Groundwater - an unknown habitat

The River Styx is one of the five rivers in the underworld of Greek mythology and a synonym for all subsurface waters. In the past it was unknown that these waters are teeming with life.

Only recently it was realised that groundwater is a habitat for a diverse fauna that evolved special features for life within the dark interstices between the grains of sand. Refering to the River Styx the invertebrates which spend their whole lifes in subsurface waters are called stygobionts. They are found not only in porous groundwater aquifers of river valleys but also in karstic areas where caves of different sizes may be found. Stygobionts are able to penetrate into man made habitats such as wells and slow sand filters.

The stygobionts show a remarkable set of morphological and physiological characteristics which can be interpreted as adaptations to life in groundwater. The most apparent insignia are the depigmented and vermiform body and the lack of eyes. Compared with related organisms in surface waters the stygobionts have very long lifes (15 times longer and some even more) and all developmental stages are prolonged.Their metabolism is reduced and the reproductive output is small compared with surface water organisms. But due to the long life span of stygobionts the absolute number of offspring may be similar to that of surface water organisms.

The main property of the groundwater ecosystem is the paucity of vitally important resources. Stygobionts are able to withstand adverse situations very well by adopting an appropriate strategy, the so called 'adversity-selection model' or A-type strategy. They are resistant against starvation and low oxygen conditions. Due to the lack of light no primary production takes place in the groundwater ecosystems. As a result the groundwater food webs are almost heterotrophic. The production depends on the input of organic matter from the surface. Unpolluted groundwater systems have scarce trophic resources and their biocoenoses are delineated by simple and few trophic links. Most of the organic matter entering the groundwater habitats is mineralised by microorganisms (e.g. bacteria, fungi). The microbes, on the other hand, serve as food for secondary consumers such as protozoans, nematodes and crustaceans. Predatory invertebrates mark the end of the food web. Although differently spezialised feeding types and/or feeding habits may be found it is important to realise that most stygobionts are omnivores and not spezialised feeders.

Guiseppe Pesce provides information and links on his Groundwater biology home page .