Cynarina Button Coral

Seen in Ikok's Rock dive site in Lankayan Island. Cynarina lacrymalis is a large, solitary coral with a single polyp nestling in a corallite, the stony cup it has secreted. It can grow to a diameter of 15 cm (6 in). It is cylindrical with a round or oval upper surface. It is usually fixed to rock but has a pointed base and can be embedded in sand or survive unattached. There are about twenty broad white radially arranged septa (ridges) joined to the corallite wall, with secondary septa between. They have large, rounded lobes and the central axial structure (columella) in the corallite is short and broad. The septa can be seen through the transparent, fleshy mantle which contains symbiotic flagellates known as zooxanthellae which give the coral its colour. This is usually pale brown or green, sometimes with a contrasting oral disc, but also sometimes pinkish or bluish. The colour depends on which species of zooxanthella take up residence. The coral has the ability to change its surface from glossy to dull but it is unclear why it does this. At night, when the polyp extends its many tentacles to feed, the coral resembles a sea anemone.