Diagnostic Investigation of Tissue Loss and Pigmented Lesions in Corals

Corals are important representatives of the One Health concept, demonstrating the interconnection between health of the environment, people, and animals. However, corals have declined significantly, with climate change, human-related activities, and disease as major drivers of this decline, placing them among the most threatened animal groups according to the International Union for Conservation of Nature (IUCN). Coral have limited ways in which to respond to disease, with tissue loss and pigmented lesions representing some of the most impactful on coral populations. A variety of unique challenges exist in disease investigation of corals, resulting in much confusion and a poor understanding of disease pathogenesis in these animals. Microscopic examination is a cornerstone of disease diagnosis in other veterinary species and one of the few reliable diagnostic tools in corals, but microscopy alone may be insufficient in determining disease etiology. We investigated 2 outbreaks of tissue-loss disease in the Caribbean, applying a standard paired biopsy histologic approach in the first and exploring potential benefits of a strip sampling method in the second. Our results indicated no benefit of the strip sampling method and divergent microscopic abnormalities underlying these outbreaks despite similar gross presentations. We also investigated highly prevalent brown lesions in Orbicella species in St. Kitts using an interdisciplinary approach pairing histologic assessment with other diagnostic tools. Our results indicated pathology of symbionts and alterations in photosynthetic pigments, but questions remain regarding the basis for brown pigmentation. Finally, we evaluated safety of 4 commonly used treatments in coral aquaculture with histopathology. No major trends were observed. Collectively, these studies highlight the importance of microscopic evaluation in coral disease and the added benefits of pairing with other diagnostic tools.