Pulsing xenia

At first glance the pulsing xenia (Xenia umbellata) does not look like a headliner: a few centimetres of pale cream or greyish-brown soft coral with small stalked umbrellas. Then you pause above it and notice what makes divers melt: the eight-armed feathery polyps open and close rhythmically, about once a second, like thousands of tiny hands clapping in unison. The effect is so hypnotic that many divers get stuck over a single colony for minutes.

Pulsing is the xenia trademark – in the animal kingdom it is extremely rare for a sessile creature to keep up constant rhythmic movement, because it burns a great deal of energy. For the xenia it pays off: the ceaseless "clapping" drives away the stagnant, oxygen-saturated water around the polyps, letting the symbiotic algae photosynthesise several times faster. The xenia essentially creates its own draught – researchers count it among the cleverest energy investments in the coral world.

It is very common on Hurghada reefs: on the shallow plateaus and reef tops it often forms carpet-like meadows that sway and pulse at the same time. You can watch it easily while snorkeling in the coral gardens of Gota Abu Ramada, El Aruk and the Giftun Islands. It is a fast-growing pioneer species, first to claim vacant surfaces – which also gives it a role in the healing of damaged reef patches.