Coral disease
Coral diseases, in some cases triggered or accelerated by warming, are changing the structure of many reefs, particularly in the Caribbean, turning large areas of once rich coral cover into areas with little cover and low diversity and productivity.
Many diseases have been identified on coral reefs worldwide. The consequences of these diseases are varied but have in some cases caused major changes to the reef structure. Most of the main pathogenic groups, e.g. bacteria, slime-moulds, cyan bacteria, have been identified as being associated with various diseases, and several diseases appear to be a combination of these pathogens. However, despite increased research efforts the disease-associated pathogens have only been identified for a third of known diseases, and often the causative pathogen is unknown.
Reef diseases has been increasingly linked to declining water quality, for instance the bacterium Serratia marsecens, which is attributed to White Pox Disease, has been clearly associated with human sewage. While other diseases are caused by organisms known to be endemic but which are triggered into pathological condition by rising temperature or other stresses. Many diseases are known by their appearance such as Red Band Disease and Yellow Blotch Disease. Some diseases are species specific, and others affect a range of species. While some diseases progress very slowly, such Black Band Disease which progresses over massive corals less than 1cm per year. Diseases are also affecting other invertebrates and algae and some are proving to have far reaching effects.
More than three-quarters of known reef diseases have been reported from the Caribbean, this may partly reflect the quantity of research done or possibly the greater stresses from pollution. The first major disease known to have widespread and profound consequences was White Band Disease, the pathogen of which is still unknown despite many years of research. The disease was first identified in the 1970s and affected Elkhorn coral Acropora palmate, reducing the cover of this previously dominant shallow water coral by between 80 -90% through out the Caribbean. Recovery of mature colonies did take place to some extent but population levels have since been kept low by another disease, White Pox Disease. Identified in 1996, White Pox is highly contagious and coral tissue loss can spread by rates of over 2cm per day. Together these diseases have decimated populations of one of the most important reef –building species in the Caribbean and caused major changes to the reef structure.
