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

Corals with rapidly progressing tissue loss marginated by a brown band on exposed skeleton are typically given a field diagnosis of brown band disease (BrB). It is associated with scuticociliates, protists capable of causing epidemic mortality in a wide range of marine organisms, but with uncertain pathogenic role in BrB. This study presents the first comprehensive pathological description of BrB affecting Acropora cf. muricata from the Maldives. Diseased coral fragments were examined using integrated gross and subgross lesion assessments, histology, and morphological and molecular (18S rRNA) identification of the associated ciliate, comparing three lesion areas: the brown band, tissue loss margin, and adjacent apparently healthy tissue. Molecular and morphological analyses confirmed Philaster guamense as the ciliate associated with the lesion. The brown band comprised dense aggregations of vegetative and encysted trophonts intermingled with necrotic coral tissues and saprophytic organisms, whereas, at the tissue loss margin, vegetative trophonts predominated in contact with dissociated tissues and within skeletal spaces adjacent to histologically intact tissues. Occasional vegetative trophonts were also found within deep skeletal spaces of apparently healthy regions. No other infectious agents or abnormalities of coral tissues were observed at the tissue loss margin or within adjacent apparently healthy tissue. These findings suggest that P. guamense is capable of invading otherwise healthy coral tissues and is closely associated with active tissue loss, supporting its potential primary role in disease initiation and progression. This study provides a histopathological framework for BrB progression and underscores the value of histology-based diagnostics in coral disease investigation.