Yellow cluster anemone

Not really an anemone at all but a zoanthid, a colonial relative that spreads as a mat of small polyps joined by a thin creeping base. Each polyp is a slim yellow or orange column about 5 mm wide and up to 2 cm tall, crowned with two rings of fine tentacles, usually twenty-four to thirty-six of them. A colony can be a scatter of a dozen polyps or a dense patch the size of a dinner plate, and on a good wall the effect is a spill of bright yellow paint across the grey rock.

It likes shade and moving water. Look for it on vertical and overhanging rock, in the entrance zones of caves and caverns, on the shaded plating and ribs of Malta's scuttled wrecks, and very often growing directly on sponges of the genus Axinella, which is where its Latin name comes from. Divers meet it from about 5 m down to well past sport-diving depth; in Malta and Gozo the reliable spots are the shaded walls at Reqqa Point, Fungus Rock and the deeper parts of Ras il-Ħobż.

The polyps are open most of the time, day and night, because the colony feeds on plankton carried past on the current rather than relying on sunlight. Touch or a passing shadow makes them shrink to squat yellow buttons within seconds, then slowly bloom again once things settle.