Structural and Functional Analyses of Motile Fauna Associated with <i>Cystoseira brachycarpa</i> along a Gradient of Ocean Acidification in a CO₂-Vent System off Panarea (Aeolian Islands, Italy)

Ocean acidification (OA), one of the main climate-change-related stressors linked to increasing CO₂ concentration in the atmosphere, is considered an important threat to marine biodiversity and habitats. Studies on CO₂-vents systems, naturally acidified environments that mimic future ocean scenarios, help to explore the sensitivity of species and to understand how benthic communities rearrange their structure and functioning under the pressure of OA. We addressed this problem by studying the benthic invertebrates associated with a habitat-forming brown alga (<i>Cystoseira brachycarpa</i>) in the Bottaro crater vents system off Panarea island (Tyrrhenian Sea, Italy), by sampling along an OA gradient from the proximity of the main venting area (station B3, pH 7.9) to a control zone (B1 station, pH 8.1). Samples were collected in September 2016 and 2018. A total of 184 taxa and 23 different functional traits have been identified, considering feeding habit, motility, size, reproductive and developmental biology, and occurrence of calcareous structures. Invertebrates are distributed according to the distance from the high venting zone and low pH levels and results very consistent between the two investigated years. In the low-pH area (B3), 43% of the species are selected. The functional traits of the fauna mirror this zonation pattern, mainly changing the relative proportion of the number of individuals of the various functional guilds along the OA gradient. Invertebrates inhabiting the low-pH zone are mainly composed of weakly or non-calcified species, with small size, burrower/tubicolous habit, omnivorous or suspension feeders, and with direct development and brooding habit. In the other stations, heavily calcified forms, herbivore and herbivore/detritivore, and with medium (1–5 cm) and large (>5 cm) sizes prevail, showing indirect benthic and planktic development. The taxonomic analysis, coupled with functional aspects, increases our prediction of which traits could be potentially more advantageous for species to adapt to the hypothesized scenarios of OA, and identify present and future winner and/or loser organisms in the future ocean of the Anthropocene.