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Coral Disease-Induced Growth Anomalies: What Causes Them?
Reference
Aeby, G.S., Williams, G.J., Franklin, E.C., Haapkyla, J., Harvell, C.D., Neale, S., Page, C.A., Raymundo, L., Vargas-Angel, B., Willis, B.L., Work, T.M. and Davy, S.K. 2011. Growth anomalies on the coral genera Acropora and Porites are strongly associated with host density and human population size across the Indo-Pacific. PLoS ONE 6: 10.1371/journal.pone.0016887.

Background
The authors write that "growth anomalies (GAs) are common, tumor-like diseases that can cause significant morbidity and decreased fecundity in the major Indo-Pacific reef-building coral genera, Acropora and Porites," and they note that "GAs are unusually tractable for testing hypotheses about drivers of coral disease because of their pan-Pacific distributions, relatively high occurrence, and unambiguous ease of identification."

What was done
In an attempt to determine the major cause or causes of GAs in Acropoa and Porites corals -- hereafter AGAs and PGAs, respectively -- Aeby et al. modeled multiple disease-environment associations that may underlie the prevalence of AGAs and PGAs based upon 937 quantitative coral disease surveys from 13 regions spread across the Indo-Pacific that were made between 2002 and 2008, utilizing nine predictor variables: Acropora cover, Acropora density, Porites cover, Porites density, depth, weekly sea surface temperature anomalies (WSSTA) during the prior four years, human numbers within 1 km, human numbers within 100 km, and ultraviolet (UV) radiation input.

What was learned
The twelve researchers -- hailing from Australia, Guam, New Zealand and the United States -- say they found that "host abundance and human population size were the optimal predictors for variations in prevalence of AGAs and PGAs," noting that although the latter association "has been widely posited, this is one of the first broad-scale studies unambiguously linking a coral disease with human population size." And they make a special note of stating that sea surface temperature anomalies showed "no such association."

What it means
On the basis of their findings, the international team of scientists concludes that "as human densities and environmental degradation increase globally, the prevalence of coral diseases like PGAs could increase accordingly, halted only perhaps by declines in host density below thresholds required for disease establishment."

Reviewed 4 May 2011