epilithic

Epilithic describes organisms that live on the external surface of rocks. The term is used in biology, geology, and ecology to refer to communities that colonize rock faces rather than growing inside or beneath them. It contrasts with endolithic organisms, which inhabit interior pore spaces or cracks, and with hypolithic organisms, which reside beneath the rock surface. Epilithic communities occur in terrestrial, freshwater, and marine settings.

Common epilithic organisms include algae (such as green algae and diatoms that inhabit submerged rock surfaces), lichens, cyanobacteria, and bryophytes. In marine environments, epilithic algae and coralline algae colonize intertidal and subtidal rock faces. In streams and rivers, epilithic biofilms develop on exposed stones and contribute to primary production. Terrestrial epilithic communities can form on sunlit cliff faces, boulders, and man-made stone structures. Adaptations often include strong adhesion through extracellular polymers, tolerance to desiccation and UV, and, in aquatic habitats, tolerance to fluctuating salinity and temperatures.

Ecologically, epilithic communities are important for rock surface processes and ecosystem function. They contribute to primary production and nutrient cycling in rocky habitats, initiate colonization of new surfaces, and influence chemical weathering through organic acids and microboring activity. These communities also create microhabitats that support a range of microfauna and invertebrates and can affect the stability and aesthetics of stone substrates in natural and urban environments.

Etymology and related terms: epilithic derives from Greek epi- “upon” and lithos “stone.” Related concepts include endolithic (inside rock) and hypolithic (beneath rock); together they describe the various life-positions of rock-dwelling organisms.