Staghorn coral

The Staghorn coral (Acropora cervicornis) is a branching, stony coral, within the Order Scleractinia. It is characterized by thick, upright branches which can grow in excess of 2 meters (6.5 ft) in height and resemble the antlers of a stag, hence the name, Staghorn. It grows within various areas of a reef but is most commonly found within shallow fore and back reefs, as well as patch reefs, where water depths rarely exceed 20 meters (65 ft). Staghorn corals can exhibit very fast growth, adding up to 5 cm (~2 in) in new skeleton for every 1 cm of existing skeleton each year, making them one of the fastest growing fringe coral species in the Western Atlantic. Due to this fast growth, Acropora cervicornis, serve as one of the most important reef building corals, functioning as marine nurseries for juvenile fish, buffer zones for erosion and storms, and center points of biodiversity in the Western Atlantic. Up until the late 1970s, much of the fore reef zones within the Atlantic around the coasts of Southern Florida and the Caribbean Islands were covered with vast, dense colonies of Staghorn coral consisting largely of single species stands; however, a combination of white band disease and various anthropogenic factors have reduced this coral coverage by over 95% in some areas. As of 2006, Staghorn coral is listed as Critically Endangered under the International Union for Conservation of Nature and are federally designated as a threatened species under the Endangered Species Act of 1973.

Staghorn coral is found throughout the Western Atlantic Ocean, from the Florida Keys and the Bahamas, to the coasts of the various Caribbean islands. It occurs in the western Gulf of Mexico, but is absent from U. S. waters in the Gulf of Mexico, as well as Bermuda and the west coast of South America; the northern limit of this species is Palm Beach County, Florida, where only small populations have been documented

Staghorn coral is most commonly found within 20 meters (65 ft) of the water's surface, in clear, non turbid environments consistent with fore, back and patch reefs in the Western Atlantic Ocean. In this environment, water temperatures typically range anywhere from 66 to 80 degrees Fahrenheit (19 27 degrees Celsius) with salinity ranging from 33 to 37 ppt (parts per thousand).

The skeleton of the Staghorn coral is made up of a specific type of calcium carbonate known as aragonite. This substance is secreted slowly by specialized calicoblastic cells, positioned in the layer directly above the coral skeleton. These branches grow in close proximity to one another, and sometimes resemble the antlers of a stag, hence its common name, Staghorn. The colour of stony corals is largely dependent on the symbiotic zooxanthellae that lives within the corals tissue, and varies widely depending on the species and environment in which it lives. When it comes to the Staghorn coral specifically, tissue color can vary from light brown and beige to green, blue or purple. This symbiotic relationship has resulted in the incorporation of specialized dinoflagellates into the tissue of various coral species, including those of the Acropora genus like Acropora cervicornis (Staghorn coral). At the same time, the coral has become dependent on the dinoflagellates for up to 90% of their total nutritional requirements, which includes various lipids, amino acids, and sugars as well as a source of oxygenation and waste removal.

Like the vast majority of coral species, Staghorn coral are both autotrophic and heterotrophic, meaning they have two main methods of acquiring the nutrients needed to sustain their growth and continued survival. The first and most important method is autotrophic and comes from the photosynthetic symbiotic zooxanthellae that inhabit the tissue of the Staghorn. This method accounts for up to 90% of their total nutritional requirements. This is the most common during night when photosynthesis is restricted and micro organisms like zooplankton are feeding. Using their long feeding tentacles, polyps are able to catch passing food, stunning it with their stinging nematocysts to prevent escape, eventually orientating it towards the mouth at the center for digestion. The contribution of nutrients gained from heterotrophy is poorly understood, however, it is thought that in stony corals, of which Staghorn is one, that it may account for anywhere from 0 to 66% of carbon fixation. The second, called extra tentacular budding, is the formation of new polyps from tissue where none already exist, this includes the space around or between existing polyps. In Staghorn corals, much like other stony corals, this appears to be the most common mode of reproduction and can result in a single coral giving rise to many new colonies, or sometimes entirely new coral reefs. Although they technically have the ability to self fertilize, research suggests that single colonies are either completely or partially self sterile, therefore, for successful reproduction of coral planula, two distinct colonies or parents are needed. Release of eggs and sperm are synchronized in a single event called broadcasting. This increases the success of fertilization and production of fertile offspring. Staghorn coral spawning is typically restricted to late summer, in the months of July and August and occurs several days following a full moon. The mechanism by which these corals choose a spawn day is still unknown, however, it is almost certainly influenced by a multitude of factors including water temperature, the lunar cycle, wave action and tidal periods. These include basic sensory abilities to avoid damaging UV radiation, as well as the ability to detect tides, water pressure, and even hear and smell environmental phenomena on nearby coral reefs. The effects of overfishing are wide ranging, and can lead to an overgrowth of sponges by as much as 25% as their predators are systematically fished out and removed from the ecosystem. As sponges grow uninhibited, they outcompete, smother and prevent the settlement of coral planula as they become the dominant habitat forming organisms on the reef. This disease is host specific, meaning it only affects certain species of coral, the Staghorn among them. First observed in 1979, this disease is often credited with the massive population decline of shallow reef building corals, like the Staghorn and Elkhorn, contributing to as much as 95% of their decline. in the past 30 years. White band disease is the result of potentially many very infectious bacterial pathogens, including some from the genus Vibrio, and can be transmitted via direct contact between coral colonies, through the water column into damaged tissues, and even by certain species of invertebrates, like the coral eating snail, Coralliophila abbreviata. In Staghorn coral, tissue necrosis can spread up to 4 cm (40 mm) on branches daily, resulting in a total loss of tissue coverage of about 21 cm² (210 mm²) per day. Once infected, a Staghorn colony loses on average 84% of its total tissue coverage, going from around 96% before infection, to about 12% after infection completion, with a mortality rate of approximately 28%. This increase in temperature has accelerated over the past decade, resulting in approximately 4.5 times greater ocean warming than the previous 100 years. These events occur where the photosynthetic dinoflagellates (zooxanthellae) living symbiotically in the coral tissues are expelled, leaving the coral completely white and without its main nutrient source. If temperatures return to normal levels, the coral is able to reintegrate the zooxanthellae back into its tissues, however, as ocean warming events become more common, as a result of climate change, bleached corals are less likely to recover completely. On December 18, 1997, both species were removed as candidates for listing due to the unavailability of evidence on their biological status or threats. On June 23, 2004, NOAA Fisheries and the National Marine Fisheries Service (NMFS) found that listing these species may be warranted and initiated a formal review of their biological status by convening the Atlantic Acropora Biological Review Team to summarize the best scientific and commercial data available in the status review report. They also found that fused Staghorn coral doesn't meet the criteria for listing, as it is identified as a hybrid and therefore doesn't fall under the ESA's definition of a distinct species. However, by September 2014, they determined that both coral species would continue to be listed as threatened.

On March 6, 2015, the National Marine Fisheries Service (NMFS) alongside the National Oceanic and Atmospheric Administration (NOAA) published a recovery plan for both the Staghorn and Elkhorn coral species. The main purpose of this plan was to rebuild the population and ensure its long term viability with the ultimate goal of removal from the Endangered Species Act of 1973 (ESA). The goal of this designation was to address the key conservation objective when it came to the Staghorn coral, that is, to facilitate an increase in reproduction, both sexually and asexually. Staghorn coral colonies were grown and cultivated in an offshore nursery, fixed on discs or hung from fishing lines, until they achieved 30 cm in diameter before being planted in natural coral reefs off the coast of Florida. Between 2007 and 2013, thousands of Staghorn colonies were planted, and survivorship remained high in the first few years, ranging between 23% to 72%. However, after five years, a decline in coral survivorship became evident, with survival rates of planted Staghorn colonies falling below 10%.