Abstract
Samples from eight species of corals (Colpophyllia natans, Dendrogyra cylindrus, Diploria labyrinthiformis, Meandrina meandrites, Montastraea cavernosa, Orbicella faveolata, Pseudodiploria strigosa, and Siderastrea siderea) that exhibited gross clinical signs of acute, subacute, or chronic tissue loss attributed to stony coral tissue loss disease (SCTLD) were collected from the Florida Reef Tract during 2016–2018 and examined histopathologically. The hallmark microscopic lesion seen in all eight species was focal to multifocal lytic necrosis (LN) originating in the gastrodermis of the basal body wall (BBW) and extending to the calicodermis, with more advanced lesions involving the surface body wall. This was accompanied by other degenerative changes in host cells such as mucocyte hypertrophy, degradation and fragmentation of gastrodermal architecture, and disintegration of the mesoglea. Zooxanthellae manifested various changes including necrosis (cytoplasmic hypereosinophilia, pyknosis); peripheral nuclear chromatin condensation; cytoplasmic vacuolation accompanied by deformation, swelling, or atrophy; swollen accumulation bodies; prominent pyrenoids; and degraded chloroplasts. Polyhedral intracytoplasmic eosinophilic periodic acid–Schiff-positive crystalline inclusion bodies (∼1–10 μm in length) were seen only in M. cavernosa and P. strigosa BBW gastrodermis in or adjacent to active lesions and some unaffected areas (without surface lesions) of diseased colonies. Coccoidlike or coccobacilloidlike structures (Gram-neutral) reminiscent of microorganisms were occasionally associated with LN lesions or seen in apparently healthy tissue of diseased colonies along with various parasites and other bacteria all considered likely secondary colonizers. Of the 82 samples showing gross lesions of SCTLD, 71 (87%) were confirmed histologically to have LN. Collectively, pathology indicates that SCTLD is the result of a disruption of host–symbiont physiology with lesions originating in the BBW leading to detachment and sloughing of tissues from the skeleton. Future investigations could focus on identifying the cause and pathogenesis of this process.
Highlights
The Florida Reef Tract spans some 577 km from the Dry Tortugas to Martin County in the north
stony coral tissue loss disease (SCTLD) comprised varying shapes and sizes of amorphous distinct areas of tissue loss revealing (1) wide areas (∼50 mm or more width) of recent necrosis indicated by intact bare white skeleton and absent turf algal overgrowth, (2) less wide (∼10–50 mm width) but still prominent areas of recent necrosis, or (3) narrow (∼10 mm or less width) to almost unnoticeable areas of recent necrosis apposed to skeleton overgrown with turf algae
The histopathology findings presented here indicate that SCTLD is a bottom-up process starting with pathological changes in basal body wall (BBW) and surface body wall (SBW) zooxanthellae with lesions first appearing in the gastrodermis of the BBW
Summary
The Florida Reef Tract spans some 577 km from the Dry Tortugas (the southernmost Florida Keys) to Martin County in the north. With a diverse assemblage of more than 6,000 marine species, the Florida Reef Tract attracts more than 30 million visitors annually, with fishing, diving, and boating expenditures generating more than $6.3 billion to the State.1 This ecosystem, has suffered significant losses of coral and associated organisms, starting with an unexplained die-off in Panama of the dominant grazing long-spined sea urchin Diadema antillarum in which more than 90% of the population throughout the Caribbean was wiped out within a year (1983–1984), and populations have yet to recover (Lessios, 1988, 2016). A majority of the dominant reef coral Acropora (A. palmata, A. cervicornis) declined by about 80%, mainly due to tissue loss diseases (TLDs), leading to marked reductions in the three-dimensional complexity and biodiversity of Florida’s coral reefs (Aronson and Precht, 2001) Since this region has experienced a plethora of coral diseases (Mueller et al, 2001; Porter et al, 2001). The demise of acroporids, continuing intermittent chronic disease outbreaks, and coral loss due to bleaching have added to the concern over the stability and health of Florida’s coral reef ecosystem (Williams and Miller, 2012) and has highlighted the previously underestimated role of disease (Miller et al, 2014)
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