The detrimental effects of exposure to air restrict much of the littoral population to sheltered parts of the shore which are not left completely uncovered at low tide. Many algae, anemones, hydroids, bryozoans, prawns and fish occur only in rock pools where they are safe from drying, and relatively protected from wide temperature fluctuations. Shelter can also be found between the fronds and holdfasts of the shore algae, under stones and boulders or in rock crevices. Some of the most numerous shore animals have a flattened shape well suited for hiding in narrow spaces, for example the crab Porcellana platycheles, the leptostracan crustacean Nebalia, amphipods, isopods and chitons. Others burrow into the shore deposits for protection, and these comprise virtually the entire population of sandy and muddy shores.
Environments with fluctuating conditions favour the evolution of species that exhibit wide variations, both physiological and anatomical. Where an unstable habitat also includes environmental gradients, different forms of a species are likely to occupy different zones. On the seashore it is obvious that many of the dominant species are highly variable, with differences which can to some extent be correlated with differences of zonation. A notable example is Littorina saxatilis which shows great variety of shell size, colour, thickness, shape and surface texture (see Section 8.8.1, page 284). Other common polymorphic species on the shore include Littorina obtusata, Nucella lapillus (Kitching, 1977) and Mytilus edulis, with some evidence of different forms in different localities.
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