The term 'mud' is loosely applied to deposits containing a high proportion of silt or clay particles (see Section 6.1.1). The fine particles settle only from still water, and shores of mud are therefore found where conditions are normally calm and without strong currents, for example within sheltered bays, at the landward end of deep inlets and in river mouths where conditions grade from marine to estuarine. The upper levels of a muddy shore often merge into salt marsh where cord grass (Spartina) and other halophytes become established. Except for drainage channels, these shores have little slope, and extensive areas of 'mud flat' may be exposed as the tide recedes. The mud surface seldom dries appreciably, and a layer of standing water may be left at low tide. In these conditions organic debris readily settles, and the organic content of the mud is correspondingly high, often about 5 per cent of the total dry weight. The deposit usually compacts into a soft, stable medium, easy for burrowing, in which permanent burrows and tubes remain undisturbed. Because there is little exchange of interstitial water the mud beneath the extreme surface layer becomes completely deoxygenated and its sulphide content is high. The infauna therefore encounter problems of deficient oxygenation and clogging of respiratory and feeding organs by fine particles. The high organic content of the substrate is a potentially rich source of food. In comparison with sands, the populations of intertidal muds are usually less diverse but often of much greater biomass.
The population of seashore mud is made up of forms which can readily tolerate silt, among which are the worms Neanthes (Nereis) virens, Sabella pavonina and Capitella capitata, the clam Mya truncata, the anemone Cereus pedunculatus and the crustaceans Upogebia deltaura and Callianassa subterranea. The populations of muddy sand merge gradually into those of finer mud, and the muddy shore population often includes the worms Arenicola marina, Amphitrite spp., and
Marphysa sanguinea and the bivalves Cerastoderma edule, Mya arenaria and Venerupis spp. The gastropods, Nassarius reticulatus and Ocenebra erinacea, both to be found on rocky shores, occur also in soft mud, ploughing their way through the superficial layers with their long siphons extruding above the surface. Hydrobia ulvae, Hediste (Nereis) diversicolor and Macoma baltica are essentially estuarine forms which are also widely distributed in intertidal mud.
Sand and mud shores have a special ecological importance as feeding grounds for a great variety of wading birds. Also certain remote areas of mud and sand flats, such as occur in the Wash, form breeding grounds for the common seal, Phoca vitulina. Unlike the grey seal (see Section 8.8.2), the pups of this species can take to water within a few hours of birth. A gravid female can land on an exposed mud bank to calve, and the young seal is able to swim off with its mother by the time the tide returns.
Was this article helpful?
The Reasons Behind And The Plan To Build A Healthy Ego. It is important for us all to ensure that our egos are strong and healthy. Our ego is our identity. It is who we believe ourselves to be and as such is a reference point and our “home in the world.” The ego is individuality. As our identity it sets us apart from other people’s identity. Get your ego in order here.