Pathology of coral growth anomalies in the Maldives: insights from Acropora, Montipora, and Pachyseris.

Coral growth anomalies (GAs) are chronic skeletal lesions that alter coral morphology and functionality, yet their etiology remains poorly understood, particularly in understudied regions such as the Maldives. This study provides the first comprehensive pathological description of 15 GAs from the Maldivian Archipelago across 3 coral genera, Acropora, Montipora, and Pachyseris, with the last still lacking a histopathological characterization to date. Despite varying gross morphologies (either bosselated, umbonate, nodular, or fimbriated masses), all lesions microscopically exhibited basal body wall hyperplasia corresponding to areas of lax, porous, and inconsistent skeletal architecture as observed by scanning electron microscopy. Common histological findings included calicoblast hypertrophy (74%), endosymbiont depletion in the surface body wall (60%) corresponding to grossly observed discoloration of the GA, absence of gonadal development (80%), and diffuse mucocyte hyperplasia (seen only in Pachyseris). Most GAs (60%) contained centrally located masses of organisms that corresponded to regions of skeletal cavitation observed on sub-gross examination. Organisms included fungi, algae, cyanobacteria, diatoms, and labyrinthulomycetes, which in 90% of cases were invading surrounding tissues, suggesting a role in pathogenesis. Additionally, some GAs had solitary endolithic sponges (26%) or degenerate sponge-like debris (13%) in gastrovascular cavities. These findings further suggest multifactorial etiologies for the lesions, reinforcing the complexity of GAs, the limitations of current diagnostic frameworks, and challenges to lesion nomenclature.