Unravelling echinoid mass mortalities: a global overview of mechanisms, spatio-temporal trends, and taxonomic insights.

Mass mortality events (MMEs) have been occurring since the dawn of time. However, in contrast to terrestrial events, most marine MMEs remain undetected, largely due to the inaccessibility of many marine environments. One of the most notorious and best-studied marine MMEs in modern times is that of the population collapse of the echinoid Diadema antillarum in the Caribbean Sea during the mid-1980s, which triggered a catastrophic phase shift in local benthic communities from which the region never fully recovered - illustrating the ecological significance of echinoids in maintaining marine habitat stability. In recent years echinoid MMEs have reached an unprecedented scale, affecting populations across multiple ocean basins, climatic regions, and taxonomic range. Most recently, pathogenic-driven mortalities spreading from the Caribbean to the Mediterranean and Red Sea and further expanding into the Indo-Pacific, pose a significant threat to ecological integrity across thousands of kilometres. The apparent intensification of global echinoid MMEs underscores the urgency of elucidating the mechanisms driving these events and their implications for marine conservation. Here we present a comprehensive review of global echinoid MMEs, including scientific literature dating back to 1888. We identify and formulate the five main mechanisms driving echinoid MMEs: (i) pathogens (33%); (ii) catastrophic events (25%); (iii) harmful algal blooms (11%); (iv) extreme temperatures and tides (24%); and (v) human activities (7%). We then explore spatio-temporal trends and the underlying functional morphology traits that drive these events.