Marine Biology

We now consider the physical conditions and the associated biology in these various life-zones.

The Littoral Zone.

Wherever there is a marked tidal rise and fall of the sea, and not too great a slope of the sea bottom, there is a littoral zone. There are coasts formed of steep cliffs and here we can usually see a more or less vertical face of rock where the sea level rises and falls and where there are different kinds of attached organisms according to the level. Often, how ever, a steep coast has a rocky terrace at the foot of the cliffs and this flat is covered and uncovered by the tide. It is rocky with ledges, pools and boulders and there are abundant algae, zoophytes, sea anemones, Polyzoa and shore fishes. The nature of the rocks that form the cliffs may sometimes be seen to affect the fauna and flora. There are other coasts that are low and consist of materials that are easily eroded, and here there is a tendency for the for mation of beaches consisting of gravels and sands; such a fore shore is relatively bare of life and the common littoral organisms may be a few Algae, with barnacles, limpets, periwinkles, mussels and little else. More often, perhaps, the coast is a low one that has been subject to prolonged marine denudation, so that exten sive sandy flats are formed. In spite of the variable nature of the coastal materials such a foreshore generally consists mainly of quartz sand, for this is an irreducible residue resulting from erosion. Such sandy flat foreshores may have very considerable extent, occupying the greater parts of bays and estuaries. The sandy zone is broken by shallow and variable channels. Mud and quicksand may be present. In such regions the forms of life are not at all obvious, nevertheless there may be great numbers of various kinds of organisms; usually there are lamellibranch mol luscs (such as cockles) in the sand; perhaps worms (such as Arenicola) which live in burrows, and everywhere in the inter stices of the sand grains there are Diatoms and other unicellular organisms. Sometimes, as off the coast of England, there may be quite important sand fisheries (for cockles).

The variety of the conditions on the foreshore is so great that we can only suggest it here. Reef-like formations may be set up

by worms which make stiff, sandy tubes. In the tropics there are huge regions of foreshore of disintegrated coral rock, all of which has been built up by organic action. In cold and temperate lati tudes large Algae tend to be prominent in the lower levels of the littoral zone, but there is an obvious scarcity of these in tropical climes where coral reefs exist, the reason being that photosyn thesis (see later) is there carried on by the coral polypes. Gen erally the characteristic faunas and floras are affected, to some extent, by the nature of the adjacent land.

Organisms that live on the littoral zone must adapt themselves to a greater range of physical conditions than do those that live in the adjacent Laminarian zone. They must be able to live out of water and so crustacea (like the barnacles) or molluscs (like mussels, periwinkles or limpets) must be able to close their shell so as to prevent their organs of respiration from being dried up when the tide is out. Respiration ceases during those hours. Molluscs like cockles or worms (such as the lug) must be able to burrow into the sand when the tide has ebbed, and many small worms and other invertebrates also do this. A rhythmic habit of sand-burrowing may thus be established and the organ isms often exhibit this rhythm, moving up and down in the sand even when they are removed from the shore and kept in an aquarium in sand covered by water kept at a constant level. Then there are much greater variations of temperature on the f ore shore than in the sea, for the exposed sand becomes heated to a greater extent by the sea during the summer, or cooled to a greater degree during the winter. The salinity of the water on the littoral zone also shows extreme variations because it is often diluted by streams and rivers entering the sea and rainfall affects it more than the sea. Heavy storms tend to shift the sand and other materials of the sandbanks and beaches, and wave action is maximal here. Therefore the littoral organisms tend to adapt themselves to such shocks : limpets and barnacles, for instance, cling very tightly to rocks and stones, and many worms form sandy reefs into which they burrow.