Effects of acute changes in salinity and temperature on routine metabolism and nitrogen excretion in gambusia (Gambusia affinis) and zebra fish (Danio rerio)

Acute stress may affect metabolism and nitrogen excretion as part of the adaptive response that allows animals to face adverse environmental changes. In the present paper the acute effects of different salinities and temperatures on routine metabolism, spontaneous activity and excretion of ammonia and urea were studied in two freshwater fi sh: gambusia, Gambusia affinis and zebra fi sh, Danio rerio, acclimated to 27 °C. The effects on gill morphology were also evaluated. Five salinities (0‰, 10‰, 20‰, 30‰ and 35‰) were tested in gambusia, while four salinities were used in zebra fi sh (0‰, 10‰, 20‰ and 25‰). Each salinity acute stress was tested alone or in combination with an acute temperature reduction to 20 °C. In gambusia, both salinity and temperature acute stress strongly stimulated urea excretion. Routine oxygen consumption was barely affected by acute salinity or temperature stress, and was reduced by the combined effects of temperature and high salinity. Gills maintained their structural integrity in all stressing conditions; hyperplasia and hypertrophy of mitochondria-rich cells were observed. In zebra fi sh, temperature and salinity acute changes, both alone and in combination, scarcely affected any parameter tested. The major effect observed was a reduction of nitrogen excretion at 20 °C–25‰; under these extreme conditions a signi fi cant structural disruption of gills was observed. These results con fi rm the high tolerance to acute salinity and temperature stress in gambusia, and demonstrate the involvement of urea excretion modulation in the stress response in this species.