Acropora coloration - it’s all about the symbionts
A beautiful and rewarding relationship between stony corals and single celled dinoflagellate algae began sometime in the late Triassic over 200 million years ago. Corals formerly relied solely on food capture with their polyps for energy, making them slow growers. Adopting these algae cells called Zooxanthellae allowed corals to utilize both photosynthesis and food capture for faster reef-building growth, which led to the creation and evolution of our now irreplaceable, essential reef ecosystems. Acropora corals first appeared during the Eocene Epoch 37 to 54 million years ago, the beginning of our story.
Acropora coloration is in part due to these symbiotic zooxanthellae, which are plant-like cells, that live within their tissues. The coral produces CO2 and H2O, and the zooxanthellae use these for photosynthesis which in turn feeds the coral polyps. In captivity, dark brown or green Acropora are usually a result of overfeeding the zooxanthellae organic matter with high concentrations of nitrate and phosphate in the tank water. Excessively bright lighting or high temperatures can cause “bleaching,” which is when the zooxanthellae leave the coral, making its tissue nearly transparent and exposing the white skeleton beneath.
It isn’t fully understood why corals produce fluorescent coloration which is visible under actinic lighting in aquariums. UV light found in some LED lights and metal halide lights can bring out the protective fluorescent blue, purple, and pink pigments of shallow-water Acropora. Deeper water corals may produce fluorescent pigments to provide more light to their photosynthetic symbionts.
In addition to Zooxanthellae, there are other symbionts working with Acropora to process nutrients and influence their coloration. Little known fungus-like protists called Thraustochytrids may be responsible for some of the fluorescent proteins found in Acropora and other corals.
It’s almost impossible to identify captive grown Acropora
Identification of captive grown Acropora corals is difficult and in most cases, futile. Even experts who have studied wild Acropora for a lifetime can’t identify a captive Acropora with any certainty. We hobbyists have managed to change the coloration and morphology of our Acropora corals using unnatural conditions, making them nearly unrecognizable. This difficulty is compounded when faced with identifying tiny frags or small colonies. When you see an Acropora identified by scientific name in the hobby, chances are it’s just an educated guess.
The fastest growing Acropora are staghorns
The general consensus among aquarists is that the Green Slimer Acropora yongei is the fastest growing Acropora coral in captivity today. Acropora yongei branches on the Great Barrier Reef have been recorded growing as much as 4.3” during a 15 week summer. That’s more than an inch a month!
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| Image found at [1]. Created by NOAA's Florida Keys National Marine Sanctuary |
Acropora palmata form the largest Acropora colonies
The largest Acropora species is the rare and endangered Acropora palmata from the Caribbean, which typically grows to over 13 feet across, 6.5 feet high, and with a base of 1.3 feet thick.
Acropora corals are easy to frag
Despite being considered one of the most difficult corals to keep in captivity, Acropora corals are surprisingly easy to frag. Thin branches can be carefully broken with your hand, but thicker branches should be cut with a pair of stainless steel “bone” or coral cutters. If you have a coral wet saw, you can frag large colonies quickly.
Small frags can be superglued to plugs. Frags will stick better if the plug is dry. If you want branches to be glued to plugs in an upright position, use plugs with holes drilled in the center that are slightly larger in diameter than the branch. It is also possible to glue a branch horizontally onto a flat plug. Hold the coral for about 30 seconds to set the glue, then carefully place the plug in a frag rack until it cures. Larger frags may need epoxy and superglue to secure them into a hole in live rock. New frags can be traded or sold when the flesh starts to encrust onto the plug.
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| Big R Corals Walt Disney Acropora tenuis photo credit Coralust |
Pedigreed Acro frags can cost more than a month's mortgage
One of the most sought after and expensive Acro frags is the Jason Fox Homewrecker Acro frag retails for $999.99. Many hobbyists were skeptical of its extremely bright coloration in photos, some calling it “Photoshopped.” Those who saw the frags in person at MACNA 2017 in New Orleans will attest to the accuracy of photos of this unbelievable coral.
A modestly expensive and sought after Acropora frag is Mike Biggar’s of Big R Corals Walt Disney Tenuis Acropora frag. A ¾” frag of this blue and pink A. tenuis with yellow polyps retails for $199.99.
Possibly the most expensive Acro frag today is a ½” RR’s Bleeding Avenger Acro which retails on Cornbred’s site for $1999.99. Two. Thousand. For a half inch.
Those on a budget should not discount the value of the $5 brown Acropora frags in the local fish store’s sale bin. With some TLC and proper conditions, that tiny brown frag could regain bright coloration and eventually become a cherished show piece.
Acropora efflorescens - an unusual growth form for an Acropora
There’s more to Acropora than branching
We tend to think of Acropora corals as being “branchy,” but the nearly 150 different species of Acropora have 7 distinct growth forms - tables/plates, staghorn, bushy, massive, bottlebrush, corymbose, and digitate. Some of the most sought after are the deepwater bottlebrush forms like Acropora echinata and the tabling forms like Acropora hyacinthus.
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| Digitate Acropora coral |
Acropora can share and adopt new Zooxanthellae
The Shedd Aquarium in Chicago reported that their Caribbean Acropora corals were adopting Zooxanthellae from the Pacific species. The spawn collected A. palmata frags they housed in the same system with Pacific Acropora started sporting new purple polyps, and DNA test results proved the Zooxanthellae were Pacific in origin. Zookeepers and public aquarists lament the delicate nature and difficulty of A. palmata and cervicornis. Could introducing Pacific Zooxanthellae to Caribbean Acropora make the corals hardier?
Acropora corals are unique, important ecosystems
Acropora are called “reef-building” corals because they form hard skeletons that create important habitats for countless reef fish and invertebrates and nurseries for reef fish and large open water fish fry. One billion people world wide depend on coral reefs economically. A collapse of coral reef ecosystems would have devastating consequences, and its effects would be felt by every living human.
Even small colonies of Acropora corals can host many varieties of species. Tetralia and Trapezia crabs are symbiotic defenders of Acropora corals, feeding on coral mucous and chasing away predators. Polychaete worms and feather dusters, barnacles, crabs, shrimps, commensal shell-less hermit crabs, tiny fish like coral gobies and coral crouchers, copepods, amphipods, flatworms, sponges, and tunicates have all been known to make their homes either inside or underneath the safety of Acropora colonies. Dr. Ronald Shimek reports in the July/August 2018 issue of CORAL magazine that in a single large Acropora hyacinthus colony is likely to house several thousand species of symbionts.
Acropora in the wild need our help
American Acropora palmata and Acropora cervicornis were once some of the most common stony corals in their respective ranges in the Caribbean, Florida keys, Gulf of Mexico and Bahamas, but populations have declined by estimates of up to 95% in the last 30 years. Their rapid decline has been attributed to climate change and pollution, which makes the Acropora susceptible to other threats like disease. Climate change also increases coral damaging hurricane frequency and severity and higher ocean surface temperatures which contribute to coral bleaching events.
Serratia marcescens, a pathogenic, gram negative bacteria commonly found in the gastrointestinal tract and other mucosal surfaces of humans, causes a disease specific to A. palmata called white pox disease. Other diseases that plague A. palmata and A. cervicornis are Black band disease and White band disease which can also decimate entire populations quickly. Black band disease is caused by Cyanobacteria and bacteria infecting coral tissue. White band disease is not well studied, but experts believe there are many types that may be caused by several species of bacteria such as Vibrio harveyi and/or ciliate parasite Philaster lucinda.
All Acropora are listed under CITES appendix II, and many are threatened. Other species of Acropora that are endangered according to the IUCN red list are A. rudis and A. suharsoni.
There is little hope for the recovery of American Acropora species, but organizations like the Coral Restoration Foundation are not ready to give up. Ken Nedimyer and his team frag, grow, and redistribute healthy Elkhorn corals. They use innovative techniques like hanging frags on elevated trees to prevent them from being smothered by sediment or exposed to pathogens. If you’d like to help out, please visit their website www.coralrestoration.org.
Veron Corals of the World Volume 1
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