Do current environmental conditions explain physiological and metabolic responses of subterranean crustaceans to cold?

Subterranean environments are characterized by the quasi absence of thermal variations (+/-1 degrees C within a year), and organisms living in these biotopes for several millions of years, such as hypogean crustaceans, can be expected to have adapted to this very stable habitat. As hypogean organisms experience minimal thermal variation in their native biotopes, they should not be able to develop any particular cold adaptations to cope with thermal fluctuations. Indeed, physiological responses of organisms to an environmental stress are proportional to the amplitude of the stress they endure in their habitats. Surprisingly, previous studies have shown that a population of an aquatic hypogean crustacean, Niphargus rhenorhodanensis, exhibited a high level of cold hardiness. Subterranean environments thus appeared not to be following the classical above-mentioned theory. To confirm this counter-example, we studied seven karstic populations of N. rhenorhodanensis living in aquifers at approximately 10 degrees C all year round and we analysed their behavioural, metabolic and biochemical responses during cold exposure (3 degrees C). These seven populations showed reduced activities, and some cryoprotective molecules were accumulated. More surprisingly, the amplitude of the response varied greatly among the seven populations, despite their exposure to similar thermal conditions. Thus, the overall relationship that can be established between the amplitude of thermal variations and cold-hardiness abilities of ectotherm species may be more complex in subterranean crustaceans than in other arthropods.