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December 8, 2023

Case History: Multifocal, progressive tissue loss was seen in several Elkhorn coral colonies within one reef off the coast of Sousa in the Dominican Republic in the summer and fall of 2021 by Aldo Croquer (The Nature Conservancy) and Someira Zambrano (Red Arrecifal Dominicana).

Samples from early lesions were taken in July, and again in October as the tissue loss progressed. Other species of coral in the area were affected by stony coral tissue loss disease (SCTLD). While Acropora spp. have not been reported previously as affected by SCTLD, the multifocal progressive pattern of tissue loss was concerning as it resembled what is seen in other affected species (Landsberg and others, 2020).

Gross Findings: Multifocal, randomly distributed areas of tissue loss are present, with newly exposed bare white skeleton indicating recent tissue loss and yellowish to greenish turf algae growth on exposed skeleton indicating older lesions (Figure 1A). Paler areas within the live tissue give a “moth eaten” appearance to the remaining colony. Tissue is transparent post-fixation, and lesions are not grossly visible in submitted samples (Fig 1B).

Photographs of elkhorn coral show white portions where tissue loss has occurred.
Figure 1. Photographs from an elkhorn coral colony (Acropora palmata) losing tissue on a reef in the Dominican Republic. (A) Colony in situ with bright white skeleton exposed by multifocal tissue loss (arrowheads) and areas of older tissue loss with algae overgrowth (*).  (B) Submitted samples. Gross lesions are obscured by with loss of pigment with fixation. Colony is composed of numerous, small, tube-shaped polyps (arrowhead). Credit: In situ coral colony, Aldo Croquer, The Nature Conservancy. Used with permission.

Histopathological Findings:

Histology: H&E and Giemsa sections examined. Epithelium and gastrodermis of the surface body wall are discontinuous and atrophied, with multifocal epidermal erosions, ulcerations, and loss of surface body wall. At the margins of some areas of surface body wall loss there is local metaplasia from simple squamous calicodermis to columnar epidermis (Figure 2A, B). Endosymbionts are sparse within the gastrodermis, and some stretches of gastrodermis within surface body wall are bereft of endosymbionts. Adjacent to surface epidermis and associated with moderate to marked multifocal epidermal erosion and ulceration are sections of flatworm (Figure 3) with a dorsal surface epithelium containing short, bright eosinophilic rhabdites (Figure 4A), finely ciliated ventral epithelium (Figure 4B) and measuring approximately 50 to 200 microns thick to 2 to 4 millimeters long. This organism contains zooxanthellae within a loose, eosinophilic parenchyma (Figure 4A) and in some sections a curved, open pharynx containing cellular debris is evident (Figure 3).

Healthy elkhorn coral tissue next to elkhorn coral with tissue loss shown by atrophic and thin body wall.
Figure 2. (A) Photomicrograph from a healthy example of elkhorn coral (Acropora palmata). Tall columnar epidermis (E), gastrodermis (g), skeletal space (s), gastrovascular cavity (gvc), and calicodermis (white arrowhead). H&E stain. (B) Photomicrograph from elkhorn coral found with multifocal tissue loss in the Dominican Republic. Epidermis (E) and gastrodermis (g) are atrophic and surface body wall is thin. Epidermis is ulcerated, exposing skeleton (s). Simple squamous calicodermis (white arrowheads) has an area of transition (black arrowhead) into columnar epidermis. H&E stain.
Photomicrograph at low magnification from elkhorn coral showing flatworm along body wall.
Figure 3. Photomicrograph at low magnification from elkhorn coral (Acropora palmata) found with multifocal tissue loss in the Dominican Republic. A flatworm (fw) is present along a multifocally ulcerated coral surface body wall (c) along one polyp. A small amount of cellular debris can be seen within the pharynx (p) of the flatworm. Coral polyp structures are visible within the tissue (cp).
Photomicrographs of from unknown species of flatworm
Figure 4.  Photomicrographs of from unknown species of flatworm. (A) Dorsal detail showing epidermis with rhabdites (black arrowheads) and endosymbionts visible within the flatworm parenchyma (fw). (B) Ventral detail of flatworm (fw) showing ciliated epithelium (arrows) adjacent to coral surface body wall (c) that is nearly bereft of endosymbionts and discontinuous over an area of ulceration (*).

Morphologic Diagnosis/es: Epidermal erosion and ulceration, multifocal, marked, with diffuse, severe surface body wall atrophy and intralesional flatworms.

Disease: Acropora eating flatworm predation.

Etiology: The species of flatworm in this case cannot be identified via histologic sections, and no flatworm has been described in the wild on Staghorn coral or other Caribbean acroporids. Prosthiostomum acroporae is a well-known aquarium pest, native to the Great barrier Reef, that consumes tissue of Pacific species of Acropora. Another distinct but unnamed flatworm has also been identified as predating on Acroporids in captivity, and flatworms of the genus Waminoa are reported as mucus eating with a wider host range, have been described in the Red Sea and in Singapore, and may be capable of inducing immune suppression. The flatworm seen in this case appears to be ingesting coral tissue and have characters in common with those described for P. acroporae, including a cleft pharynx, inclusion of endosymbionts in tissue, dorsal epithelium with rhabdites, and a ciliated ventral epithelium, however none of these features is unique to or diagnostic for P. acroporae and further work is needed to characterize this organism.

Distribution: Acropora eating flatworms are common aquarium pests and have previously been identified as a threat in captive Acropora, however, they have not previously been associated with outbreaks of tissue loss disease in the wild. They can be found in aquaria in any part of the world.

Seasonality: none known.

Host range: Elkhorn coral is found in coral reefs throughout the Caribbean.

Transmission: Larvae of some species of flatworm are pelagic and can be transmitted through water currents. Others may travel along the substrate.

Clinical signs: Multifocal small areas of tissue loss that may coalesce to engulf the colony.

Pathology: No previous reports are available describing the histopathology of flatworm predation, but they are described as causing tissue loss by direct assimilation of coral tissue starting with the surface body wall. In this case, cellular changes consistent with wound repair of surface body wall defects were also observed.

Diagnosis: Acropora eating flatworm, unknown species.

Public health concerns: Not transmissible to humans. Loss of reefs can increase costal flood risks.

Wildlife population impacts: Affected elkhorn corals in this outbreak all succumbed to complete tissue mortality. In aquaria it is not unusual for flatworm infested colonies to be lost, and concern has been expressed that P. acroporae might escape aquaria to become invasive on reefs where it is not native. In the wild flatworms may be present on corals with no grossly visible damage. More investigation is needed to be able to interpret the significance of this flatworm in Caribbean reefs.

Management: In aquaria, appropriate quarantine of new specimens and biosecurity measures to prevent contamination between tanks is important for prevention and control. No recommendations for wild populations exist at this time, but continued monitoring may produce data to inform future recommendations. Aquaria and aquarium water containing saltwater corals should be disposed of safely away from bodies of water where coral reefs are present.


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