Behavioral, Ventilatory, and Metabolic Responses to Severe Hypoxia and Subsequent Recovery of the Hypogean Niphargus rhenorhodanensis and the Epigean Gammarus fossarum (Crustacea: Amphipoda)

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Abstract
Two aquatic amphipod crustaceans were investigated: Niphargus rhenorhodanensis (a hypogean species, which has to cope with severe hypoxic conditions during about 6 mo per year during the hydrological cycle) and Gammarus fossarum (an epigean species, which lives continuously in well-oxygenated water). The lethal times for 50% of the population ( ) for these closely) related species were 46. 7 and 6.3 h, respectively, in very severe hypoxia (<0.2 Torr) at 11°C. Therefore, the aim of this study was to examine some possible reasons why the subterranean Niphargus survived severe hypoxia longer than Gammarus and numerous other epigean crustaceans. During severe hypoxia Niphargus reduced its energetic expenditures compared with Gammarus. This reduction was associated with changes in locomotion and ventilation. In both species, anaerobic metabolism was fueled in severe hypoxia by the breakdown of glycogen and arginine phosphate. In Niphargus, however glutamate was also utilized. This resulted in the production of L-lactate as the major end product in both species. Alanine was found to be a minor end product in Gammarus, while alanine and succinate were found to be the minor end products in Niphargus. Compared to Gammarus, and to most other epigean crustaceans, Niphargus showed high amounts of stored glycogen and arginine phosphate. During severe hypoxia, both organisms excreted lactate into the medium, which is unusual for crustaceans. The differences in the stores of phosphagen and glycogen, and in behavioral and ventilatory responses, are probably the main reasons for their different resistance to severe hypoxia. During recovery, both species displayed a characteristic hyperventilation and their metabolism was predominantly aerobic. Gammarus excreted a great part of the lactate accumulated during severe hypoxia, whereas Niphargus remetabolized it, the latter possessing a higher glyconeogenesis capacity from lactate. After 24 h recovery, energy charge in both amphiods was not completely restored, and only Niphargus showed a total removal of the accumulated end products. Niphargus showed a faster replenishment of ATP concentration.

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