Acan corals of the genus Acanthastrea, commonly called Acans, are an easy addition for many reef tanks to add color and depth. Acans are hardy and a great option for beginners. They enjoy moderate lighting and flow, and aquariums with a small amount of dissolved organics. While they can be confused with other species in the Lobophylliidae family, Acans are identifiable by their larger fleshy polyps, distinct shape, coloration, and skeletal structure.
The scientific classification of corals is always evolving. Recent molecular techniques have shown that many species, previously thought to be related because of morphological (physical) similarities, are only visually similar because of convergent evolution. Convergent evolution is when two species evolve similar adaptations independent of the other because they evolved in similar conditions.
New molecular studies have shown many families of coral are actually polyphyletic. This means members of the family are derived from different ancestors, and are actually less related than we thought.
The Acan corals belong to the genus Acanthastrea, first described in Western science in 1848 by French Zoologists Edwards & Haime. Before 2009, the genus Acanthastrea was in the family Mussidae. However, in 2009, the families Faviidae, Merulinidae, Mussidae and Pectiniidae were revised because many species were found to be polyphyletic. A new coral family called Lobophylliidae was formed from the closely related species of the Indo-Pacific. This new family includes many popular genera of coral including Lobophyllia, Micromussa, and Acanthastrea.
Acanthastrea now includes 13 species. Several species were reclassified in the 1970s and 80s as Micromussa. This article will focus on the Acan corals as a genus.
Acan corals are large polyp stony corals. A colony can have anywhere between one and several dozen polyps. The individual polyps of Acans are noticeably larger than similar corals, like the Micromussa species.
Polyps range in color including purple, red, green, blue, orange, brown, rust, and pale tan and gray.
The polyps form inside their individual corallites which, together, form the colony skeleton. Unlike the Euphyllia species, the skeleton does not form a wall or branching structure, but more of a flat, broad, stone-shaped skeleton.
Acans have a row of short feeding tentacles that line the perimeter of their mouths, inside each polyp. These tentacles are often visually present, and extended to some degree throughout the day. During feeding and at night, these tentacles extend further.
Acans also have sweeper tentacles and can be relatively aggressive. Most reef keepers recommend keeping Acan colonies at least six inches from any other coral, including other Acans. This distance is usually sufficient to prevent corals from stinging each other in territorial disputes.
The several species of Acanthastrea are found throughout the Indo-Pacific region including the coastal waters of the Red Sea, Madagascar, the east coast of Africa, Sri Lanka, southern India, southern Japan, Indonesia, Malaysia, Vietnam, Papua New Guinea, the Solomon Islands, and Australia.
Acanthastrea evolved in depths up to 50 meters. They grow in almost any reef habitat and prefer moderate lighting and flow.
Acanthastrea should be placed in moderate lighting and flow. Acans are less likely to open up fully in bright lighting. A photosynthetically active radiation (PAR) level of 30-50 is considered moderate. A PAR closer to 90 is too intense for most Acans.
An ideal sample lighting scheme might look something like this.
Flow is important for all corals as most are filter feeders. Acans are large polyp stony corals. These types of corals' polyps are fleshy and more sensitive to damage. Keeping flow just high enough to slightly move the polyps without whipping them around, or bashing them against the skeleton is important. You want just enough flow to allow organic material to flow past the polyps continuously.
Because Acans are aggressive and will extend their sweeper tentacles to defend themselves, they should be placed at least six inches from other corals. Be sure to consider the growth of the coral over time when placing them. If you are adding a small Acan frag, leave plenty of room in between corals for them to grow out.
Aquarium Water Parameters
Acanthastrea consume dissolved organics and do well in tanks with around 5 ppm of nitrate. Aquariums that are too clean can lead to stunted growth in Acan corals.
1.022 - 1.025 sg
7.8 - 12
400 - 480 ppm
1300 - 1400 ppm
76 - 78 degrees Fahrenheit
Approx. 1 meter
Hundreds of Years
10 + U.S. Gallons
Acanthastrea, like nearly all corals, have symbiotic algae in their cells called zooxanthellae. These algae photosynthesize within the coral polyps and provide important sugars for the coral’s energy and growth requirements. They are also responsible for most of the color and fluorescence of the coral.
While providing light for the zooxanthellae is important for coral health, corals are still animals, with mouths and stomachs. The additional feeding of animal or plant matter (depending on the type of coral) is crucial to coral health and growth.
Acanthastrea will benefit from target, or spot feeding. This is a method where food such as animal and phyto (plant) plankton are applied directly to open coral polyp mouths. They should be fed at night, when their feeding tentacles (nematocysts) are visible. Acans will also adapt to feed during the day if you are consistent with your feeding times. They can be fed this way multiple times a week. For specifics, check out this article on coral feeding.
Acanthastrea also filters dissolved organics in the water. A small, but measurable amount of nitrate (5ppm), and phosphate (.01 ppm) is important for Acan coral health and growth.
Acclimating New Corals
Acanthastrea are hardy corals and won't easily succumb to some of the common coral ailments. They can still become stressed from high lighting and transport. Acclimating Acans with a dip and lowered light intensity will insure their successful addition to your aquarium.
Dipping corals removes unwanted parasites, bacteria, and protozoans which can cause damage to your new and already established corals.
Acclimating new corals lowers the stress put upon them and decreases the chances of them succumbing to an infection. Acclimation usually includes a 30 - 45 minute drip of your aquarium water into the water your coral was transported in. Check out this article on drip acclimation for how to perform one successfully.
Acclimation also includes keeping flow and light intensity turned down for several days after adding the coral to your aquarium. Slowly bring the intensities back to normal over the course of a week. If your coral reacts poorly to the lighting or flow (polyps recede, coral loses color), then find a new placement for them.
Acans are generally hardy corals. However, some of the problems you can come across are polyp nipping from non-reef safe fish and invertebrates, no dissolved organics, and high lighting and flow.
If your coral does not fully extend or begins to recede and expose its skeleton, or starts to bleach (loose color), these are signs of stress. You should take steps to determine which of the above stimuli are causing the stress. Start by measuring your nutrient levels and determine if your organics are too low. Next, you can change the flow or lighting by adjusting equipment or moving the coral to a new placement. Try to only change one variable at a time. If there is a change in behavior, you will know which variable caused it.
Acanthastrea is commonly confused with the Lobophyllia, Micromussa, Favia, and Moseleya corals. Scientists even grouped some of these generas’ species together before molecular testing was possible.
Some specimens just look too similar to be 100% sure, especially when their polyps are not fully extended. It is important to know which genus you’re buying because the care requirements are different and you don’t want to put a coral that requires pristine water in an aquarium with elevated organics. However, as long as you're confident in the genus your buying, the care requirements of species within that genus are usually fairly similar, and specific taxonomic identification can occur later, when the coral is plump and happy in your home tank.
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IUCN Red List. Acanthastrea - Genus. https://www.iucnredlist.org/search?taxonomies=131459&searchType=species. Accessed 6/27/22.
Corals of the World. Acanthastrea echinata. http://www.coralsoftheworld.org/species_factsheets/species_factsheet_summary/acanthastrea-echinata/. Accessed 6/27/22
WRMS. world Register of Marine Species. Acanthastrea. https://www.marinespecies.org/aphia.php?p=taxdetails&id=206405#notes. Accessed 6/27/22