In the middle of January, 1983, researchers in Punta Galeta, on the Caribbean coast of Panamá, noticed signs of severe illness in large numbers of Diadema antillarum individuals (Lessios et al. 1984b). Each individual began to develop mucilage on its spines, then shed the spines, and then die. An example of the progression of this illness through one population is given in the graph in Figure 3. Figure 4 shows the appearance of an urchin in the second stage of illness, with spines scattered around its body. Within a few days, over 95% of the D. antillarum individuals in the area had died (Lessios et al. 1984a). The graphs in Figures 5 and 6 chart the progress of the mass mortality through two major study sites in Panamá (Punta San Blas and Cayos Limones, respectively). Figures 7 and 8 illustrate the dramatic shift from a population density exceeding 20 urchins/m2 to a massive aggregation of dead urchin shells (Carpenter 1988).

Soon, scientists at other sites throughout the Caribbean began to report similar illness and mortality in other D. antillarum populations. By January, 1984, the mortality had spread through the entire range of D. antillarum, affecting every known local population (Lessios et al 1984b). The directional progress of the mortality is illustrated in a map of the Caribbean in Figure 9. The population reduction of more than 95% in the entire range of this species (Hughes 1994) makes this mass mortality event the most extensive epizootic on record for a marine invertebrate (Lessios 1984b).

Incidentally, there is some confusion of terms among different sources referring to mass mortalities. Several sources, including Lessios (1984b) and Miller & Colodey (1983) use terms such as 'epidemic' and 'symptom' where 'epizootic' and 'sign', respectively, would be more appropriate. Occasionally, a source (e.g., Lessios 1984b) will use the different terms interchangeably. As Williams et al. (1993) point out, the derivation of epidemic (Greek demos -- people; as opposed to zoon -- animal) restricts it to human diseases, and the standard definition of symptom is something communicated by a human patient, accompanying signs which are witnessed by an objective observer. Therefore, the mass mortality of D. antillarum is correctly described as an epizootic, and the loss of spines prior to mortality in individual urchins is correctly called a sign of infection.

The consensus among all sources consulted is that this mass mortality was caused by a pathogen, perhaps similar to the one that caused the mass mortality of the green sea urchin Strongylocentrotus droebachiensis in Nova Scotia (described in the next chapter) (Lessios et al. 1984b; Miller & Colodey 1983).

Lessios et al. (1984b) give three lines of evidence to support the hypothesis that this mortality was caused by a pathogen, rather than by a pollutant or by a mass of water with physical abnormalities. First, the mortality spread throughout D. antillarum's range without any dissipation of its effects, while other sea urchin species in the area were unharmed. All available data say that every known population of D. antillarum was affected by this mass mortality, and that only a few individuals in each population survived. The potency over a wide range suggests a biological, rather than chemical, cause, and the immunity of other species further suggests a highly specific pathogen targeting unique traits of D. antillarum. The lack of any significantly resistant population implies that the pathogen was something to which the urchins were not frequently exposed prior to the sudden mass mortality. The pathogen may have mutated or immigrated to the Caribbean in early 1983, thereby exposing the urchins to a previously unencountered threat.

Second, water temperature and salinity in Panamá were within normal limits even during the worst of the mass mortality. Third, coral bleaching (by expulsion of symbiotic zooxanthellae) and death did not occur in the same pattern and sequence as Diadema mortality. Thus, coral die-offs and urchin die-offs cannot be attributed to a single cause. All these observations support the hypothesis that a single, species-specific biological pathogen caused the mass mortality of D. antillarum in the Caribbean.