Coral Health Assessment and Disease Identification

Coral Health Assessment and Disease Identification

The ability to distinguish a healthy coral colony from a diseased or stressed one is a foundational skill for anyone working in reef restoration. Misidentifying a condition—or missing it entirely—can lead to the spread of disease across a restoration site, wasted propagation effort, and harm to wild reefs. This lesson introduces six major coral disease and condition categories, describes how to read tissue signs in the field, and explains the biosecurity practices that protect both corals and restoration workers.

What a Healthy Colony Looks Like

A healthy coral colony displays fully extended polyps during feeding periods, consistent pigmentation across the entire colony surface, and a firm, intact tissue layer overlying the calcium carbonate skeleton. The tissue-skeleton interface is sharp and well-defined. Mucus production is normal and not excessive. Any deviation from these baselines warrants closer inspection and documentation.

Six Key Disease and Condition Categories

1. White Band Disease (WBD)

White Band Disease is characterized by a distinct, progressive band of exposed white skeleton that advances from the base of a colony upward. The boundary between living tissue and bare skeleton is sharp and well-defined. WBD has been closely associated with significant declines in acroporid corals (staghorn and elkhorn) throughout the Caribbean. The causative agents are still under investigation, but bacterial involvement is strongly suspected.

2. Stony Coral Tissue Loss Disease (SCTLD)

First documented in Florida in 2014, SCTLD is one of the most devastating coral diseases on record. It affects more than 20 species of stony coral and is recognized by multiple discrete lesions that appear simultaneously across the colony rather than a single advancing margin. Tissue loss can be rapid—consuming a colony in days to weeks. The lesions leave bright white, clean skeleton. SCTLD is considered an emergency-level event; affected colonies at restoration sites must be reported immediately and may require amoxicillin paste treatment under established protocols.

3. Black Band Disease (BBD)

Black Band Disease is caused by a microbial mat dominated by cyanobacteria along with sulfate-reducing and sulfide-oxidizing bacteria. It appears as a dark, brown-to-black band—often one to several centimeters wide—migrating slowly across the colony surface. The band produces hydrogen sulfide, which kills tissue ahead of its path. Bare white skeleton is left behind. BBD is associated with elevated temperatures and poor water quality.

4. Growth Anomalies (Tumors)

Growth anomalies appear as irregular, knob-like protrusions or skeletal overgrowths on the colony surface. The tissue covering them may appear pale or disorganized. Unlike diseases caused by pathogens, growth anomalies are abnormal tissue or skeletal proliferations. Their causes are not fully understood, but genetic factors and environmental stressors have been implicated. They reduce reproductive output and can impair overall colony fitness.

5. Coral Bleaching

Bleaching occurs when a coral expels its symbiotic zooxanthellae (dinoflagellates of the family Symbiodiniaceae) in response to thermal stress, UV exposure, or other stressors. The colony turns white or pale because the zooxanthellae provide most of the coral's color. A bleached colony is stressed but not yet dead—it can recover if conditions improve. Partial bleaching (sections of a colony bleached while others remain pigmented) is common. Extended or severe bleaching leads to starvation and death.

6. Predator Damage

Crown-of-thorns sea stars (Acanthaster planci), corallivorous fish (such as parrotfish bite scars), and fireworms can all leave characteristic marks. Parrotfish scrape semicircular bite scars that expose skeleton in a scattered, non-progressive pattern. Crown-of-thorns feeding leaves patches of bare white skeleton with sharp edges. Unlike disease lesions, predator damage does not advance on its own once the predator moves on, and tissue margins around the damage are typically healthy.

Using a Field Identification Key

A structured field key walks you through observable characteristics in sequence: (1) Is bare skeleton present? (2) Is the margin sharp or diffuse? (3) Is there a band or mat visible? (4) Are lesions single or multiple? (5) Is the colony uniformly pale or normally pigmented at margins? Work through the key systematically rather than jumping to a conclusion. Photograph every suspect colony from multiple angles before touching it.

Disease Transmission and Biosecurity

Corals can acquire disease through direct contact, waterborne transmission, and—critically—through equipment that moves between colonies without disinfection. This is called iatrogenic spread: disease introduced by the actions of the restoration worker rather than natural transmission. To prevent this:

• Disinfect all tools (cutters, scrapers, measuring tapes) between colonies using a dilute sodium hypochlorite solution (commonly used in field protocols; follow the concentration and contact time specified in your site's standard operating procedure).
• Work healthy colonies first, then move to suspect colonies—never the reverse.
• Change gloves or rinse them thoroughly between colonies.
• Do not touch tissue lesion margins unnecessarily; mechanical disruption can accelerate tissue loss.

Water quality matters: elevated sea surface temperatures, elevated nutrients, and low oxygen conditions all promote disease spread. Document water conditions whenever you record a disease observation.

Safety Considerations

When working around diseased corals, wear appropriate PPE including dive gloves and exposure suits to prevent skin contact with corals and the chemicals used for disinfection. Sodium hypochlorite solutions are corrosive; avoid contact with eyes and skin, and follow your site's SDS guidance. Dispose of disinfectant solutions according to site protocols to avoid introducing chemicals to the reef.

Documentation Is Non-Negotiable

Every observation must be recorded: GPS coordinates or site map position, date, time, species (if identified), percentage of colony affected, condition category, and supporting photographic evidence. Accurate records allow managers to track disease progression, evaluate treatment outcomes, and meet reporting requirements for programs such as Florida's SCTLD response network.