Bodies of waters in caves and in crevices of rocks are distinguished from the other subsoil water ecosystems (“eustygon”, “stygorhithron”, “stygopotamon”) under the names “troglostygon” and “petrostygon”. The colonisation of subsoil water biotopes involves a fundamental principle which controls the development of the main biotopes for the stygobiont undergroundwater organisms. According to this ecological rule, which is described in detail and formulated, the several interstitial biotopes (for example “eustygopsammal,” “rhithrostygopsammal,” “potamostygopsephal”) are to be considered as the real biotopes of the stygobiont subsoil water organisms; waters in caves, on the contrary, are secondary biotopes of these animals. Caves which contain marine water are described as ecostystem “Thalassotroglon” in their relation to “limnotroglon” (= “stygotroglon”). In this why the contact between “limnospeology” and “thalassospeology” is established, and the limnic and marine microcavernal biotopes - ”thalassopsammal” and “thalassopsephal” - are also taken in consideration. “Limnospeology” and “thalassospeology” as limnological and thalassological investigations of subsoil water are characterized as biological fields of work, which serve for the investigation of an ecological unit.