The Excretion of Magnesium by Carcinus Maenas

Abstract

1. Measurements have been made of the concentrations of sodium, calcium and magnesium in the blood and urine of Carcinus maenas after transfer to 50 or 150% sea water. Inulin clearance studies were also made. 2. Magnesium is concentrated in the urine by a secretory process when the crabs are in 100% sea water or 150% sea water. There is evidence that the rate of secretion declines when the crabs are in 50% sea water or in 100% magnesium-free sea water. 3. There is no evidence for active withdrawal of magnesium from the urine. When the blood is depleted of this ion the U/B for magnesium drops to a value similar to the U/B for inulin. 4. The clearance of inulin is faster when Carcinus is in 150% sea water than when it is in 100% sea water, and it is calculated that the animal excretes magnesium at a faster rate than it does in 50% sea water. 5. The excretion of magnesium by Carcinus and Pachygrapsus is compared, and it is shown that magnesium is conserved more effectively in dilute media by Carcinus than by Pachygrapsus. This probably reflects differences in the ecology of the two species.