and only a few centimetres thick. The living tissue is a hot orange-to-egg-yolk yellow, and when the polyps are open each one puts out a short ring of translucent tentacles around a slit-shaped mouth. Closed up in daylight the colony looks like a sheet of orange honeycomb glued to the rock.

It carries no symbiotic algae, so it has no reason to seek the sun. Instead it takes the shaded places: the roof and entrance of every cavern, the underside of overhangs, the dark side of boulders and the inner walls of swim-throughs, anywhere from a metre below the surface down to about 50 m. Around the Maltese islands it is one of the first things a torch beam picks out at Reqqa Point, in the Dwejra caves and along the shaded ledges of Comino's caverns, often sharing the rock with yellow cluster anemones and encrusting sponges.

The polyps feed at night on zooplankton drifting past, so a night dive or a deep cave shows the colony fully open and furry-looking rather than shut tight. It grows very slowly, which is exactly why a fin kick that snaps a crust off the rock is worse than it looks.