Littoral caves: unique biodiversity hotspots and potential climate refugesUnderexplored Reefs

Jacques Grall¹, Anaelle Bizien², Anna Capietto³, Mathieu Helias^2,4, Sylvie Castay², Jeanne Cassandre², Edouard Paillard², Jean-André Pratt⁵, Nicolas Royal², Thomas Burel^2,1

¹Insitut Universitaire Européen de le Mer, Université de Bretagne occidentale, France - ²LEMAR IUEM, France - ³PNRA, France - ⁴Institut National des Sciences et Techniques de la Mer (CNAM Intechmer), France - ⁵Parc National Marin d’Iroise, France

Littoral caves are natural hollow structures formed in coastal rocks through wave action, erosion, or geological activity. Despite variations in size and shape, these caves create unique habitats characterized by stable temperatures, low light, and limited water circulation. These conditions support highly specialized reef habitats, including communities of sublittoral origin within the intertidal zone. We address the lack of quantitative studies on the biodiversity of marine caves, focusing on identifying associated species, understanding biodiversity organization, and investigating whether physical cave characteristics (shape, depth, orientation) explain observed patterns. In the context of climate change and increasing heatwave frequency, we explore how marine caves might act as refuge habitats for rare and uncommon taxa. Sampling involved random quadrats and measurements of light intensity and rock temperature. A total of 195 fauna and 90 macroalgal species were recorded, predominantly sessile fauna (sponges, bryozoans) and red algae. Results revealed three macroalgae-dominated zones structured by light gradients: (1) the entrance zone, with high light levels supporting photophilic species, consumers, and predators; (2) a medium-light zone (25%-3%), harboring sciaphilic species and grazers; and (3) a low-light zone (3%-1%) dominated by encrusting algae (e.g., Lithophyllum) and sessile fauna (sponges, barnacles). Beyond light, pool size significantly influenced community composition and organization. Biotic interactions, such as competition and the presence of engineer species, also shaped these communities. Theese outcomes highlight the potential of littoral caves to provide microhabitats that shield subtidal species from stressors like heatwaves or direct human pressure. Which They finally emphasizes the ecological importance of these habitats, encouraging further study into their role as biodiversity reservoirs and refuge zones.