Water loss

Organisms which live on the shore surface in situations where they are frequently uncovered to air must have a protective covering to prevent excessive drying. The exposed algae have mucilaginous outer layers which reduce evaporation (Chapman, 1979). The beadlet anemone, Actinia equina, often inhabits uncovered rocks and is coated with slimy exudations which presumably help to conserve water. Most of the surface-dwelling animals have a strong shell, the

Table 8.1 Uric acid content of nephridia of Littorina spp. (From Nicol, 1967 by courtesy of Pitman)

Species

Dry weight (mg/g)

Melaraphe (Littorina) neritoides

25

Littorina saxatilis

5

Littorina obtusata

2.5

Littorina littorea

1.5

orifice of which can be kept closed while they are uncovered. The limpet, Patella, draws down the rim of its shell so close to the rock surface that only a very narrow gap is left, sufficient for oxygen and carbon dioxide to diffuse through without allowing much evaporation. Winkles, top shells, dog whelks and serpulid worms close the shell aperture with an operculum. In barnacles, the movable plates of the shell, the terga and scuta, are kept shut most of the time the animals are uncovered, occasionally opening momentarily for renewal of the air enclosed within the shell. To some extent animals adjust to the drying conditions at different shore levels by metabolic adjustments. For instance, specimens of Patella vulgata living at high shore levels during summer have lower respiratory rates, lower rates of water loss and can tolerate greater percentage water losses than those low on the shore (Davies, 1966-70).

In addition to losses by evaporation, animals also lose water by excretion. The majority of marine creatures excrete ammonia as their chief nitrogenous waste product. This is a highly toxic substance which has to be eliminated in a very dilute urine, involving the passage out of the body of a copious amount of water. On the seashore, this must present a difficulty to animals already in danger of desiccation, and some of the littoral gastropods reduce their excretory water-loss by excreting appreciable amounts of uric acid, a less soluble and less toxic substance than ammonia which can be excreted as a semi-solid sludge, thereby conserving water. The different species of Littorina form a series, those which live highest on the shore excreting the greatest amount of uric acid in the most concentrated urine (Table 8.1).

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