BEHAVIORAL, VENTILATORY, AND METABOLIC RESPONSES TO SEVERE HYPOXIA AND SUBSEQUENT RECOVERY OF THE HYPOGEAN NIPHARGUS-RHENORHODANENSIS AND THE EPIGEAN GAMMARUS-FOSSARUM (CRUSTACEA, AMPHIPODA)

Two aquatic amphipod crustaceans were investigated: Niphargus rhenorho danensis (a hypogean species, which has to cope with severe hypoxic co nditions during about 6 mo per year during the hydrological cycle) and Gammarus fossarum (an epigean species which lives continuously an wel l-oxygenated water). The lethal times for 50% of the population (LT(50 )) for these closely related species were 46.7 and 6.3 h, respectively , in very severe hypoxia (<0.2 Torr) at 11 degrees C. Therefore, the a im of this study was to examine some possible reasons why the subterra nean Niphargus survived severe hypoxia longer than Gammarus and numero us 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 o f glycogen and arginine phosphate. In Niphargus, however glutamate was also utilized This resulted in the production of L-lactate as the maj or end product In both species. Alanine was found to be a minor end pr oduct in Gammarus, while alanine and succinate were found to be the mi nor 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 excrete d lactate into the medium, which is unusual for crustaceans. The diffe rences in the stores of phosphagen and glycogen, and in behavioral and ventilatory responses, are probably, the main reason for their differ ent resistance to severe hypoxia. During recovery, both species displa yed a characteristic hyperventilation and their metabolism was predomi nantly aerobic. Gammarus excreted a great part of the lactate accumula ted during severe hypoxia, whereas Niphargus re-metabolized it, the la tter possessing a higher glyconeogenesis capacity from lactate. After 24 h recovery, energy, charge in both amphipods was not completely res tored, and only Niphargus showed a total removal of the accumulated en d products. Niphargus showed a faster replenishment of ATP concentrati on.