Elkhorn Coral

Figure 3: Spawning of a colony of elk horn coral. Source: National Park Service

Ecology: Like most stony corals, elkhorn coral possess a unique mutualistic relationship between polyps and zooxanthellae algae. Individual polyps act as suspension feeders, utilizing nematocysts to catch food as it passes by. Food captured by polyps only accounts for a small percentage of the energy intake of the coral. The algae contained within the tissues of the coral are photosynthetic and supplies the vast majority of the food for the organism. They pass many of the products of photosynthesis on to the polyps. In return the algae receives shelter, carbon dioxide, and animal waste products.

Elkhorn coral is one of two species of primary reef building corals that are found in the Caribbean. A primary reef builder is a species of coral that lays the framework for coral reefs with their ability to colonize rough, shallow underwater environments. With its fast growth rate, elkhorn coral creates an environment for fish, crustaceans, and countless other organisms. Other species of coral, stony and soft, are able to become integrated once primary reef builders such as elkhorn coral have become established.

Reef development relies on ideal conditions for corals to flourish. Critical environmental factors such as water chemistry, movement, temperature, salinity, clarity, and the substrate dictates the ability for corals to survive. Water temperatures must remain between eighty five and seventy degrees Fahrenheit. Water clarity is extremely important, as the zooxanthellae algae require light to passage for photosynthesis. Turbulence is also important as it supplies planktonic food and ensure a high level of dissolved oxygen.

Figure 4: Macro photo of coral polyps with arms extended. Source: Florida Keys National Marine Sanctuary.

Threats to Elkhorn Coral:Even though elkhorn corals grow quickly and can withstand great forces, they are very sensitive to change within their environment. Fluctuations in underwater conditions have proven fatal for elkhorn coral. Increased sedimentation is one of many killers. Construction, urbanization, and increased boat traffic has caused a reduction in water clarity. Clarity is vital for corals, as sunlight must penetrate the water for the zooxanthellae algae to carry out photosynthesis. Ocean acidification also has a devastating effect on stony corals. Lower pH impacts their calcification rates. This weakens elkhorn corals skeletons and can cause them to become eroded away. Another big factor affecting elkhorn coral is the increase in water temperature. If the water is too warm, the coral will expel its algae in a process known as bleaching. The bleached coral does not die immediately, as the polyps will remain intact. In some rare instances, the algae can return to the coral. If they do not, the polyps will starve, and the coral will die. Increased water temperatures also acts as a vector for coral disease. White band, black band, and white pox disease outbreaks have proven to be the most devastating. These diseases are responsible for wiping out seventy to ninety percent of elkhorn coral in the Caribbean. Major outbreaks of coral disease in the Caribbean weren't identified until the mid nineties and have killed off corals at an exponential rate.

Figure 5: Healthy elkhorn reef devastated in just a couple years.

Source: Biosphere Foundation.

White Pox Disease: White pox disease has been the most destructive force in the death of elkhorn coral. The disease is caused by a common bacteria, Serratia marcescens, found within the intestines of humans. This is the first known instance in which a bacteria species associated with the human gut has been shown to act as a pathogen to marine invertebrates. It is believed Serratia marcescens was introduced by leaking septic tanks, but the exact source is unknown. Increased water temperatures allow this bacterial species to thrive. Periods of higher water temperatures correspond with instances of disease outbreak. The oldest colonies of elkhorn coral have been selectively compromised, indicating that the exposure to this bacteria is a new phenomenon. White pox begins by causing small white lesions of tissue necrosis (figure 8, left image). These spots may appear to be very small at first, but can expand very quickly (figure 7.) The tissue necrosis due to the bacteria spreads at a rate of 2.5 cm² to 10.5 cm² per day. The disease has been shown to be very contagious, and quickley spreads to neighboring colonies of elkhorn coral. Figure 6 displays the rapid decrease in the cover of elkhorn corals within different reef systems of the Caribbean.

Figure 7: Elkhorn coral severely affected by white pox disease located in the Molasses reef. Source: Wikipedia

Figure 6: Rapid decrease in percent cover of Acropora palmata

in established reefs. Source: Proceedings of the National Academy

of Science of the United States of America.