Why Won't My Coral Open?

When your coral doesn't open, it can cause some concern, particularly if you are new to the aquarium hobby. While coral polyp extension is an indication of good health, polyp retraction doesn't always mean there is a major issue with your coral. 

However, a coral that remains closed for a long time may indicate that something inside your aquarium needs attention. 

What does normal coral behavior look like?

Before worrying about closed coral polyps, you should understand the basics of coral behavior. As mentioned, some coral retraction is normal coral behavior - many coral species naturally close up in response to different stimuli. For example, LPS corals such as Euphyllia, are known to briefly retract their polyps when being fed or if a fish gets too close. Other corals like soft corals, are also known to go through waves of expanding and contracting their polyps throughout the day. 

Saying that, if your coral has been closed for an extended period, especially during times when you usually see it open, it is time to take a look at potential causes. 

Why do coral polyps close up?

There are eight reasons why your corals may not be opening:

• Water Parameter Issues
• Poor Lighting
• Poor Water Flow 
• Chemical Warfare
• Coral Competition
• Feeding Issues
• Pests 
• Coral Infections

Water Parameter Issues

Problems with water parameters, is one of the most common reasons corals close their polyps in reef aquariums. Corals are sensitive to their surrounding environment, therefore, even the smallest change in water parameters can cause issues such as polyps closing. 

You should frequently check the following parameters:

• pH: Most corals thrive with a pH range of 8.1 - 8.4. Low pH levels can cause calcification and coral polyp retraction. 
• Temperature: Saltwater aquariums should have a stable temperature of 75-78 ℉. If the temperature inside your aquarium fluctuates, it can cause extreme stress and closing of coral polyps.
• Salinity: Aim for salinity (or specific gravity) levels between 1.024 to 10.25. Salinity levels outside this range can severely stress your corals, which often leads to them closing the polyps.
• Alkalinity: Saltwater aquariums should have an alkalinity reading of 8-12 dKH. Unbalanced alkalinity levels contributes to increased stress and polyp closure. 
• Magnesium: This levels should be kept at 1250 - 1350 ppm. Magnesium is key in maintaining good alkalinity levels and allows your corals to function properly. 
• Calcium: This level should be maintained at 400-450 ppm. Problems with calcium levels can impair coral growth, causing them to retract their polyps, plus it can affect their natural behavior. 

Poor Lighting

Inadequate lighting is another cause for coral polyp retraction. Both too much and too little lighting can trigger this behavioral response. 

Corals may suddenly close if you have recently upgraded your lighting system, if you have moved them to a different position in the aquascape, and after replacing a light bulb. 

You may also have noticed reduced polyp extensions or color loss in addition to retracted polyps. In extreme cases, lighting issues can also cause tissue recession. 

Solutions? We recommend positioning corals in an area where they will receive their specific lighting requirements. If you are unsure what lighting requirements your coral species requires, take a read of our aquarium lighting guide.

If the issue persists, you may need to replace the light bulbs. To keep an eye on lighting intensity we recommend using a PAR meter. 

Poor Water Flow

If the water flow is too strong or weak, it can cause coral polyp retraction. There are three main types of corals - SPS, LPS, and soft corals. These all require different flow requirements:

• SPS: Moderate to high water flow
• LPS: Moderate water flow
• Soft corals: Low to moderate water flow

Signs of poor water flow:

• Tissue damage 
• Bacterial film developing
• Asymmetrical polyp extension
• Debris collection on the coral's tissues

Chemical Warfare

When corals are not comfortable, they are capable of chemical warfare. When corals release allopathic compounds into the water, it can affect neighboring corals and other tank inhabitants. So, if your corals are suddenly closing their polyps after adding a new coral species, your new addition may be to blame.  

Preventing chemical warfare

• Monitor new additions and change coral placement if corals start to become aggressive 
• Ensure water flow is adequate to prevent chemical build-up
• Use a chemical filtration system such as activated carbon
• Allow plenty of space between coral specimens

Feeding Issues

Poor feeding can also contribute to coral polyp closure. This can be from poor nutrition or feeding-related stress.

Try and give your corals a variety of foods, this includes:

• Coral pellets/powders
• Phytoplankton
• Zooplankton
• Amino acid supplements

Frequency and timing can also help prevent polyp closure. We recommend feeding LPS corals 2-3 times a week (directly), SPS corals 1-2 times a week (direct spot-feeding), and soft corals 2-3 times a week (this can be directly or indirectly). 

When feeding corals we suggest feeding during low flow and with a pipette for accuracy. You should never force feed corals, so ensure that their polyps are naturally extended before feeding. 

Pests

Even though aquariums are typically closed systems, pests can still make their way into your tank. Common aquarium pests that can cause coral polyp closure include: 

• Flatworms
• Coral-eating nudibranchs
• Coral-eating fish such as some wrasses, angelfish, and butterflyfish species

To prevent pests causing issues inside an aquarium, regularly expect any new corals or live rock before introducing them to your reef aquarium. Always follow the correct quarantine procedures and pest removal methods. 

Coral Infections & Diseases

Bacterial and viral infections like Zoa pox can sometimes cause coral polyp closure. Look out for:

• Changes in color
• irregular polyps
• Tissue recession
• Overproduction of mucus
• Uncomfortable behavior

How to reopen coral polyps?

If your coral will not open, there are a few steps to help it recover. 

First, test the water parameters mentioned earlier. Ensure you are using a reliable water test kit (like the Trident Water Analyzer) and keep detailed records of the readings so that you can look for patterns and sudden changes. If you need to make any changes to the parameters, you must always adjust gradually to prevent stress. 

Next, look at your coral's environment. This includes the light intensity, water flow, and temperature stability inside the aquarium. 

Finally, inspect for any pests. Hopefully, you quarantined any new additions, but sometimes, these pesky critters are expert hiders! Take a flashlight during the evening to see if any make an appearance. Pests also cause infections and infestations, so take a look for any visible and behavioral signs. If any corals have been infected beyond repair, consider removing them to prevent spreading. 

Final coral closure tips

• maintain stable water parameters through regular testing, frequent water changes, and performing proper equipment maintenance. 
• ALWAYS quarantine new additions and observe for potential issues. 
• Track water parameter changes and document coral behavior. 
• Invest in reliable testing kits, proper filtration systems, and good lighting systems. 

Conclusion

When corals close their polyps, it can be very alarming, particularly if you are a newbie reefer. Understanding the common reasons why corals retract their polyps will help you make adjustments so your corals can thrive. 

If you are facing persistent issues with closed coral polpys do not hesitate to contact our experienced team at Reefco Aquariums for advice. We are always happy to help!