Gross and Microscopic Pathology for Quantifying Responses of Coral Reefs to Remediation Efforts

Coral reefs are widely valued as shelters for food and ornamental fish, attractions for tourists, barriers to prevent land erosion, reservoirs of genetic diversity, and sources of novel chemicals that translate into biotechnology and pharmaceutical products. They are so valuable that the Copenhagen Consensus Center determined that $104 would be generated for each dollar spent on stemming their loss by 50% before 2030. ⁵ Some also argue that, independent of their importance to humankind, coral reefs have an intrinsic right to exist unperturbed, while to others they have regionally specific cultural worth. To slow the current rate of loss, which stands at 2%, ⁵ actions being taken include decreasing the rate of climate change and ocean acidification and reversing overuse, overfishing, and pollution.

To assess the impacts of these actions, the scientific community should define methods to quantify the health of coral reef ecosystems. Already, the efficacy of conservation efforts is measured by multiple agencies using live coral cover as a proxy for ecosystem health. ^4,5 Serial surveys of gross and microscopic lesions, such as that published here by Work et al, ⁸ can serve as more sensitive tools, biomarkers if you will, to quantify decline of coral colonies before they are lost from numerous causes. One of the most important causes of coral loss is disease.

Biologists began documenting isolated cases of unusual lesions and naming putative coral diseases in earnest approximately 40 years ago. Because each researcher was focused on a limited region, reports were difficult to contrast and compare with those from other regions. For example, several “white diseases” were described, including white syndrome ⁷ in the South Pacific, white plague ¹ in the Western Atlantic, and ulcerative white spot disease ⁶ in the Indo-Pacific. Disease entities were not only defined by geographic distribution but also according to host specificity and temporal and spatial characteristics of lesion progression. Agents for several diseases then became known by fulfilling Koch postulates, and etiologic diagnoses were assigned, giving birth to a new field: coral disease biology.

More recently, scientists have begun to assess the contribution of disease to larger-scale loss of coral ecosystems through regionwide disease surveys—most of these have been based only on gross findings. The coral disease biology community has also recognized a deficit of standardized practice and has begun to define the nomenclature for gross and microscopic lesions. ³ In this regard, the current manuscript provides comprehensive baseline data on the extent of lesions in the relatively underexplored Indo-Pacific region. Also useful is the authors’ complementary use of microscopic anatomy.

When the microanatomy of coral is being examined, it is important to understand that the animal is triploblastic—made up of epidermis and gastrodermis, separated by mesoglea forming a thin layer of tissue thrown into folds over a calcareous skeleton that it secretes. The body shape is radial, consisting of a central gastrovascular cavity separated from the water column by an oral disk. ² Within the cavity and attached to the body wall are tissue partitions (mesenteries) with mesenterial filaments along their free edges, which are topped by specialized cnidoglandular bands replete with granular gland cells and stinging cells (cnidocytes). Corals live in symbiosis with intracellular dinoflagellates, called zooxanthellae, which in turn harvest energy through photosynthesis to benefit the animal. The surface epithelium is covered by a mucopolysaccharide layer with abundant microbiotic consortia. Microbiota also take root in the calcareous skeleton, and other invertebrates use the skeleton for shelter. Additional specialized cells that one may observe in tissue sections include neurons, amoebocytes (a primitive immune cell), calicoblasts, and desmocytes. ³

Using microscopic anatomy, the pathologist can precisely define disease entities. Moving beyond generic categories toward precise definitions provides a means to track prevalence and gives information that can be used to generate hypotheses about pathogenesis. Subsequent experimental results provide tools with which to intervene. If, like Work et al, others adopt such an approach, the results will inform government policy as well as transplantation, protection, quarantine, and remediation efforts crucial to stem the loss of these important ecosystems.