Spatially aggressive peyssonnelid algal crusts (PAC) constrain coral recruitment to Diadema grazing halos on a shallow Caribbean reef

Abstract

Peyssonnelid algal crusts (PAC) have rapidly spread on coral reefs throughout the Caribbean since 2010 and have become dominant space holders in multiple locations. In 2019, PAC covered 31–86% of the shallow reefs (< 6-m depth) at two sites in St. John, US Virgin Islands, but within halos around aggregates of the echinoid, Diadema antillarum, PAC was absent and small corals were common. To test the hypothesis that the density of small corals is greater in Diadema halos versus on adjacent substrata, and that this effect is accentuated by high coverage of PAC, the shallow reefs of St. John were surveyed in July/August 2019 and January 2020. Densities of small corals (pooled among taxa) in Diadema halos were equal to, or greater than, densities on adjacent substrata, and were highest in Diadema halos adjacent to areas with high coverage of PAC. Where the cover of PAC was ≥ ~ 10%, the mean (± SE) density of small corals was 23.3 ± 2.1 colonies m−2 in Diadema halos, but was 67% lower at 7.5 ± 1.2 colonies m−2 on adjacent substrata; where the cover of PAC was < ~10%, densities of small corals were nearly identical in Diadema halos (9.1 ± 0.9 colonies m−2) and on adjacent substrata (9.0 ± 1.1 colonies m−2). Similar results were obtained when the analyses were repeated for the common corals, Porites spp., Siderastrea radians, and S. siderea. As Diadema halos remained in the same location for at least 6 months, they may function as refuges for coral recruitment in locations where PAC is becoming spatially dominant.

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