Help! My Xenia polyps are being eaten!

January 23, 2000
From: Rich Staley

I work at a retail pet store, and one of my customers approached me with a problem. He had harvested over 50 "sea slugs" that had colonized his aquarium and had feasted on his xenia polyp colony. The slugs are 0.5 cm approximately, white, with a faint green saddle. They apparently have only bothered his Xenia colony, and they do not seem to appear elsewhere in his aquarium. He keeps his tank stocked with Fiji live rock, probably from Walt Smith Inc. His water parameters generally test fine, with a ph around 8.0-8.2 being the only "problem" test. I would provide more info., but this is all I know.

If you can help me, please let me know what these are, what else they might eat.

Thank you,


Staley, R., 2000 (Jan 23) Help! My Xenia polyps are being eaten!. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Rich,
Without some idea of shape, it is difficult to even guess at what your friend has. I guess that because he doesn't mention any papillae, gills or other 'frilly' bits that his animal doesn't have them. My bet then is that he has an infestation of a flatworm rather than of a sea slug. There are a group of acoel flatworms which occur in large numbers on soft corals and other colonial animals. There is a picture of one in a recent message from Atsushi Ono.

If the animals are nudibranchs, the most likely culprits are aeolids such as Phyllodesmium crypticum which live and feed on Xenia and its relatives.

And before you ask - I know of no 'cure'. The animals either came as microscopic larvae on the Xenia or else on the 'live rock'. Xenia have symbiotic algae in their tissues so unless your friend has good plant 'grow lamp' lighting, the Xenia may already be in a fragile state. It may survive, but I guess the important thing is to keep an eye on it and remove it before it starts rotting. If he has acoel flatworms it is posssible they will move to other soft corals he has in his aquaria so he will have to decide whether he wants healthy soft corals or healthy flatworms.

Let me know if my guess is correct
Best wishes,
Bill Rudman.

Rudman, W.B, 2000 (Jan 23). Comment on Help! My Xenia polyps are being eaten! by Rich Staley. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from



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