# Title of Dataset --- Data for the sea urchin Diadema antillarum population density and size structure from 1983 to 2022 and across sites on the south shore of St. John, US Virgin Islands. These sites and many locations across the Caribbean experienced two mass mortality events of this sea urchin in 1984 and 2022. Also included are juvenile coral abundance in and out of zones occupied by D. antillarum in the summer of 2021, just prior to the 2022 mass mortality event. ## Description of the data and file structure The data are presented in one excel file. This file has separate tabs each type of data collection (listed below). Sea urchin density: Columns for census year, site, depth and density per meter squared. This data set represents sea urchin counts from a variety of census sites in St. John US Virgin Islands. The sites are YawziT1, YawziT2, YawziT3, VT1, VT2, VT3, VT4, TektiteT1, TektiteT2, TektiteT3, WPRecruit, YawziRecruit, WTektiteRecruit, ETektiteRecruit, DonkeyBiteRecruit, CabritteRecruit, Site5, Site6, Site15, Site11, Site2, Site9, SQST, DOBI,WELL, NELL, TECL. Site Decriptions can be found in Levitan et al. Oecologia. 175, 117-128 (2014) Density of Adults and Recruits: Columns are year, size of area monitored, # of Adults (> 20 mm test diameter) and Recruits (<= 20 mm test diameter), total number, and density of adults, recruits and total sea urchins per meter squared. This data set represents the number of sea urchins as either adults or new recruits at five 1000m2 sites; SQST, DOBI, WELL, NELL and TECL. Sea urchins per 100m2: The number of Diadema within 10m x 10m quadrats (N = 10 per site) for the five sites in 1984 (following the 1984 mortality event) and 2022 (following the 2022 mortality event). This data set represents the distribution of sea urchins (# of sea urchins in a 100m2 area) to determine how aggregated these sea urchins were before and after the 1984 and 2022 mass mortality events. Size distribution of sea urchins in 10mm test diameter bins in 1983 and 1984 (before and after the 1984 mortality event) and in 2021 and 2022 (before and after the 2022 mortality event). This data represents the size distribution of sea urchins to determine the number of new individuals that recruited to these sites after each mortality event. Juvenile coral abundance in 0.5 x 0.5 quadrats inside and outside of algal-free zones: Columns are site (Halo), location inside or outside the algal free halo, and then abundances of 7 taxa; Porites astreoides, P. porites, Siderastrea radians, S. sideria, Agaricia spp., Millipora alcicornis and Favia fragum. This data set provides information on how grazing by Diadema influences that success of juvenile corals. Halo size and Diadema numbers: Columns are Halo site number, count of Diadema in the halo, area of halo (m2) This data set provides information on the number of sea urchins inside these grazing locations (halos).