THE WATER ECONOMY OF THREE SPECIES OF THE AMPHIBIOUS CRAB, POTAMON

Abstract

1. The rate of water loss in air from three species of the fresh-water crab Potamon has been determined at various relative humidities and a constant temperature of 20° C. Water is lost, not only from the gill cavity but also through the integument. The rate of water loss decreases with desiccation. This decrease is probably due to decreasing permeability of the integument as desiccation progresses.

2. Of the three species studied, P. sidneyi has the lowest rate of water loss. There is no significant different between the rate of water loss of P. depressus and P. warreni. Evidence is presented that this greater resistance of P. sidneyi to desiccation is not due to a difference in permeability of the integument.

3. The survival time and the water loss at death have been determined for all three species. The lethal water loss is the same in all conditions of desiccation studied. P. sidneyi has a higher lethal water loss than the other two species. This, together with its lower rate of water loss, results in it having a longer survival time in dry air than the other species.

4. The lethal water loss of Potamon is low compared with most other terrestrial arthropods. Death is due to water loss and not to asphyxiation following desiccation of the respiratory surfaces.

5. Potamon depressus, when offered a choice, prefers a more humid environment. The intensity of the preference shown is affected both by the magnitude and absolute value of the gradient offered.

6. Evidence suggests that the intensity of the humidity preference of fully hydrated P. sidneyi is less than that of P. depressus or P. warreni.

7. The percentage of time spent in water by crabs offered a choice of water or ‘land’ has been determined. P. sidneyi spends more time in the water than do the other two species. In conditions of alternating light and dark, P. sidneyi spends relatively more time in water during the light periods: the behaviour of the other two species is not affected by this factor.

8. The rates of water uptake, following desiccation, of normal and eyestalkless Potamon sidneyi have been compared. In winter removal of the eyestalks increases the rate of water uptake: in summer it does not. The rate of water uptake of normal summer crabs is similar to that of eyestalkless winter crabs.

9. The water drive of eyestalkless winter crabs is probably less than that of normal crabs.