Why do some Soft Corals Pulsate?

 

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 or Heteroxenia fuscescens and the Umbrella xenia, Xenia umbellata.

Scientists have long assumed that these Xenids rhythmically pulsate to create a current to assist feeding and respiration. This is a very costly strategy, though, in terms of energy used. Until now it wasn’t known exactly how much, and exactly what, benefit the coral gets from working this hard.

This week the mystery has been solved by researchers from the Hebrew University of Jerusalem and the Technion-Israel Institute of Technology. Their work appears in the current issue of PNAS (Proceedings of the National Academy of Sciences). They made several discoveries about the Pulsating Xenid and found that thanks to pulsation, the ratio between photosynthesis to respiration in Heteroxenia fuscesens is the highest ever measured in both hard, and non-pulsating soft, corals.

The elegant motion of Xenids has been fascinating the 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 it had not been explained. Now, in the study of Kremien, Genin and Shavit, it is known that the pulsation motions augment a significant enhancement in the binding of carbon dioxide to the photosynthetic enzyme RuBisCo, also leading to a decrease in photorespiration. This explanation justifies the investment of energy in pulsation — the benefit overcomes the cost.

The next time I dive in the Red Sea I’ll look more carefully at these unique animals.

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