Fauna cavernícola en zona litoral en la arenisca de Ulía (Formación Jaizkibel).

In the coastal zone of Mount Ulía (San Sebastian, Basque Country) there are enclaves with numerous coating caves, between and under big blocks of sandstone of Eocene age (Jaizkibel Formation). Some of them are in continuity with interstate galleries and mesocaverns, with zones in darkness or accentuated penumbra. This habitat is extensive and constitutes an ecotone or transitional zone by double entry, since it is located between the marine and terrestrial environment, and between surface habitats and hypogeums. Although little known, it has a considerable ecological and evolutionary interest, since it constitutes one of the paths followed for the colonization of the subterranean environment and differentiation of cave troglobitic species, aquatic and terrestrial. Explorations in the Bajo Aundi sector have allowed us to study terrestrial cave invertebrate populations, associated with marine species, which include isopods, arachnids, tysanura (Machiloidea y Zygentoma), collembolans and other groups. It describes the main features of this fauna and its curious ecology.