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1 Oregon Institute of Marine Biology, University of Oregon, Charleston, Oregon 97420
1. The crab Hemigrapsus nudus regulates the osmotic pressure of its blood in media down to less than 60% seawater, and is iso-osmotic in 100% seawater and higher salinities.
2. Measurements of simultaneous uptake of tritiated water (THO) and deuterated water (DHO) give uptake values for THO about 96% those obtained with DHO.
3. The Q10 of uptake of both isotopes is about 1.62, and the relation of uptake of both to body weight is similar.
4. The results are consistent with, but do not prove, the concept of a small isotope effect in the uptake of THO and DHO. Published reports of higher water fluxes based on methods not involving isotopes of water are consistent with the argument for an isotope effect. It is suggested that water fluxes based on methods using THO:DHO:H2O as ordinarily employed are of the relative magnitudes 91: 95:100.
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