Invasion context matters: Vulnerability of the sea urchin Paracentrotus lividus to ingestion of Rugulopteryx okamurae increases with invasion time

Abstract

The current spread and proliferation of the invasive macroalga Rugulopteryx okamurae in South European coastal waters is a major ecological problem with a high socioeconomic impact. First identified as invasive in the Strait of Gibraltar (S Iberian Peninsula) in 2014, R. okamurae continues its expansion along the Atlantic and Mediterranean coasts. Despite its extended presence, the biotic control mechanisms and long-term impacts of R. okamurae consumption by native herbivores remain unexamined. This 24-week experiment investigates the physiological responses of the common sea urchin Paracentrotus lividus from three different locations following an invasion gradient by the time when R. okamurae was first detected. Sea urchin individuals were fed two diets: a strict diet of 100 % R. okamurae or a mixed diet with 50 % R. okamurae and 50 % Ulva sp. We evaluated the response of P. lividus individuals based on ingestion rates, relative weight (g), mortality, Ivlev's electivity index, and gonadosomatic index (GSI). Individuals from locations with more time since invasion fed on 100 % R. okamurae showed higher weight loss, higher mortality, and lower GSI than individuals from short-term invaded locations. Those negative effects were reduced for individuals fed a mixed diet. Within the mixed diet, Ivlev's electivity values of individuals from “10 years of invasion” reduced from random selection to partial avoidance over R. okamurae across the experiment. Our results reveal long-term underlying effects of R. okamurae ingestion, suggesting implications for the conservation of P. lividus populations in invaded locations, with potential ecosystem-level consequences as it is a key controlling species in coastal systems.