Sea urchin Diadema africanum mass mortality in the subtropical eastern Atlantic: role of waterborne bacteria in a warming ocean

A widespread mass mortality event of the sea urchin Diadema africanum was detected in the subtropical eastern Atlantic, extending from Madeira to the Canary Islands, covering a straight-line distance of >400 km. This is the first disease-related die-off of a diadematid documented in the eastern Atlantic Ocean. Diseased individuals showed deterioration of the epidermis and water-vascular system, resulting in epidermal necrosis and release of spines. Despite some spatial heterogeneity in disease incidence at the study sites, there was a 65% overall reduction in urchin abundance after the mortality event compared to numbers pre-mortality. However, the reduction in urchin numbers did not compromise the species' reproductive success; the settlement peak following the mortality event was of a similar magnitude to that in prior years. Bacterial isolation and culture techniques, and subsequent laboratory infection experiments, strongly suggested that Vibrio alginolyticus was involved in the disease. We hypothesize that the mass mortality event developed due to anomalously high seawater temperatures recorded in the 2 studied archipelagos and that warmer temperatures enabled infection of D. africanum by waterborne pathogenic bacteria. Fluctuations in urchin populations are key in determining the structure and functioning of benthic ecosystems: under the current seawater warming scenario, disease may result in more frequent phase shifts, aiding the persistence of macroalgae.