Pulsating Xenia Coral, Xenia umbellata

5 November 2022

If you've dived in the Red Sea or Indo-Pacific you will have seen pulsating soft coral repetitively "grabbing" at the water. They are so common that we take them for granted. But only one family of coral does this - the Xeniidae. Even within this family, only a few members pulsate. Those that do include the Pulsating Xenid, Heteroxenia fuscescens and Umberella Xenid, Xenia umbellata. Continuous pulsating must be very costly in terms of energy used - is it worth it? Well, Israeli researchers found that it is. Indeed, the corals are the most efficient of any so far measured.

Pulsating Xenia coral — Umberella xenia, *Xenia umbellata*

After watching several coral colonies with an underwater infrared-sensitive camera night and day, the researchers made a surprising discovery: Heteroxenia corals cease to pulsate and take a half-hour break every single day in the afternoon. At this stage, the afternoon "siestas" remain unexplained.

Volunteer divers helped with the investigation. They found that if a diver lightly touched the coral, the polyps "close" and remain motionless for a few minutes, after which the coral returns to its normal pulsation activity. The researchers used this behaviour in order to repeatedly measure the flow field around the Heteroxenia during pulsation and rest.

Xenia umbellata — The elegant motion of Xenia has fascinated scientists for over 200 years.

Even though the polyps' motions are uncoordinated (i.e. each polyp starts its period of motion at a different time), the accumulated effect of the polyps' activity is a significant enhancement of the water flow around the colony, particularly sweeping water away from the coral and reducing the probability of re-filtration of the same water.

Corals "host" photosynthetic algae in their tissues. The symbiotic algae provides the coral with essential nutrients and lives off the waste of the coral. The pulsation motions enhance the coral's photosynthesis rate.

Xenia coral, Red Sea — Some xenia corals cease to pulsate and take a half-hour break every afternoon. Martin Hablutzel/DepositPhotos

A previous study found that the motion of water around corals is essential in order to enhance the efflux (removal) of oxygen from the coral tissues. Without water motion, the oxygen concentration in the coral tissues would rise and the photosynthesis rate would drop.

The elegant motion of Xenia has fascinated scientific society for over 200 years. The first Xeniidae colonies were collected during Napoleon's invasion of Egypt during the years 1798-1799 when specimens of Xenia umbellata were brought back to Europe. Both specimens were given brief descriptions by Lamarck in 1816 and accompanied by detailed drawings of the colonies and their polyps. In spite of this the pulsations had not been explained until now.

Pulsating Xenia Coral, Xenia umbellata

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