Desiccation

Abstract

When aquatic animals colonised the terrestrial habitat they had to respond to a number of constraints posed by the replacement of the aqueous external medium by air. These included (1) loss of physical support by the surrounding medium, (2) respiratory problems associated with the collapsible nature of aquatic gills exposed to air, (3) difficulties in the excretion of nitrogenous by-products of protein metabolism, (4) evaporative water loss, (5) the necessity for animals to seek out water to replace that lost by evaporation from the body surface, by respiration, excretion and defaecation. The last two constraints are closely linked and stem directly from the desiccating tendency of air. In air of less than 100 per cent relative humidity all animals tend to lose water by evaporation. Kensler (1967) pointed out that if the surface of an animal was subjected to unrestricted evaporation, the rate of water loss would be proportional to the vapour pressure of water (p₀ mmHg) at the experimental temperature. Equally, the rate of condensation of water on to the animal’s surface would be proportional to the vapour pressure of water in the air surrounding the animal (i.e. p₀ × relative humidity RH). The net loss of water to the environment (evaporation — condensation) will therefore be proportional to ø (the ‘desiccation potential’ or ‘saturation deficit’ in mmHg) where:

$$\varphi = p_0 (1 - {\rm{ \backslash textupRH}})$$

((4.1))

and an RH value of 1 corresponds to 100 per cent relative humidity.