Achieving maximum polyp expansion is super desirable for coral keepers everywhere.


Nice skeleton colors are not enough we want to see those tentacles waving too!

Softies and LPS corals of course are the easiest corals to achieve optimal polyp expansion if those corals are healthy with appropriate lighting, stable and high quality water- you’re pretty much guaranteed consistent polyp extension (PE).

SPS coral keepers on the other hand see polyp extension a lot less frequently and often have a hard time visualizing their polyps at all.

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Generally speaking withdrawn polyps indicates the coral is in a state of stress as unhealthy corals rarely extend their polyps.

But that said, there are many reasons why you may not be seeing PE much, not to mention that some varieties of SPS corals extent their polyps less further and less frequently than others.

extend coral polyps

Polyp expansion (PE) is the holy grail of SPS reefing

Usually you would view polyp extension at night time, when the corals feed on plankton and to a lesser extent in the day time because of respiration.

When it comes to the most popular SPS; Acropora – the axial (growing tip) and radial (surrounding the tip) coralite polyps are a good indicator that the specimen is thriving.

When you see no PE at all at any time day or night in a 24 hour cycle that indicates there is something wrong!

hard coral PE

Soft corals and LPS corals exhibit much more PE than do SPS corals, but reasons for lack of PE are the same (image credit: Roberto Perez Bahena)


So why do coral polyps extend/retract?


  • Feeding: extended polyps are the only way corals can capture plankton out of the water column which supply much needed protein for tissue regeneration. Polyps also contain zooanthellae so can help with photosynthesis if the coral needs it.
  • Respiration: corals extend their polyps for gas exchange taking in O2 and putting out CO2 at night.
  • Protection from predation: if coral polyps are being nipped at by fish, crustaceans or preyed on by parasites such as flatworms, red bugs PE will be greatly reduced or stopped entirely. This will in turn limit the corals feeding and growth.
  • Protection from environment: unstable water parameters, inappropriate flow and/or lighting are major environmental reasons for lack of PE
Coral polyps won't extend

Just by target feeding you can expect to see triple the amount of PE than without it (image credit: dizzylizzy1227)


Why your coral polyps aren’t extended…


  • Your water conditions are unstable
  • You have parasites (flatworms, nudibranchs, sea spiders, acro red bugs, some crabs, some snails) and/or tank-mates that are attacking polyps themselves.
  • Your corals are starved, so don’t have the energy to expand their polyps.
  • Your phosphate levels are too high
  • Your lighting is new or your corals are still acclimatizing to lighting
  • Your levels of nitrates are too high
  • Build-up of algae or detritus on polyps
  • Too much water flow
  • Too much light
  • Chemical warfare from your soft corals
  • Stray voltage
  • Bacterial of fungal infection
coral polyps don't extend

There are many reasons for lack of PE, the most difficult to fix is coral parasites which is why all corals should be quarantined and dipped (image credit: Stéphane Duquesne)

Polyp extension seems to be most pronounced in established, nutrient poor aquariums that are well skimmed and target fed plankton.

In SPS corals parasites are a major reason for lack of PE and hard to remove once in your aquarium.

To ensure optimal PE all new corals definitely need removal of parasites by thorough inspection and dipping with both ReVive and Lugols solution followed by a gradual acclimation and quarantine period.

Don’t forget, if you would like access to my free ebook “9 Saltwater Aquarium Success Tools” to minimize mistakes and get your tank thriving simply click the button below.

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