The Vertical Extent of Groundwater Metazoans: An Ecological and Evolutionary Perspective

We investigated the upper and lower vertical limits of the distribution of inhabitants of the most abundant freshwater habitat—groundwater. Distribution in photic habitats is limited by competition, predation, and risks of exposure to ultraviolet radiation. Nonetheless, a number of eyeless, depigmented subterranean species occur in twilight habitats, taking advantage of the higher food resources available and modulating their distribution by photophobic behavior. We argue that the upper boundary is an interesting system in the study of classic ecological and evolutionary questions. The lower boundary of the distribution of groundwater species (approximately 2000–4000 meters) is likely controlled by physicochemical parameters, including temperature, pressure, and oxygen. The lower boundary warrants further research, and it is one of the most poorly explored areas of the biosphere.