Understanding the Factors that Trigger Zoanthids to Close


Zoanthids are a popular choice among reef aquarists due to their stunning colors and unique appearance.

These corals, also known as button polyps, are relatively easy to care for and can thrive in various aquarium conditions.

However, one of the common concerns among zoanthid keepers is why these corals close up and retract their tentacles.

In this article, we will explore the factors that cause zoanthids to close and how to prevent them from happening.

Understanding Zoanthid Anatomy and Behavior

Before we delve into the reasons behind zoanthid closure, it’s essential to understand the anatomy and behavior of these corals.

Zoanthids are colonial animals that belong to the phylum Cnidaria, which includes other corals, anemones, and jellyfish.

These corals consist of a base called the “foot” that attaches to a surface, such as a rock or a substrate and a polyp that extends from the foot.

The polyp has a central mouth surrounded by tentacles that are used to capture planktonic food particles.

Zoanthids are photosynthetic, which means they can produce their food through a symbiotic relationship with algae called zooxanthellae.

These algae live within the coral’s tissues and provide the coral with nutrients through photosynthesis.

Additionally, zoanthids can feed on tiny food particles, such as brine shrimp, plankton, and other tiny organisms.

Zoanthids can exhibit different behaviors, including opening and closing their polyps, retracting their tentacles, and expanding or contracting their colonies.

These behaviors are affected by various environmental factors, which we will discuss in the following sections.

Factors That Cause Zoanthids to Close

Light Intensity and Duration

Zoanthids require adequate lighting for their zooxanthellae to carry out photosynthesis effectively. However, too much light or prolonged exposure to light can cause zoanthids to close up and retract their tentacles.

This response is the coral’s way of protecting itself from photodamage, which can occur due to excess light exposure.

Water Flow

Zoanthids require moderate to high water flow to bring in food particles and remove waste. However, strong water flow can cause the polyps to close up and retract their tentacles.

This response is due to the coral’s inability to capture food particles effectively in high water flow conditions.

Water Parameters

Zoanthids require stable water parameters to thrive, including temperature, salinity, pH, and alkalinity.

Fluctuations in these parameters, especially sudden changes, can cause zoanthids to close up and retract their tentacles.

Chemical Imbalances

Chemical imbalances in the aquarium, such as high levels of nitrates, phosphates, or metals, can stress zoanthids and cause them to close up.

Exposure to harmful chemicals or medications can also cause zoanthids to react negatively and close up.

Physical Agitation

Physical agitation of the coral, such as during water changes or tank maintenance, can cause zoanthids to close up and retract their tentacles.

This response is due to the coral’s natural defense mechanism against potential damage.

How to Prevent Zoanthids from Closing

Provide Adequate Lighting

Provide zoanthids with the appropriate lighting intensity and duration to prevent photodamage.

Use a timer to control the lighting cycle and ensure the coral receives adequate light without excess exposure.

Adjust Water Flow

Adjust the water flow in the aquarium to prevent excessive water movement around the zoanthids.

Use a powerhead or wavemaker to create a more gentle flow that allows the coral to capture food particles without being swept away.

Maintain Stable Water Parameters

Monitor and maintain stable water parameters to prevent fluctuations that can stress zoanthids. Test the aquarium water regularly and make necessary adjustments to maintain optimal conditions.

Avoid Chemical Imbalances

Avoid chemical imbalances in the aquarium by using high-quality filtration systems, performing regular water changes, and avoiding exposure to harmful chemicals and medications.

Handle Zoanthids with Care

Handle zoanthids with care during aquarium maintenance and avoid physical agitation of the coral. Use tools such as tongs or forceps to move the coral gently and avoid touching the polyps or tentacles.

Is It Normal for Zoas to Close at Night?

Yes, it is normal for zoanthids (commonly called “zoas”) to close their polyps at night. Zoanthids are photosynthetic organisms with symbiotic zooxanthellae within their tissues, providing the corals with energy through photosynthesis.

However, zoanthids also feed on plankton and other small organisms at night when photosynthesis is impossible. To capture prey efficiently, the polyps extend their tentacles and capture food particles in the water column.

Once they have finished feeding, the polyps will retract and close up until the next feeding opportunity. So, if you notice your zoanthids closing up at night, it is perfectly normal behavior.

How Do You Keep Zoas Happy?

To keep zoanthids (zoas) happy, you need to provide them with the right environment and care. Here are some tips for keeping your zoanthids healthy and thriving:

  1. Water parameters: Zoanthids prefer stable water conditions, including a pH between 8.0-8.4, salinity between 1.024-1.026, and temperature between 75-80°F. It’s important to monitor these parameters regularly and adjust as needed to ensure they stay within the appropriate range.
  2. Lighting: Zoanthids require moderate to high lighting levels, ideally between 100-200 PAR. You can achieve this by using LED or T5 lighting fixtures. Be sure to acclimate your zoas to the lighting gradually to prevent stress.
  3. Water flow: Zoanthids prefer moderate water flow, which helps to carry nutrients and waste away from their polyps. Aim for a gentle to moderate water flow rate in your tank, and avoid strong and turbulent currents that may damage the zoas.
  4. Feeding: Zoanthids are photosynthetic organisms that capture food particles in the water column. Supplementing their diet with small pieces of meaty foods, like shrimp or mysis, can be beneficial. However, be cautious not to overfeed, which can lead to water quality issues.
  5. Tankmates: Zoanthids can coexist with other reef-safe fish and invertebrates, but avoiding aggressive or coral-eating species is important. Also, be cautious when handling zoanthids as some species can release toxins that harm other tank inhabitants and humans.

How long does it take for new zoas to open?

The length of time it takes for new zoanthids (zoas) to open can vary depending on several factors, including the health of the coral, the lighting and water flow in the tank, and the acclimation process.

Generally, it can take anywhere from a few hours to a few days for new zoas to fully open after being introduced to a new tank.


Zoanthids are fascinating and colorful corals that can add vibrancy to any reef aquarium. However, they can be sensitive to various environmental factors that cause them to close up and retract their tentacles.

By understanding these factors and taking appropriate measures to prevent them, aquarists can help their zoanthids thrive and flourish.


  • Can zoanthids close up if they are healthy?

Yes, zoanthids can close up even when they are healthy. They may close up in response to changes in lighting, water flow, or other environmental factors.

  • Can zoanthids die if they close up for too long?

Zoanthids can die if they remain closed up for extended periods. This can occur if they are not receiving enough light or stressed due to poor water conditions.

  • How long does it take for zoanthids to open up after closing?

The time it takes for zoanthids to open up after closing can vary depending on the reason for the closure. If it’s due to changes in lighting or water flow, they may open up quickly.

However, it may take longer due to stress or poor water conditions.

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