Reading Coral Polyps: Health and Behaviour
Open or closed polyps are your coral talking to you. Learn what healthy extension looks like, when retraction is normal, and when persistently closed polyps signal a husbandry problem.
A coral is a colony of many tiny polyps, each a sac-like animal ringed with tentacles that carry stinging cells for capturing food. Those polyps extend and retract using muscle fibres, and how they behave is one of the most useful health signals a reef keeper has. Learning to read polyp extension lets you catch trouble early, often before any tissue is lost.
What healthy looks like
A content coral generally shows good polyp extension for its type. Large-polyp stony corals inflate and put out tentacles, small-polyp stony corals show fuzzy extended polyps, and soft corals and zoanthids open up, with some such as pulsing xenia visibly pumping. Many corals naturally extend their tentacles at night to catch plankton and retract them during the day, so timing matters when you judge them.
When closed is normal
- Recently moved or newly added corals often close up for a while as they acclimate.
- Day-night rhythm: many corals retract by day and extend at night.
- Leather corals periodically retract and develop a waxy mucus coating that they shed, which is healthy behaviour.
- A brief feeding or defensive response to being touched.
Common causes of retraction
- Unstable parameters: corals dislike wildly fluctuating temperature, salinity, pH, calcium and alkalinity; alkalinity swings over a short period are especially likely to change polyp extension.
- Nutrients out of range: when phosphate or nitrate run too high, stony corals such as Acropora tend to retract.
- Light problems: a sudden jump in intensity causes photo shock; corals can also retract if placed too close to or too far from the light.
- Flow problems: too little flow lets detritus settle on polyps, while too much can damage the flesh; both cause retraction.
- Pests and pestering: acro flatworms and red bugs reduce extension, and fish such as dwarf angels that nip corals keep them closed.
- Chemical warfare: stinging or allelopathic neighbours can suppress a coral nearby.
A diagnostic approach
Work through the likely causes in order. Test the big three and nutrients first, since stability is the single biggest factor; review placement for light and flow; inspect closely for pests and check whether a fish is harassing the coral; and consider whether a neighbour is stinging it. A well-established, well-fed reef kept at stable, low-but-not-zero nutrients tends to give the best, most consistent polyp extension.