Coral Reefs: A Reef Resilience Toolkit Module

Modes of Transmission

There are several known ways in which pathogens infect corals. It is important for managers to be aware of the various modes of disease transmission, as these inform monitoring efforts and parameters, which can ultimately help guide management strategies.

Black band disease advancing from right to left in the coral Diploria strigosa. Photo © Sven Zea, Universidad Nacional de Colombia/Marine Photobank

Similar to human populations, coral species living at higher local abundances have been shown to be more susceptible to disease.1,2 For example, there has been an increase in the prevalence of the coral disease white syndrome in areas where coral cover is greater.1 The underlying mechanism is that disease can spread more readily within crowded populations, suggesting that locally common species have a greater disease potential than rare species.3

A potential vector for disease transmission that is exacerbated in dense populations is coral predation. Predators act as vectors by oral or faecal transmission of pathogens.3,4. Some diseases, such as black band disease, are thought to flourish in the presence of corallivorous fishes and the gastropod Drupella spp. are suspected to increase the rate at which the disease is spread from infected to non-infected colonies.3,6 Other coral-disease vectors identified are the fireworm Hermodice carunculata, whose gut has been found to harbor Vibrio shiloi (the pathogen inducing bacterial bleaching in a Mediterranean coral),7 and damselfish, which harbor one life-history stage of a digenean (trematode) that infects Porites.8

Predators also increase disease potential by making scars in corals that allow pathogens to penetrate and infect tissues.2 Certain diseases (such as skeletal eroding band) require a tissue lesion in order to infect corals.2 For example, lesions may be driving the positive association between coral polyp-eating butterfly fishes and disease prevalence.5