EFFECTS OF AMBIENT AMMONIA LEVELS ON BLOOD AMMONIA, AMMONIA EXCRETIONAND HEART AND SCAPHOGNATHITE RATES OF NEPHROPS-NORVEGICUS

During commercial handling of Nephrops norvegicus (L.) there are a num ber of situations when the prawns may be exposed to very high ambient ammonia levels. These experiments evaluated the effects of increased l evels of ambient total ammonia (TA = NH3 + NH4+) on blood ammonia, amm onia efflux rates and on the cardio-ventilatory performance of N. norv egicus. When prawns were taken from <1 to 2000 mu mol TA l(-1) medium, blood TA concentrations increased rapidly for the first 2 h but tende d to drop thereafter. Original blood TA levels were restored 6 h after the prawns were transferred back from seawater containing 2000 to <1 mu mol TA l(-1). Sudden exposure to 500, 1000, 2000 or 4000 mu mol TA l(-1) medium induced blood TA concentrations to increase respectively to 50, 30, 33 and 36% of external concentrations (normally, internal T A values are much higher than external levels). Immediately after tran sfer back to seawater with low ammonia concentration (<1 mu mol TA l(- 1)), excretion rates were higher than those of control prawns, and the absolute amounts of TA excreted were considerably higher than those c alculated to have accumulated in the haemolymph. Heart rate (HR) and s caphognathite rate (SR) were not altered when prawns were subjected to sudden alterations in ambient ammonia (<1 to 2000 to <1 mu mol TA l(- 1)). When water ammonia concentrations were altered more gradually, bo th rates increased, but only at 4000 mu mol TA l(-1). These results sh ow that N. norvegicus is able to remove ammonia from the haemolymph an d/or transform ammonia into some other substance when subjected to inc reased levels of ambient ammonia. Possible mechanisms involved (e.g. a ctive transport across the gills; storage in some other tissue; glutam ate synthesis) are discussed.