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December 14, 2023

USGS National Wildlife Health Center pathologists and partners have authored a new reference guide on the lesions and histology of scleractinian corals.

Why this matters: Coral diseases contribute to coral reef degradation and loss, but the causes of most diseases remain unknown. Histopathology provides major contributions to the understanding of human and animal diseases by identifying characteristic cellular and tissue level changes, and in some cases directly identifying pathogens. It has the potential to provide similar insights into coral diseases, but few veterinary practitioners have familiarity with corals. To address this knowledge gap, a publication targeted to veterinary pathologists (Hawthorn et al. 2023), was developed that compiles systematic descriptions of common gross and microscopic lesions of corals.

Image: Colony of Boulder Brain Coral (Colpophyllia natans)
A large colony of Boulder Brain Coral (Colpophyllia natans, Order Scleractinia).

Stony corals, Order Scleractinia, are animals that create coral reefs crucial for fish nurseries, flood mitigation, and tourism, and provide critical marine habitats that house the highest density of ocean biodiversity. They are also one of the most rapidly declining taxonomic groups of animals on the planet. While substantial attention has been given to thermal bleaching, these corals are also experiencing unprecedented levels of disease, which is often the ultimate cause of death in stressed corals. Numerous diseases have been documented, but the causes of most remain unknown and additional research will be required to better understand and mitigate immediate and long-term disease threats to coral.  

One barrier to better understanding coral diseases is they are visually indistinguishable at the macroscopic level. Histopathology, an important tool in disease investigations, has not been extensively applied to coral diseases, in part because it requires familiarity with the appearance of both normal tissues and lesions in the organism being studied. In order to facilitate wider use of histopathology, we have compiled a body of work to serve as a reference and introduction to the study of scleractinian coral pathology (Hawthorn et al. 2023). Gross lesions associated with diseases and injury are described, and corresponding histological lesions are characterized and correlated to coral diseases in which they have been observed. Defining microscopic changes in corals is an evolving process, and literature gaps are highlighted. This reference may stimulate additional work where pathologists contribute to defining both normal and abnormal states in this colonial animal that depends on intimate relationships with microorganisms to survive. Fundamental aspects of physiology and anatomy, sampling, and ancillary diagnostics relevant to coral diagnosticians are also summarized.

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