Ciliate Confirmed as Primary Driver of Deadly Coral Brown Band Disease

On the reefs surrounding Magoodhoo Island in the Republic of Maldives, a fast-moving killer stalks the branching corals that build the region’s most complex three-dimensional habitats. Corals struck by brown band disease develop a distinctive brown ribbon of organisms that creeps across exposed skeleton, leaving behind stark white, denuded branches. For more than three decades, scientists have debated whether the scuticociliate protists that make up this band are the true killers or merely scavengers arriving to feast on tissue already destroyed by bacteria or other stressors. A new study published in the journal Coral Reefs provides the most detailed histopathological account of the disease to date, and its findings tilt the argument decisively toward the ciliates.

An international research team led by Chiara Bises of the University of Milano-Bicocca, working with veterinary pathologist Michelle M. Dennis of the University of Tennessee and colleagues, examined diseased fragments of Acropora cf. muricata collected by SCUBA in May 2022 from reefs at depths of 7 to 15 meters in Faafu Atoll. Brown band disease had been recorded in the Maldives since 2012, but its underlying pathology had never been described at the microscopic level. The researchers sampled three colonies showing the classic field presentation: a variably intense brown band paralleling an annular zone of acute tissue loss, marked by bright white but non-eroded skeleton. From each colony they collected biopsies capturing three distinct regions along the disease trajectory: the brown band itself, the tissue loss margin, and the bordering apparently healthy tissue.

Identifying the culprit required both morphology and genetics. Ciliates were gently dislodged from coral fragments using menthol treatment, fixed in ethanol, and subjected to DNA extraction and amplification of the 18S rRNA gene using primers originally developed for brown band investigations on the Great Barrier Reef. The resulting sequences, deposited in GenBank, were assembled into a 1300-base-pair alignment alongside reference sequences of the genus Philaster and appropriate outgroups. Maximum likelihood and Bayesian phylogenetic reconstructions, each strongly supported, placed the Maldivian ciliates in a well-defined clade with Philaster guamense isolates previously recovered from diseased Acropora muricata in Australia. Morphological examination of seventy individuals corroborated the molecular identification: oval to elongated cells averaging roughly 336 micrometers in length, with rows of cilia, a centrally positioned elongated macronucleus, and a colorless to brownish-yellow appearance imparted by ingested coral cells and their endosymbiotic algae.

The histological story that emerged from the six biopsies is one of invasion, consumption, and dormancy unfolding in sequence across the lesion. In the brown band region, the researchers found densely packed aggregations of both vegetative and encysted trophonts, the feeding and resting stages of the ciliate, intermingled with fragments of necrotic, dissociated coral tissue representing the basal body wall, surface body wall, and mesenteries. Necrosis in this zone was severe, affecting more than three-quarters of the tissue in the examined fields. Special stains revealed that the cyst capsules stained strongly with Alcian blue and periodic acid Schiff, consistent with acidic mucopolysaccharides, while failing to react with silver or trichrome stains. In five of six biopsies, other organisms, including fungi, flatworms, and Labyrinthulomycetes, clustered within the band, yet none of these saprophytes invaded host tissues or contacted living polyp structures.

The critical evidence came from the tissue loss margin and the tissue beyond it. At the advancing front, vegetative trophonts predominated, lying in direct contact with both intact coral tissues and fragments of dissociated tissue. Where the coral surface was still intact, ciliates occupied deep skeletal spaces and the gastrovascular cavity, pressing against the basal body wall, surface body wall, or mesenteries. Most strikingly, in the bordering regions that appeared completely healthy under gross examination, occasional vegetative trophonts were found deep within skeletal spaces in contact with the basal body wall, and in one case within the gastrovascular cavity touching the mesenteries. These polyps showed intact architecture, regular cell morphology, no necrosis, no degeneration, and no loss of endosymbionts. In other words, the ciliates were infiltrating tissue that was, by every histological measure, still healthy.

This pattern carries profound implications for the pathogenesis debate. One long-standing hypothesis held that bacteria initiate the injury, damaging coral tissue and opening the door for ciliates to consume the compromised remains. The histology does not support this scenario. Bacteria were not microscopically evident within coral tissues, and the only bacterial structures observed were cell-associated microbial aggregates confined to the healthy surface body wall of a single coral, structures generally regarded as potentially symbiotic rather than pathogenic. No degenerative changes suggestive of microbial injury preceded the ciliate invasion. The findings instead align with a model in which Philaster guamense invades coral tissue from the underlying skeleton, attacking deep tissues directly, a strategy that parallels the calcified-matrix invasion documented in shrimp and sea urchins afflicted by related scuticociliates.

Time-lapse observations of diseased fragments in the laboratory added a behavioral dimension to the pathological picture. Tissue loss proceeded from the base of branches toward their tips at approximately 1.2 millimeters per hour, with ciliates advancing from bare skeleton onto intact tissue and then aggregating and encysting on exposed skeleton once tissue resources were depleted. Encystment, the researchers suggest, is triggered by unfavorable conditions such as nutrient depletion or crowding, and involves cytoplasmic condensation and shrinkage within a protective mucinous capsule. The abundance of empty cysts in the brown band may explain why the band’s pigmentation varies in intensity, appearing lighter when many trophonts have exited or died. This clustering and encystment behavior, the authors note, has not been documented in other scuticociliatoses affecting crustaceans, bivalves, echinoderms, or fish, hinting at a pathogenesis unique to the coral system.

The study also carries practical consequences for how coral diseases are diagnosed and named. Gross visual signs alone are notoriously unreliable, since many coral diseases produce overlapping appearances, and the researchers found that tissue appearing normal to the naked eye can harbor substantial microscopic pathology. They therefore propose a formal case definition for acroporid scuticociliate tissue loss disease, requiring rapidly progressing tissue loss, the presence or absence of a brown-pigmented skeletal deposit, microscopic confirmation of invasive ciliates within the skeleton of otherwise normal polyps, dissociation of coral tissue in contact with histophagous trophonts, and molecular or morphological confirmation of scuticociliate identity. Such definitions, grounded in histology rather than appearance, could bring much-needed consistency to surveillance and research across the Indo-Pacific and beyond.

Caveats remain. The sample size was limited to three colonies and six biopsies, constrained by the low prevalence of active lesions, the remote location, and ethical reluctance to place additional destructive sampling pressure on stressed reefs. Definitive proof of causation will ultimately require experimental infection trials in healthy corals, along with control biopsies from colonies far from any tissue loss. The role of Philaster lucinda, a related species sometimes co-occurring with P. guamense in Great Barrier Reef outbreaks but absent from the Maldivian samples, also remains unresolved. Nevertheless, the consistency of the pathological findings across all biopsies, combined with the absence of any alternative pathogen or pre-existing injury, positions Philaster guamense as a credible primary driver of brown band disease. As coral reefs face intensifying thermal stress and disease outbreaks worldwide, understanding which organisms truly kill corals, and at what stage of degradation they intervene, is essential for designing effective conservation responses. This study provides both a histopathological framework for tracking disease progression and a compelling case that, in brown band disease, the ciliate is not a scavenger but the executioner.

Subject of Research: Histopathology of brown band disease caused by the scuticociliate Philaster guamense in Acropora corals from the Maldives

Article Title: Histopathology of scuticociliate tissue loss associated with Philaster guamense (brown band disease) in Acropora cf. muricata from the Republic of Maldives

Article References: Bises, C., Dennis, M. M., Gobbato, J., Maggioni, D., Galli, P., & Montano, S. (2026). Histopathology of scuticociliate tissue loss associated with Philaster guamense (brown band disease) in Acropora cf. muricata from the Republic of Maldives. Coral Reefs. https://doi.org/10.1007/s00338-026-02954-4

Image Credits: AI Generated

DOI: 10.1007/s00338-026-02954-4

Keywords: brown band disease, Philaster guamense, coral disease, Acropora, histopathology, scuticociliate, Maldives, coral reefs, tissue loss, ciliate pathology, Indo-Pacific, coral health