Overlapping niches among sea urchin species reveal Sargassum-induced degradation at Caribbean shallow coral reefs

Abstract

Massive influxes of pelagic Sargassum into Caribbean coastal ecosystems have disrupted environmental conditions and reshaped benthic communities. We evaluated how varying levels of Sargassum exposure (2015-2021) influenced the isotopic niche structure of five reef-associated sea urchin species across three reef lagoons in the Mexican Caribbean. Exposure scenarios were classified as Low (2016-2017), High (2015, 2018), and Prolonged (2021) based on satellite-derived biomass and its effect on lagoon reefs over last years. Dissolved oxygen (DO) measurements characterized lagoon conditions, and stable isotope analysis (δ¹³C and δ¹⁵N) assessed shifts in niche position, breadth, and overlap, using standard ellipse areas (SEAc). Results revealed species-specific responses, including niche expansions, contractions, and increased overlap under prolonged exposure. Diadema antillarum showed marked flexibility, expanding its niche under high Sargassum exposure and contracting under prolonged exposure. Echinometra lucunter exhibited the broadest niche at Xcalak during prolonged exposure and the narrowest at Mahahual under low exposure. Echinometra viridis maintained a broader niche under low exposure at Xahuayxol but experienced the greatest restrictions during high exposure years. Eucidaris tribuloides and Tripneustes ventricosus expanded their niches under high and prolonged conditions, with T. ventricosus displaying notable adaptability at Xahuayxol. Prolonged Sargassum exposure promotes the homogenization of resource use, reshaping ecological interactions. These findings offer novel insights into the trophic responses of echinoids facing prolonged environmental disturbance, contributing to a better understanding of the ecological consequences of Sargassum influxes on Caribbean reef ecosystems.

Keywords: Echinoids; Mexican Caribbean; Niche breadth; Reef lagoons; Stable isotopes; Trophic interactions.