Night surveys reveal abundant populations of sea urchins with high erosive potential in Cayo Arenas, Banco de Campeche

Abstract

Sea urchins perform key ecological functions in coral reefs, such as herbivory and erosion of calcareous structures, which intensify at night. The Cayo Arenas reef, Campeche Bank, Mexico, is located far from the coast, which has helped to protect it from direct anthropogenic threats. However, no information is available on the status of sea urchin populations and the bioerosion processes they perform in the reef. For this reason, we conducted diurnal and nocturnal surveys to characterize urchin density as a function of the diel cycle and to calculate their bioerosive potential. The surveys were conducted at 3 sites with different survey depths (10–19 m) and varying reef heterogeneity related to 2 components: coral cover and sand cover. All surveys were conducted in July 2023. In general, an abundant population of urchins was observed, which was mainly composed of Diadema antillarum. At night, the abundance (14.5 ± 3.8 ind) and bioerosion rate (1.55 ± 0.53 kg CaCO3·m–2·y–1) were higher than during the day (2.6 ± 0.8 ind and 0.2 ± 0.06 kg CaCO3·m–2·y–1). In addition, depth and reef geomorphology seemed to influence the pattern of urchin density. At sites with high reef heterogeneity, sea urchin density was higher at night, which could be associated with a greater availability of refuge from predators. Our results show an underestimation of urchin population densities in daytime surveys and, therefore, their impact on the ecosystem as bioerosion agents because sea urchins are mostly nocturnal but are commonly surveyed during the day.

Keywords

• Echinoderms
• Geoecological functions
• Keystone species
• Carbonate budget
• Diadema antillarum
• Campeche Bank