ABSTRACT
In Carcinus, in dilute media, internal volume and pressure were found to be regulated by the intermittent release of urine which maintained internal pressure at a mean value of 15 cm water, compared to a mean value of 10 cm for crabs in sea water. Following transfer of crabs from 100 % to 50% sea water, urine production (measured by a continuous collection method) was observed to increase within 5 min of dilution of the medium, by which time the estimated increase in internal pressure would have been only 1·3 cm water. Such a pressure increase is considered inadequate to stimulate a rise in urine production, since artificially raising internal pressure of crabs in sea water did not result in an increase in urine flow, until the pressure was in excess of 20 cm water.
The results support the hypothesis that the stimulus to increase urine production by crabs in dilute media is not raised internal pressure due to osmotic influx of water. An alternative hypothesis, namely that of changes in urine production being in response to direct monitoring of the external environment, is suggested.
