Animal life

The animal population of sandy shores mainly dwells below the surface, but includes some species which at times emerge to crawl or swim. Along the upper parts of some sandy beaches is a zone where the sand dries out and air penetrates during intervals between periods of submergence. This is the littoral fringe of the sandy shore, which in the British Isles is often inhabited by the burrowing amphipod Talitrus saltator, occurring in great numbers where there is much organic debris deposited along the strand line. It is an air-breather, usually restricted within a narrow strip of upper shore except when it moves over the surface for feeding at night (see Section 8.7.5, Vision). The strand line and adjacent sand also house a diverse population of insects, for example Bembidion laterale, Chersodromia arenaria and larvae of Coelopa frigida.

Lower on the shore the superficial layers of sand may dry briefly during low tide, but capillary forces hold a water table some height above sea level depending on the size of spaces between the sand grains. In sand containing much fine material the deposit remains waterlogged throughout the tidal cycle, but in coarse sands the water table may drop considerably as the tide recedes. Even where there is air in the sand its humidity is high, and burrowing creatures are not in appreciable danger of desiccation. The deeper-dwelling forms are also well insulated from surface fluctuations of temperature and salinity, which seldom produce much effect below a depth of a few inches. However, many burrowing animals descend deeper in the sand during winter than in summer. Populations of sandy beaches also exhibit considerable seasonal changes of biomass and composition associated with breeding and growth periods (Harris, 1972).

Factors which vary with shore level, and influence the zonation of burrowing populations, include the duration of submergence which for many species determines the time available for feeding and respiratory exchange; also the particle size of the substrate (Longbottom, 1970) (see Section 6.2.1) and the range of hydrostatic pressure. A general indication of the zonation of some sand-dwelling species is given in Figure 8.14 and some interconnections of the food web are illustrated in Figure 8.15.

Coarse sand

Wave action exerts a major control over the distribution of sand populations because it influences, directly or indirectly, many important features of the substrate, including its stability, particle size, gradient, drainage, oxygenation and organic content. Beyond a limit, strong wave action produces a virtually barren shore because no sizable organisms can withstand the crushing effects of deep churning in a coarse shifting beach. In less wave-beaten conditions the sand is more stable, and survival becomes possible first for creatures that can burrow in fairly coarse material and do not require any permanent tube or burrow. These forms must be sufficiently robustly built to withstand pressures from sand movement, or must be able to avoid them by burrowing at a safe depth, or

Figure 8.14 Zonation of some common inhabitants of intertidal deposits.

escaping by swimming. Coarse sand is also found in calmer conditions where the water is fairly fast-moving, for example the shores in tidal channels.

Among the inhabitants of the coarser sands of British shores are the molluscs Donax vittatus, Fabulina fabula, Lucinoma borealis, Ensis siliqua and Lunatia (Natica) alderi, the crustaceans Portumnus latipes, Nototropis swammerdami, Eurydice spp., Bathyporeia spp., Urothoe spp. and Haustorius arenarius, the echinoderms Astropecten irregularis and Echinocardium cordatum, and sand eels Ammodytes spp. Sandy shore pools often contain large numbers of shrimps (Crangon vulgaris) and small flatfish. Some sandy shores are important nursery areas for young plaice (Pleuronectes platessa), dabs (Limanda limanda) and soles (Solea solea), also sand gobies (Pomatoschistus microps) and whiting (Merlangius merlangius). The venomous weever fish (Echiichthys vipera), whose poison spines on its gillcovers and first dorsal fins can give the unwary an agonizing sting, burrows superficially in clean sand. It is an inhabitant of shallow water which sometimes occurs on the lower shore where there are abundant shrimps and small fish on which they feed.

Figure 8.15 Simplified food web of a depositing shore.

Muddy sand

With increasing shelter, movements of sand by wave action extend less deeply until only a superficial layer is disturbed. Below this the substrate is quite stable. Sheltered conditions permit the settlement of smaller particles and also favour the deposition of organic material, the latter providing food for a more numerous and diverse population than occurs in coarser, less stable sand. Sulphide-darkening of the subsurface layers indicates the activity of anaerobic bacteria in a deoxygenated medium. The burrowing population is then dependent upon oxygen obtained from the overlying water by drawing a current through the burrow, or extruding the respiratory organs above the surface of the sand. Where the surface dries at low tide, there must be some means of storing sufficient oxygen to survive this period.

The macrofauna of muddy sands of British shores includes the worms Arenicola marina, Lanice conchilega, Neoamphitrite figulus, A. edwardsi, Nerine foliosa, Notomastus latericeus, Scolecolepis fulginosa, Glycera convoluta, Perinereis cultrifera and Nephtys spp., the molluscs Cerastoderma (Cardium) edule, Venus spp., Venerupis decussata and V. pullastra, the holothurian Leptosynapta inhaerens, the crab Carcinus maenas and the shrimp Crangon crangon. There is also a diverse microfauna and meiofauna of flattened and threadlike forms living in the interstices of the sand (Harris, 1972; Swedmark, 1964) including the smallest known species from most of the invertebrate phyla.

Was this article helpful?

0 0

Post a comment