Corals are marine animals composed of thousands of tiny creatures called polyps. Like other animals, they are susceptible to infections from pathogens like bacteria, fungi, and viruses. These illnesses appear as changes in the coral’s color or damage to its structure, often involving tissue loss. Understanding the visual signs of these diseases is a primary step for aquarium hobbyists to identify health issues in their tanks.
Visual Guide to Common Coral Diseases
A frequent and fast-acting ailment is Brown Jelly Disease, which presents as a light brown, gelatinous mass that consumes the coral’s tissue. This jelly-like substance is composed of bacteria and protozoans, often has a distinct rotten smell, and can spread rapidly. It is common in large polyp stony (LPS) corals, such as those in the Euphyllia genus.
Black Band Disease is characterized by a dark, sometimes reddish-brown, mat of microbes that moves across the coral’s surface. This band separates healthy tissue from the exposed white skeleton left in its wake. The microbes produce sulfur-rich toxins that are lethal to the coral tissue, causing its death as the band advances.
White Plague is identified by the rapid loss of coral tissue that exposes the white skeleton, without a distinct band present. The tissue loss begins at the base of the coral and progresses upwards. Several types of this disease exist, with some starting at the branch tips. The pathogen is believed to be bacterial, but a specific agent has not been definitively identified.
Rapid Tissue Necrosis (RTN) is characterized by the extremely fast peeling of coral tissue from its skeleton. This condition can cause a coral colony to perish in a matter of hours or days. RTN is associated with significant stress events and can be caused by both pathogenic microorganisms and physical injury.
Primary Causes of Coral Infections
Coral diseases are the result of environmental stressors that compromise the animal’s natural defenses. Pathogens that may be harmless in a stable environment can become infectious when a coral is weakened.
Poor water quality is a significant contributor to coral stress. Elevated levels of nitrates and phosphates from waste and overfeeding in an aquarium can fuel the growth of harmful bacteria and algae. These conditions provide an ideal environment for pathogens to thrive.
Sudden changes in the aquarium’s environment can also weaken corals. Fluctuations in temperature, salinity, and lighting can shock a coral’s system, impairing its physiological functions and ability to fight off infection. Corals thrive in consistency, and abrupt shifts from their ideal conditions make them more susceptible to pathogens.
Physical damage creates direct entry points for infectious agents. Handling corals, rock falls, or bites from fish can cause cuts and abrasions on the coral’s surface tissue. These breaks in their protective layer allow bacteria and other microorganisms to invade the coral’s interior, leading to infections.
Management and Treatment Strategies for Aquariums
When an infection is identified, several strategies can be used for management and treatment:
- Isolate the affected coral in a separate quarantine tank to prevent the spread of pathogens. A quarantine period is also a preventative measure for any new coral to observe for latent diseases before introducing it to an established aquarium.
- Improve water parameters to address environmental stress. Regular monitoring and maintenance of temperature, salinity, nitrate, and phosphate levels help restore a stable environment where corals can better utilize their own immune responses.
- Use therapeutic coral dips for direct treatment. This involves briefly immersing the infected coral in a solution containing iodine or other coral-safe antiseptic compounds to clean the surface and reduce the infection load on the animal.
- In severe cases, fragging may be the only option to save a part of the colony. This process involves carefully cutting away diseased portions of the coral, and the remaining healthy fragments can be mounted to grow into new colonies.