The soft coral that pulses. Each polyp opens its eight feathery tentacles and closes them again, over and over, all day, and a colony of hundreds does it out of step - one of the few genuinely mesmerizing things on a reef and one of the strangest, because a colony that never moves has no obvious reason to work this hard.
The best answer is that it is farming. Xeniids get most of their food from symbiotic algae in their tissue, and photosynthesis stalls when oxygen builds up in the thin layer of still water against the surface. The pulsing sweeps that layer away, oxygen leaves, carbon dioxide arrives, and the algae keep working - measurements show pulsing colonies photosynthesize substantially better than the same colonies held still. It is the most energetically expensive behavior known in any coral, and it pays for itself.
They brood their larvae internally and release them ready to settle, and colonies also spread by simply splitting and creeping, which is why xenia takes over a patch of reef quickly.
What lives on it: small commensal shrimps that sway in the same rhythm.
- soft coral with polyps that open and close in a continuous slow pulse
- eight feathery tentacles on each polyp, in cream to pale brown
- colonies form low clumps rather than branching trees
- colonies to 20 cm across
