April 8, 2026 By Julian Sprung
Euphyllia and Fimbriaphyllia spp. aka "Torch," "Hammer," and "Frogspawn" corals, among other common names, sometimes suffer from parasitic diseases that result in the rapid or slow loss of tissue in meandroid species or entire polyps in branched species. The most common affliction is "brown jelly" infection, which is driven by huge populations of ciliated protozoan parasites that directly consume coral tissue, forming a gelatinous "front" at the site of infection, with healthy tissue behind the front and bare skeleton in front of it. These Brown jelly infections may involve not only protozoans, but also bacteria, as experiments have shown that antiprotozoal and antibiotic treament may both help. Less common, but not rare, is sudden, unexpected polyp bail out or polyp destruction, with no brown jelly present. This normally only affects one or two polyps at a time, but it can proceed to wipe out an entire colony or colonies, slowly but surely. This issue has been brought to my attention by a few hobbyists who asked me about a possible cause, and what came to mind was either bacterial infection or a predator (assuming brown jelly infection had been ruled out). It turns out that predator was the correct diagnosis, but what a surprise it was to discover the way it works when I saw this in one of my own displays. The predator is the common bristle worm, but in this case the worm does not simply browse the aquarium for a tasty snack, the way we commonly think of bristle worms as opportunistic clean up crew with occasional taste for coral. On the contrary, in this case the bristle worm takes up residence INSIDE the skeleton of Euphyllia and/or Fimbriaphyllia! 
And not just one worm, several living together in the relatively soft, hollow skeletons that characterize these corals. From this position the worms then eat the coral polyp from below. I only discovered this by cutting open an affected coral, revealing them deep inside. Initially I observed one of them crawling UP and OUT through the center of one of the polyp skeletons, which was a "what the heck?!" moment. Then I sliced the skeleton some more to reveal the rest of its bristly family inside.
This habit of living within the coral skeleton instead of simply under it or just nearby reminds me of what bristle worms do to large tridacnid clams... they take up residence within the hinge and feed on the clam from below, through the byssus opening. Their excavation of the hinge eventually compromises it and leads to broken hinge syndrome, which is ultimately fatal to the clam.
