Cluster Coral

Milli, Millepora Acropora, Cluster Acropora

Family: Acroporidae Picture of a Cluster Coral, Acpropora millepora, also known as Milli, Millepora Acropora, and Cluster AcroporaAcropora milleporaPhoto © Animal-World: Courtesy Greg Rothschild

   The Cluster Coral, highlighted by a rather 'scaly' look, is a gorgeous Acro but more difficult to keep!

   The colors of A. millepora are very attractive. They are most commonly found in green with orange tips, but can also be bright salmon-pink, blue, green, pale green, bright orange, or pink. The Cluster Coral is a great addition to any Acro community. They are a more difficult stony coral to keep however, so are not suggested for the beginner.

   The Cluster Coral Acropora millepora is popularly known as Milli, or the Millepora Acropora. This Acropora comes from the cluster shaped grouping and forms a nice, neat appearance. Their corallites have no upper wall, but a prominent lower wall, and rounded or flared lips. This gives them the letter "C" shape. Their short branches are surrounded with these corallites giving them a scale like appearance.

   Acropora corals are some of the largest, most contributing corals for reef formations in the world. In fact, between the Acropora and Montipora corals, they make up one-third of all reef building coral species. In the wild they are the most tolerant of water temperatures, salinity changes, water movement, and lighting, but in captivity they can prove to be very difficult to keep. In the ocean, they are the first to arrive at a reef and spread quickly. Other corals that arrive later, then tend to move in.

   The A. millepora have been propagated in captivity thus helping to spare wild populations in the world's reefs. Buying captive propagated A. millepora helps the aquarist, since they are a little more hardy than their wild siblings, but they can still succumb to diseases typical to Acros.


  • Kingdom: Animalia
  • Phylum: Cnidaria
  • Class: Anthozoa
  • Order: Scleractinia
  • Family: Acroporidae
  • Genus: Acropora
  • Species: millepora
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Scientific name    Family: Acroporidae
   Species: Acropora millepora

Distribution / Background    Acropora Coral Information: The Cluster Coral Acropora millepora was described by Ehrenberg in 1834. They are in what is called the aspera group of Acroporas. This group also includes A. aspera and A pulchra, each of which can have different shapes. But these all have corallites with no upper wall, but a prominent lower wall. These corallites have rounded or flared lips, resembling the letter "C", with the open side being the part that lacks upper wall. Some of their growth forms have been included in common names such as Millepora Acropora, Milli, Cluster Acropora, and Plate Acropora.

   An interesting factoid is a study done on the eggs and larvae of A. millepora and the Finger Staghorn Coral A. humilus. They are the preferred food for planktivorous fish like angelfishes. The angelfish enjoyed pellets made with these coral's eggs, but when offered eggs or larvae made from Pachyseris speciosa (Lettuce Coral) the fish taste and then spit them out. This shows that not all coral eggs and larvae are palatable. The rates of survival vary from each coral species and with the types of predatorial fish in the area.

   Where Acropora Corals Are Found: A. millepora are found from Sri Lanka and Thailand to the Tonga and the Marshall Islands; all around Australia except for Lord Howe Island, then south to the Houtman Abrolhos Islands.

   Acropora Coral Habitat: The A. millepora are found shallow waters, reef flats, lagoons and upper reef slopes. In the wild, Acropora spp. corals are found across the reef in various locations from turbid waters (with sediment or foreign particles stirred up or suspended in the water) to those with strong waves and high currents; from areas where there is little light to being fully exposed to the sun (and the air) at low tide.

Status    The Acropora millepora is on the IUCN Red List for Endangered Species as Near Threatened (NT).

Description    What do Acropora Corals look like: All Acropora Spp. corals have very porous and lightweight skeletons. They have short branches. The corallites surrounding the branches having a scale like appearance. They have corallites with no upper wall, but a prominent lower wall and rounded or flared lips, giving the letter "C" shape. The corallites at the tip of the branch are similar. Acropora millepora colors can be bright salmon-pink, blue, green, pale green, bright orange or pink. They are most commonly found in green with orange tips.

   Acropora corals are characterized by fast growing terminal (or axial) polyps at the tips of the branches. The polyps at the tips secrete this corallite, constantly forming new growth in a cluster shape. These terminal polyps do not have zooxanthellae. They grow quickly because the terminal polyps are fed by the rest of the colony. This allows Acropora to outgrow other corals on the reef.

   Acropora Coral Life Cycles: The Cluster Coral A. millepora grow from 12 to 24" (30 - 61 cm), but can get up to 6.5 feet (2 m). They reach sexual maturity within 3 to 5 years, with a branch diameter of 1.5-2.75" (4-7 cm). They can live 4 to 7 years.

Difficulty of Care    Acropora Coral Care: Acroporas are among the more difficult corals to keep, which is surprising as their natural habitats have a wide array of conditions. In captivity they require stable tank conditions, sudden changes may result in death. They are sensitive to temperature changes, sedimentation, chemical and other environmental stresses. They will stress very easily if the light is too low, or the water movement is not sufficient.

   Acropora need to be carefully acclimated to their new homes. They need to be placed in their permanent position within a tank after acclimation. It takes about 5 to 6 months for the A. millepora to regain their normal growth rates after being added to an aquarium. Moving Acropora from place to place will stress them, and possibly cause death.

   At times a healthy system has an additional Acro added and all the others die from polyp bail out or what people refer to as RTD (rapid tissue degeneration) and a whole tank of acros can be wiped out within a matter of hours. They are unsure why this happens, but it would probably be a good idea to have all the Acros you are going to want, and put them together

Foods / Feeding    Acropora Coral Feeding: In the wild, Acropora corals have developed several feeding strategies. Through a symbiotic relationship with a marine algae, known as zooxanthellae, they receive the majority of their nutrients. They also capture planktonic organisms and microscopic food particles from the water column and can absorb dissolved organic matter.

   In captivity, feeding zooplankton once a week is the preferred choice. Copepods, Artemia, and nauplii are too large for them to ingest. But new forms of prey are being developed including invertebrate larvae and new strains of rotifers. Many feel that Acros in captivity need to be in a tank that has dissolved or solid organic matter to survive and thrive. Without this, they may seem fine for a while, but over a few months, without any visible indication, they may end up dead because of starvation. Signs of lack of food would be no new growth, polyps extending and some tissue recession.

Aquarium Care    Excellent and stable tank conditions are required to keep all Acropora spp. corals. Doing water changes of 10% every 2 weeks is needed, although it is suggested that doing 5% water changes once a week will bring about amazing results. Keep the nitrate levels low. Tanks should offer high calcium levels and strontium should be added.

   Suggested levels for Acropora species are:

  • Calcium:  400 to 450 ppm (closer to 450). If the Acro does not have enough calcium, it will not grow. There will be no tissue recession, but the polyps will be extended.
  • Alkalinity:  3.2 TO 4.5 MEQ/L (8 to 10 dKh - 10 is recommended)
  • Phosphates:  0, zero. Phosphates are the worst of all and all corals hate them.
  • Magnesium:  1350-1500. Magnesium makes calcium available, so if your calcium is low, check your magnesium levels before adding any more calcium.
  • Strontium:  10

Aquarium Parameters

Quick Reference Chart
Lighting: Prefers Low Lighting Levels Prefers Medium Lighting Levels Prefers High Lighting Levels
Water Flow: Prefers Low Water Flow Levels Prefers Medium Water Flow Levels Prefers High Water Flow Levels
Temperament: Peaceful Temperament Semi-Aggressive Temperament Aggressive Temperament

   A typical live rock/reef environment is what is needed for your Cluster Coral, along with some fish for organic matter production. A mature tank (well over a year old) is advised to increase the chance of successfully keeping Acropora.

  • Minimum Tank Size / Length: 100 gallons (380 L) or larger
  • Marine Lighting: High, metal halides are suggested
  • Temperature: 72° - 78° F (22.2° - 25.5° C)
  • Salinity / Specific Gravity: 1.025
  • Water Movement: Strong and surge-like. They prefer a strong, random, mixing-type current, therefore a rotating powerhead or wavemaker setup is recommended. Tunze stream pumps, though they have a comparatively high initial purchase price, are popular with Acro keepers.
  • Water Region: Top of middle of the aquarium

Compatibility and Social Behaviors    The Acropora corals are peaceful, but watch out for crabs. Many experienced aquarists do not believe any crab should be kept in a closed system with Acros. Crabs are opportunistic predators, with the exception some of the symbiotic crabs like commensal crabs, and gall crabs.

   Acroporas are best kept in a small polyp stony (SPS) tank with only other SPS corals. They can send out digestive strands called 'acontia' that are actually used to digest neighboring corals, so keep an eye out for any problems. The exception to this would be if your system has an incredible filtration system and the Acros are at least 10" away from other corals such as zoanthids, large polyp stony (LPS) corals, and other invertebrates.

   Do not keep soft leather corals with your Acropora species as they are aggressive and release terpins that will eventually kill your expensive investment. Leather corals are dangerous to Acros, even in the best filtered aquariums. Some large polyp stony (LPS) corals can stretch out their tentacles and kill your Acros too, so caution is needed if you have a mixed reef.

Sex - Sexual differences    No sexual difference in appearance is known.

Breeding and Reproduction   Acroporas grow rapidly and are considered to be among the most rapid growing of the stony corals. They will reach sexual maturity within 3 to 5 years, with a branch diameter of 1.5 - 2.75" (4 - 7 cm). They fragment easily and the fragments can form new colonies. This makes them ideal candidates for captive breeding/propagation.

   The Acropora Spp. are male and female and can reproduce both sexually and asexually. In the wild they reproduce sexually by releasing eggs and sperm at the same time, resulting in a fertilized egg which then forms into a free-swimming planula larva. Eventually the planula larvae settles onto the substrate, becoming plankters. This then forms a tiny polyp which begins to excrete calcium carbonate and develops into a coral. Planula larvae are extremely vulnerable to predation, and very few survive. In the wild Acros reproduce asexually as well. The polyps at the tips of branching species secrete corallite around itself, forming longer branches. Acros also spread from breakage due to storms and fragmentation.

   Propagation is rather simple for Acropora corals. First you need to choose a healthy coral that is not showing any signs of distress. Then, simply cut a branch at least 2" long and glue the frag to a plug or rock. You can use the 2-part epoxy or underwater putties. A little tip, don't glue frags upright since they will grow faster on their sides.The slime that the coral will exude should not come in contact with any other corals and gloves are suggested. Give the frag ample water flow.

Potential Problems    Signs of stress are bleaching and receding and they will diminish and die rapidly. Transporting Acropora spp. corals can be a problem because of their shapes, the easily broken outer edges of the coral skeleton, and the ease with which they become stressed. They are also susceptible to almost every coral disease including white-band disease, black-band disease, necrosis, etc. Acropora is particularly susceptible to rapid tissue necrosis (RTN), especially specimens from the wild.

   For more information on disease see Keeping Acropora Corals: SPS Coral Ailments

   There are many animals that will prey on Acropora spp. corals. Some Acropora have symbiotic relationships with crabs and shrimps that may help them against predators. Other hitchhikers may actually be predators so careful observation must be done. Chocolate sea stars and others from the same genus are known to eat Acropora.

Availability    Acropora Corals for Sale: The Cluster Coral A. millepora are very easy to find at pet shops and on line. Online they can run about $36.00 USD or more depending on size and color.

   Many Acropora spp. corals have been propagated by fragmentation. It is not difficult to find captive-bred colonies of Acropora corals for sale or trade. Captive-bred corals may tolerate less intense lighting and water movement as well as possibly being easier to care for than those taken from the wild.

References

Author: Clarice Brough, CRS; Carrie McBirney
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