Microbial mats discovered in a ground-water ecosystem in southern Romania contain chemoautotrophic bacteria that fix inorganic carbon, using hydrogen sulfide as an energy source. Analysis of stable carbon and nitrogen isotopes showed that this chemoautotrophic production is the food base for 48 species of cave-adapted terrestrial and aquatic invertebrates, 33 of which are endemic to this ecosystem. This is the only cave ecosystem known to be supported by in situ autotrophic production, and it contains the only terrestrial community known to be chemoautotrophically based.

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Reduction and total loss of characters are common evolutionary phenomena.Vestigialization of any morphological, physiological, or behavioral feature can be expected upon relaxation of selection on the trait. Direct selection of vestigialization is rarely documented. Most explanations of evolutionary re ductions invoke indirect selection through energy economy or antagonistic pleiotropy arguments, while some invoke the effects of accumulation of neutral mutations. A few documented cases of trade-offs between fitness and wing reduction or pesticide resistance in some insects, and between fitness and resistance to phages or antibiotics in bacteria suggest that indirect selection is a plausible mechanism for evolutionary reductions. Expression of presumably useless genes suggests that neutral mutation arguments require a longer time than is available for the observed reductions. Rapid decay of useless behaviors may require explanations in terms of trade-offs among neural pathways for information processing.

The amphipod Gammarus minus is present in both caves and springs, with cave populations showing elaborated (size and antennae) and reduced (eye) characters relative to spring populations. Earlier studies have shown that cave populations resulted from independent invasions of hydrologically isolated subterranean drainages and that there is genetic variation for both elaborated and reduced characters. In this study we tested the hypothesis that a similar pattern of selection on isolated cave populations is responsible for the parallel evolution of cave morphologies. We used variation in mating success and fecundity to test for the presence of directional selection on eye, antennal, and body size characters in a set of cave and spring populations during a series of seasonal cross-sectional samplings. We found significant directional selection for smaller eyes in caves and larger eyes in springs, which supports the hypothesis that selection is responsible for reduced eye size in cave populations. We also found selection for larger body and antennal size in cave populations, which is consistent with the hypothesis that parallel patterns of selection in caves are responsible for the elaboration of body and antennal size. However, we found selection for larger body and antennal size in spring populations that is not consistent with the observed divergence of spring and cave populations. We suggest that unmeasured components of viability selection could be more important in springs than in caves and may act against the selection for larger size found in spring populations.

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