OSMOTIC AND IONIC REGULATION IN THE HORSESHOE CRAB<i>LIMULUS POLYPHEMUS</i>(LINNAEUS)

Abstract

1. Osmotic and ionic regulation of Limulus has been studied over a wide range of salinities. The blood is isosmotic in sea water of 100-200% (950-2084 milliosmoles) within 1.5%. It is hyperosmotic in dilute sea water of 10-66%. After a period of acclimatization in waters of 50-10%, some small horseshoe crabs (prosomal width about 7 cm) remained healthy in 5% sea water (48 milliosmoles. about 1.7‰ salinity) for 2-5 days, the blood in 4 specimens being kept at a level of 229-335 milliosmoles.2. Analyses of blood-serum as originally obtained and after dialysis against sea water give the following measure of ionic regulation: Na 101%, K 119%, Ca 91%, Mg 82%, Cl 100% and SO4 63%.3. Weight changes in waters of different salinity have been studied and the final water content of the muscles. The latter ranges from 67% in specimens from 200% sea water to 83% in specimens from 10%.4. From experiments with injected inulin solutions, the average extracellular volume in crabs of 100-634 g weight is 52% of the body weight.5. Estimates of extracellular water in the muscles (22.8% of total muscle water) have been used for calculations of intracellular concentrations of inorganic ions, phosphates and various non-protein nitrogenous compounds. Two-thirds of the osmotic concentration of the muscle cell is made up by the latter, of which free amino acids constitute only a small fraction.6. In whole muscle of specimens kept in 20% and 10% sea water the absolute amounts of ions and molecules fall in conformity with blood concentration, the ions increasing as a proportion of the serum concentration from 47% in 100% sea water to 58% and 63%, respectively. In 200% sea water ions in muscle as a proportion of those in serum also rise, to 62%.