Advancing histopathological insight into coral tissue loss and growth anomalies across tropical and temperate reef systems

Coral tissue loss conditions and growth anomalies are increasingly reported in coral reefs worldwide, yet their underlying causes, cellular dynamics, and ecological consequences remain poorly understood. To address these knowledge gaps, we coupled gross and microscopic examinations of lesions from both tropical and temperate reef-building anthozoans, including Indo-Pacific scleractinians and Mediterranean gorgonians. Across systems, lesions manifested as distinct expressions of growth anomalies developed in tissue or skeleton, acute tissue-loss lesion, and ciliate-associated pathologies, especially Brown Band Disease (BrB). Despite wide variation in their gross appearance, histopathology consistently revealed alterations of the basal body wall, depletion or redistribution of symbionts, inflammatory responses, and taxon-specific features such as mucocyte hyperplasia or epidermal thinning. In tissue-loss conditions, microscopic analyses highlighted divergent etiologies: some lesions reflected multifactorial stress responses, while others were driven by primary invasive agents, such as scuticociliates infiltrating intact tissues ahead of visible necrosis. Opportunistic endolithic organisms were frequently present within lesion matrices but showed inconsistent patterns of tissue invasion, underscoring their uncertain role in disease progression. Together, these findings illustrate the value of comparative histopathology for disentangling the diverse mechanisms underlying coral tissue degradation, from chronic proliferative disorders to rapidly advancing pathogen-driven diseases. By identifying conserved and taxon-specific lesion signatures across biogeographic regions, this work highlights how microscopic diagnostics can refine coral health assessments, inform disease surveillance, and support the development of early-warning indicators within resilience-based reef management frameworks.