SALT AND WATER BALANCE IN TWO MARINE SPIDER CRABS,LIBINIA EMARGINATAANDPUGETTIA PRODUCTA.III. SOME FACTORS INVOLVED IN SHORT-TERM ADAPTATION TO A DILUTE MEDIUM

Abstract

1. Libinia emarginata and Pugettia producta respond to 80% sea water (SW) in similar ways. In both species, during the first hour of exposure to 80% SW an increase in volume, 0.56 and 1.4% body weight (bw) for Libinia and Pugettia respectively, accommodates the major portion of the influx of water.2. Urine production increases from about 6 to 30% bw/day in both species during the first hour of exposure to 80% SW. Elimination of salts and water via the isosmotic urine helps to decrease the potential for an increase in hydrostatic pressure. The hydrostatic pressure in Libinia changes from 7.5 to 14 cm H2O during the first hour of exposure to 80% SW.3. In Pugettia, most of the ions in the blood decrease at a rate proportional to the concentration difference between the blood and the medium. The ordered rates of concentration changes are as follows: potassium > calcium > sodium ≃ chloride >> magnesium. Blocking the nephropores has a greater effect on the rate of magnesium concentration change than on the other ions. With the exception of magnesium, all other ions which were measured are lost from the blood mainly via extra-renal routes.4. During the second hour of exposure to 80% SW, the volume of both species declines, probably as a result of the increase in urine production and the reduction in the rate of water influx caused by the reduction in the concentration gradient. By contrast, the volume of nephropore-occluded crabs of both species continues to increase, reaching at least 5% bw after 4 hr of exposure to 80% SW.5. When transferred from 80 to 100% SW, specimens of Libinia and Pugettia lose 0.75 and 1.8% bw, respectively. Blocking the nephropores has no effect in this case and the gain in salts must take place entirely across the body surface, since urine flow is unidirectional.