An Overview Of The Cluster Coral (Acropora millepora)

One of the most spectacular species of coral when healthy and well established, A. millepora is always worth the wait.

Orange Skunk Clownfish. ( Acropora millepora  Via MDC Seamarc Maldives/Wikipedia
Orange Skunk Clownfish. ( Acropora millepora Via MDC Seamarc Maldives/Wikipedia

It is not hard to understand the popularity of small-polyped stony (SPS) corals. Beautiful branching growth forms and vibrant colors are the norm for Acropora spp. Among one of the most common, yet highly prized, species among aquarists is A. millepora. Though it has been described by some as being difficult to keep in captivity, its care is rather straightforward and its growth rapid if proper attention is paid to lighting, water movement, water quality and nutrition.


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Some divide the species into several subspecies. It is in the Aspera group of acroporids, and is believed to be most closely related to A. aspera and A. pulchra, though it is similar in appearance to A. convexa and A. prostrata. Commonly referred to as cluster coral (or milli, plate acro, etc.), it has a very recognizable growth form and corallite shape. It is a short-branched, true stony coral. Its colonies are corymbose cushions with uniformly sized individual polyps enclosed in a tightly compacted corallite. Corallites lack an upper wall while having a pronounced lower wall to form a small cup around the often “hairy” polyp. Like others in its genus, it has extremely rapidly growing terminal polyps on the tips of its branches. Terminal polyps are mainly azooxanthellate, but are nevertheless quite prolific due to sharing of nutrients by the rest of the colony. Colonies are usually richly pigmented, with green, pink, purple, blue varieties are known, often with orange tips. Variation in coloration and colony shape arises from differences in depth and water velocity as well as geographic region.

A. millepora is widely distributed across the tropics in brightly sunlit, shallow reefs such as upper reef slopes, lagoons and especially reef flats. It may be found in both intertidal and subtidal areas at depths of 2-12 meters. It occurs throughout the southeast Atlantic Ocean, the east and west Indian Ocean, and the northwest, southwest and west central Pacific Ocean. It is fairly common in Indonesia as well as around Australian reefs, with the notable exception of the Lord Howe Island group. While it is known for having a high tolerance for turbidity, it cannot for long survive smothering by sedimentation. Heat stress and bleaching are particularly strong stressors for the species and require long periods of recovery for individuals not killed outright. Where conditions are acceptable and stable, it can dominate vast areas of reef habitat.

As it is in the wild, so it is in captivity that this coral can grow aggressively, but only where specific environmental conditions are created. By all reports, it demands as much patience as attention to the environment itself. All changes, including acclimation to new water or lighting, should take place very slowly and in steps. Even under the best of circumstances, it may take several months for this species to regain its footing and reach its maximum growth rate after being introduced to a new aquarium system. Unless necessary, do not move an established colony, even within the same tank.

A. millepora might tolerate being placed in the middle of the tank, but is best situated at the very top with plenty of room to grow. A mature system is best. Maintain a temperature of 72°-78°F. Do not keep them with large numbers of soft corals, which may release compounds that are harmful to acroporids. As they require very intense illumination, use a powerful light with a high PAR rating. Water flow should likewise be very strong. Use of multiple pumps on timers is ideal, as it tends to prevent the accumulation of detritus between the branches. Frequent water changes are said to do wonders for cluster corals, and simply cannot be overdone, as excellent water quality must be maintained at all times, with special attention to calcium and alkalinity. Many swear by supplemental addition of strontium and amino acids. Nighttime offerings of small, plankton and marine snow also accelerates growth.

Acquiring captive propagated specimens (which are far more adaptive) will almost certainly help to get new colonies off to a great start. With time and a properly maintained aquarium system, one can propagate massive and richly colored cluster coral colonies. One of the most spectacular species of coral when healthy and well established, A. millepora is always more than worth the wait.

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