Rapid and Slow Tissue Necrosis (RTN/STN) in SPS Corals
Rapid and slow tissue necrosis strip living tissue off SPS corals such as Acropora, leaving bare white skeleton. Learn how to tell RTN from STN and from bleaching, the temperature-driven bacterial link, and the emergency and prevention steps reef keepers use.
RTN vs STN
In rapid tissue necrosis (RTN), coral tissue sloughs off the skeleton extremely fast — classic events can wipe out entire colonies, and even entire tanks, within hours. In slow tissue necrosis (STN), the coral slowly bleaches, usually from the base, and the tissue is consumed upward over days or weeks, leaving bare white skeleton below a clear demarcation line.
How it differs from bleaching
The critical distinction is this: in bleaching the coral turns white but living tissue is still present (it has lost its symbiotic algae and is starving). In RTN or STN the tissue itself is gone, exposing bare skeleton. RTN and STN are an emergency; bleaching is often recoverable if the stressor is removed.
The temperature-driven bacterial link
Any spike in water parameters — alkalinity or temperature instability — can trigger RTN, often in stressed or unquarantined corals. Vibrio bacteria have been found dominant in some outbreaks. Vibrio coralliilyticus is capable of invading and lysing coral tissue above 27°C but is avirulent at or below 24°C, so a stable, moderate temperature directly lowers the risk.
Response and prevention
- Identify true RTN or STN (rapid sloughing, or an advancing bare-skeleton line) versus minor tissue loss.
- Isolate the affected colony to protect its neighbours.
- Frag healthy tissue well above the necrosis line and mount it separately to salvage the coral.
- Stabilise alkalinity and temperature and check for parameter swings.
- One documented hobby treatment reported by Reef Builders used ciprofloxacin dosing (500 mg per 500 gallons every other day for 10 days, escalated in stubborn cases) — an aggressive, last-resort measure best guided by an experienced reefer or veterinarian.
Sources: reefbuilders.com ; pmc.ncbi.nlm.nih.gov