Causes and consequences of excess resistance in cryptobiotic metazoans.

Despite more than 200 yr of recognition that some microscopic metazoans survive environmental conditions far beyond those experienced in nature while in a cryptobiotic state, this phenomenon has received little attention from evolutionary biologists. The excess environmental resistance exhibited by cryptobiotic organisms cannot be viewed as an adaptation within current evolutionary biology. Rather, excess resistance may have evolved as a by-product of natural selection for tolerance to desiccation or other naturally occurring environmental agents. The combined effects of desiccation, metabolic arrest, effective stabilization of dry or frozen cells by protectant molecules, and efficient DNA repair mechanisms may have led to a protection of the organism against conditions far beyond those experienced in nature.