Brown jelly disease (BJD) produces a distinctive brown, slimy mucus that coats the surface of infected coral tissue. It looks like a translucent to opaque brown gel clinging to the coral’s flesh, and it spreads outward from the point of infection as the tissue beneath it breaks down. If you’re staring at your coral wondering whether that brownish film is normal detritus or something worse, the key visual clue is the consistency: BJD has a fluid, gelatinous texture that looks wet and slimy, not dry or crusty.
The Hallmark Brown Mucus
The defining feature of brown jelly disease is a layer of brown, jelly-like mucus sitting on top of the coral. It ranges from a light tan to a dark chocolate brown, and it has a slippery, almost liquid appearance. The mucus is produced as a combination of bacterial and protozoan organisms consume the coral’s living tissue, essentially digesting it in place. What you’re seeing is partly the pathogens themselves and partly the liquefied remains of the coral’s soft tissue.
The jelly tends to accumulate in the recesses of the coral first, pooling between polyps or along the base of tentacles before spreading across the colony. In corals with large fleshy polyps, like hammer corals or frogspawn, you might notice the mucus gathering where the tentacles meet the skeleton. As the disease progresses, the brown slime expands outward and the tissue underneath disappears entirely, leaving bare white skeleton behind the advancing edge of jelly.
How It Differs From Other Problems
Several coral ailments can look alarming, but BJD has features that set it apart from the most common lookalikes.
- Normal tissue necrosis: When coral tissue dies from stress (temperature swings, chemical burns, physical damage), it typically recedes or peels away from the skeleton. There’s no thick, slimy coating over the dying area. BJD specifically produces that gelatinous brown layer on top of the tissue as it breaks down.
- Fungal infections: These tend to appear as white, cotton-like growths on the coral’s surface. The color and texture are clearly different from BJD’s brown, smooth gel.
- Algae overgrowth: Algae is usually greenish, attaches firmly to the coral surface, and has a more structured, filamentous look. BJD’s mucus is fluid and slimy rather than rooted or hairy.
One practical test: gently direct a small stream of water (using a turkey baster or powerhead) toward the affected area. Brown jelly mucus will partially lift and drift in the current because of its loose, gelatinous consistency. Algae stays put, and necrotic tissue tends to flake rather than flow.
Which Corals Get It
BJD most commonly strikes Euphyllia corals, the group that includes hammer corals, frogspawn, and torch corals. These large-polyp stony corals have fleshy, exposed tissue that seems particularly vulnerable. The disease also occasionally shows up on other large-polyp species like Galaxea and Goniopora, but Euphyllia colonies are by far the most frequent victims in home aquariums.
Infected Euphyllia colonies often show behavioral changes before the brown mucus becomes obvious. Polyps may retract partially or fully, tentacles can look shriveled or limp, and the coral may stop extending at its usual times. If you notice a Euphyllia looking “off” for a day or two and then spot a brownish film forming at the base of the polyps, that progression is a strong indicator of BJD.
What Causes the Jelly
The brown jelly itself is produced by a combination of bacteria and single-celled organisms called ciliates. Researchers studying a related condition on wild reefs (called brown band disease) identified the ciliates as tiny, elongated organisms roughly 200 to 400 micrometers long, far too small to see individually with the naked eye. These ciliates are packed with symbiotic algae cells they’ve consumed from the coral tissue, which is what gives the mucus its brown color. Essentially, the brown tint comes from the coral’s own photosynthetic algae being eaten and concentrated by the pathogens.
The infection is contagious. The jelly can slough off an infected coral, drift through the water column, and settle on a neighboring colony. This is why hobbyists who spot BJD on one coral often find it appearing on adjacent Euphyllia within days.
What to Do When You Spot It
Speed matters with BJD because the tissue destruction is fast, sometimes consuming an entire colony within 24 to 48 hours. The first step is isolation. Remove the affected coral from your display tank immediately and place it in a separate container with matched water parameters. This limits the risk of the jelly spreading to nearby corals.
Once isolated, use a turkey baster or gentle stream of saltwater to blast as much of the brown mucus off the coral as you can. The goal is physical removal of the pathogen load. Some hobbyists follow this with a brief dip in a coral-safe iodine solution or diluted hydrogen peroxide bath, though results vary and there’s no universally proven chemical cure. After dipping, place the coral in clean water and monitor it closely over the next several hours for any return of the mucus.
If the disease has already consumed most of the tissue and only a small living section remains, you can try fragging the healthy portion away from the infected area. Cut well ahead of the visible jelly margin, since the pathogens can extend slightly beyond what’s visible. Discard the infected fragment and treat the salvaged piece as you would a fresh frag, keeping it isolated until you’re confident no new mucus appears.
Antibiotic treatments have shown mixed results for coral diseases in general. A recent study on a different coral disease found that amoxicillin applied directly to infected tissue did not slow disease progression and may have disrupted beneficial microbes in surrounding healthy tissue, potentially making things worse. This doesn’t rule out antibiotics entirely for BJD, but it underscores that physical removal and isolation remain the most reliable first response.
Preventing Outbreaks
BJD typically strikes corals that are already stressed. Poor water quality, unstable temperatures, aggressive tankmates, and recent shipping trauma all lower a coral’s immune defenses. Keeping your water parameters stable, particularly temperature, alkalinity, and nutrient levels, is the single best preventive measure.
Quarantining new corals before adding them to your display tank gives you a window to spot early signs of BJD before the pathogen reaches your established colonies. Even a week of observation in a separate system can catch problems that aren’t visible at the store. If you’re adding a new Euphyllia, watch the base of the polyps carefully during quarantine for any brownish film or unusual tissue recession.