SODIUM TRANSPORT lN THE FRESHWATER ASIATIC CLAM CORBICULA FLUMINEA

¹ Department of Zoology and Physiology, Louisiana State University, Baton Rouge, LA 70803

The Na transport mechanism was examined in pondwater-acclimated (PW) and salt-depleted (SD) specimens of Corbicula fluminea. The Na influx in 0.5 mM Na₂SO₄ of 7.90 ± 0.79 µM Na/(g dry tissue·hr), higher than most freshwater animals, increased to 18.53 ± 2.10 µM Na/(g dry tissue·hr) in SD animals.

Saturation of the transport system is typical of Michaelis-Menten enzyme kinetics. Maximum influx of PW clams was 12.90 ± 3.01 µM Na/(g dry tissue·hr), with a K_m of 0.05 mM Na/l. The maximum rate in SD clams was 28.66 ± 2.17 µM Na/(g dry tissue·hr), with little change in K_m.

Sodium movement in C. fluminea may be partitioned into passive diffusion, excretion, exchange diffusion and active transport. Exchange diffusion comprises a substantial portion of Na movement: 5.91 ± 0.80 µM Na/(g dry tissue ·hr) in PW animals and 16.05 ± 0.67 µM Na/(g dry tissue·hr) in SD clams. Passive inward diffusion of Na was 0.50 µM Na/(g dry tissue·hr) for PW clams and 1.17 µM Na/(g dry tissue·hr) for SD clams.

The primary exchange ion for Na is H, although a Na/NH₄ exchange is functional in SD animals. In PW clams, 2.41 µM H/(g dry tissue·hr) is transported in a 1:1 exchange with Na. In SD clams, the net NH⁴ flux quadrupled contributing to a Na: (H + NH₄) exchange.