This chapter offers a summary of current knowledge of the major adaptations that characterize cave organisms, with special emphasis on their origin and evolution. Fauna that do not show typical ‘cave’ adaptations (blindness and depigmentation) are included, with an explanation of why they are so important for understanding cave biology.

What is a hypogean/cave organism?

Although some of the terminology commonly used in biospeleology is worthy of criticism, because it furthers typological thinking about nature, it is imperative to begin by clarifying some of the nomenclature frequently used in cave science. Appendix 1 contains a more or less complete list of terms commonly used in biospeleology.

In general, biospeleologists divide the world into two environments: the epigean and the hypogean ones. The former refers to the environment outside caves, which is exposed to light directly or indirectly on a regular basis; the latter represents any part of the biosphere that is found underground. The hypogean (sometimes called endogean) environment includes the following ecosystems: soil or interstitial (both are sometimes used interchangeably, and the latter term is often used by aquatic biologists only when dealing with spaces filled with water), phreatic or artesian, and cave. Phreatic waters are those water deposits in compact rocks that can be studied only indirectly through wells. These are extremely important: 97% of the world's freshwater is underground (Marmonier et al. 1993). The term ‘cave’ refers to an underground habitat that can be directly explored by humans (for further subdivisions, see Appendix 1).