b l u e crabs absorb Na a g a i n s t a gradient, b u t n o t those i n which crabs remain i n s a l t balance w i t h t h e medium. Net a c i d o u t p u t does n o t c l e a r l y vary w i t h s a l i n i t y . The balance between ammonia and urea o u tp u t s h i f t s towards ammonotelism a t : ow s a l i n i t y . These r e s u l t s , and the temporal responses o f b l o o d NH change, support the hypothesis tha? excess ammonia i n b l o o d p l a y s an i m p o r t a n t r o l e i n osmoregulation as w e l l as opposing t h e e f f e c t of s a l t r e d u c t i o n on t h e oxygen t r a n s p o r t system. The output+of ammonia increases i n d i l u t e waters i n which , Na+ and C 1 -a f t e r a s a l i n i t y I n t h e e s t u a r i n e b l u e crab C a l l i n e c t e s sapidus Rathbun, b l o o d PH v a r i e s i nv e r s e l y w i t h a c c l i m a t i o n s a l i n i t y . The decrease i n H+ c o n c e n t r a t i o n a t low sal i n i t y o l j p o s e s t h e e f f e c t o f s a l t r e d u c t i o n on t h e oxygenation o f hemocyanin, and t h e n e t r e s u l t i s an e n a n t i o s t a t i c c o n d i t i o n o f t h e oxygen t r a n s p o r t system (Weiland and Mangum, '75). c o m i t a n t changes i n b l o o d H+ and s a l t , i t was suggested t h a t (1) t h e e l e v a t e dblood PH a t low s a l i n i t y r e s u l t s l a r g e l y from t h e b i n d i n g o f H+ t o t h e NH3 p r oduced i n c a t a b o l i s m o f f r e e amino acids, as i n t r a c e l l u l a r f l u i d s e q u i l i b r a t e w i t h a d i l u t e blood, and ( 2 ) by evidence o f increased ammonia c o n c e n t r a t i o n s a t low s a l i n i t y (Gerard and G i l l e s , ' 7 2 ) and by t h e c o r r e l a t i o n o f H+ changes w i t h osmotic and s a l t con-I n a study o f t h e r e s p i r a t o r y consequences o f con-129

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In the estuarine crab, Callinectes sapidus, net NH4+ efflux was twice as high in animals acclimated to 17% salinity seawater (SW) (0.495 +/- 0.084 mumol . h-1 . g wet wt-1, n = 7) than in animals acclimated to full-strength 35% SW (0.212 +/- 0.028 mumol . h-1 . g-1, n = 7). Amiloride (3 X 10(-4) M) in the external SW reversibly inhibited these effluxes by 63 +/- 6% (n = 6) and 67 +/- 6% (n = 5), respectively. Unidirectional Na+ influx was reversibly inhibited 42 +/- 6% (n = 7) by amiloride in 17% SW-acclimated crabs and 49 +/- 7% (n = 8) in 35% SW-acclimated crabs. This mutual sensitivity to amiloride is evidence for a Na+/NH4+ exchanger. Inhibition of unidirectional Na+ efflux by Na+-free SW indicates the presence of an obligate Na+/Na+ exchange component that accounts for at least 42% of the Na+ flux and is also amiloride sensitive. The lack of inhibition by amiloride of the net H+ efflux does not support the presence of a Na+/H+ exchanger. Acclimation of the crab to dilute SW may involve an increase in the activity of the Na+/NH4+ exchanger in the gills, but this mechanism contributes only a small fraction of the total Na+ transport.