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Abstract
Recurrent sea urchin mass mortality has recently affected eastern Atlantic populations of the barren-forming sea urchin Diadema africanum. This new episode of die-off affords the opportunity to determine common meteorological and oceanographic conditions that may promote disease outbreaks. The population dynamics of this sea urchin species are well known-urchin barrens have persisted for many decades along most of the coastlines off the archipelagos of Madeira, Selvages, and the Canary Islands, where they limit macroalgae biomass growth. However, this new and explosive mortality event decimated the sea urchin population by 93% on Tenerife and La Palma Islands. Two severe episodes of southwestern rough sea that led to winter storms, in February 2010 (Xynthia) and February 2018 (Emma), preceded both mass mortality events. The autumn and winter months of those years were anomalous and characterized by swells with an average wave height above 2 m that hit the south and southwest sides of the islands. The amoeba Paramoeba brachiphila was the only pathogen isolated this time from the moribund and dead sea urchins, suggesting that the amoeba was the primary cause of the mortality. This new sea urchin die-off event supports the "killer-storm" hypothesis that has been already described for western Atlantic coasts. These anomalous southwest storms during winters generate pronounced underwater sediment movement and large-scale vertical mixing, detected in local tide gauge, which may promote paramoebiasis. This study presents valuable insights about climate-mediated changes in disease frequency and its impacts on the future of coastal marine ecosystems in the Atlantic.
FIGURE 1. Map showing Madeira and the Canary Islands archipelagos with coordinates. The red circle marks the buoy position and the blue star the mareograph position, the thin black line indicates the monitoring sites in Tenerife and La Palma Islands and the arrow indicates locations, on other islands, where D. africanum die‐off was detected by divers
FIGURE 2. (a) Shaded areas indicate the annual percentage of days with southwesterly waves due to winter storms; colors indicate wave height (see the scale). The bars indicate the mean density ± standard deviation (SD) of Diadema africanum before and after the mass mortality events on La Palma and Tenerife. Averages were calculated using the Abades, Boca Cangrejo, and Teno site density data for the studied years on Tenerife Island and the Pta. Fraile, Playa del Pozo, Pta. Fuencaliente, and La Bombilla sites on La Palma Island. Red arrows indicate the mass mortality year. (b) D. africanum individuals showing first signs of the disease on the ambulacral plates column, and a general view of the Las Galletas site on the southwest coast of Tenerife (Photos: José Carlos Hernández and Antonio Espinosa)
FIGURE 3. (a) Compass rose showing average wave height and direction from the buoy data for the two winter storm periods: December 2009–February 2010 and December 2017–February 2018. These storm periods were characterized by southwesterly and southern waves with heights of 1–2 and 2–3 m. (b) Compass rose showing average wave height and direction from the buoy data for the winter periods of 2012, 2013, 2014, and 2015. These years were normal and not characterized by winter storms. Most of the days during these study periods were characterized by 0.2–1 m and 1–2 m northeasterly and eastern waves
FIGURE 4. Mareograph data showing the tide height in meters from January 2009 until December 2018. Periods when the die‐off were first registered in the Canaries and Madeira archipelagos are shaded in orange; and it can be seen it coincides with anomalous peaks of tide height registered by the mareograph
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