Complex sentence: Stygofauna are any fauna that live within groundwater systems, such as caves and aquifers, or more specifically small, aquatic groundwater invertebrates, though terrestrial air-breathing subterranean animals are also sometimes included.
Split into simpler sentences: Stygofauna are any fauna that live within groundwater systems, such as caves and aquifers. More specifically, it often refers to small, aquatic groundwater invertebrates, though terrestrial air-breathing subterranean animals are also sometimes included.