In the sea, the factors controlling the distribution of organisms can be classified as:
1. Biological: food supply, competition from other organisms, and sometimes protection by or dependence on other organisms.
2. Physical: the sedimentary environment, turbulence, currents, temperature, light intensity.
3. Chemical: salinity, water and substrate chemistry.
Some of these controls are related to depth (light, turbulence, temperature fluctuations and suspended organic matter all show a general decrease with depth), but hydrostatic pressure only has marked effects in the deepest environments, so depth alone is not a significant control.
Latitude is closely related to temperature, but temperature can vary according to the distribution of oceanic currents. Many marine organisms are restricted by temperature and other factors which may change with latitude, but latitude by itself is not a primary control.
Every animal needs to feed, and different methods of feeding can restrict animals to certain habitats. Marine animals (and it is these we are chiefly concerned with in this book) can be classified into four types of feeders (Raup and Stanley, 1971):
1. Grazers, which remove algal and other encrusting organic material from rocks and other hard surfaces.
2. Deposit feeders, which ingest sediment and feed on the organic matter coating the grains or mixed with the grains.
3. Suspension feeders, which select organic matter suspended in water.
4. Carnivores and scavengers, which eat other animals alive or dead.
Grazers normally inhabit rocky areas which are coated by organic films, they are thus generally confined to shallow water areas where bare rock is exposed, but they can occur in deeper water if suitable substrates are present.
Deposit feeders are often burrowers in muddy sediments, especially if the mud is mixed with a little sand or silt so that stable burrow systems can be developed. Organic matter in sediments normally occurs as a thin coating on individual grains of sediment. With decreasing grain size, the proportion of surface area in a given volume will increase. Thus fine grained sediments contain more organic matter and can support greater densities of deposit feeders. Fine grained sediments can occur in all depths of water.
The infauna are protected by their burrows; many of them have no external hard parts, and those that do often have thinner shells (or other external covering) than the epifauna exposed above the substrate. Many burrowing animals (especially worms and some decapod crustaceans) are thus rare as fossils although their burrow systems may be quite common. Deposit feeders often have characteristic complex burrow systems, which are developed as they search for food (these are in marked contrast to the simple burrows produced by infaunal suspension feeders).
Suspension feeders may be epifaunal, infaunal or pelagic. They include sedentary animals like corals, bryozoans, brachiopods, many bivalves and the crinoids, all of which have well-developed hard parts and are thus common as fossils. Their main food supply is probably diatoms and other protists which rely on photosynthesis, and which can thus only develop in abundance very near the surface of the sea (Ryther, 1963). The pelagic larval stages of many marine invertebrates also feed on these protists and live with them near the surface of the oceans. These larvae are also an important part of the marine food chain. Much more of this (protist and larval) food supply is thus available on the sea floor in shallow water areas; these are the areas, both today and in the Palaeozoic, where the majority of suspension feeders live.
Although well endowed with food supplies, the shallow water faunas have to contend with high environmental stresses, including much variation in sedimentary deposition rates, the unsettling effects of storms, and (except in low latitudes) fluctuations in temperature. So the only places for a quiet life (in a stable environment) are those where food is scarce. In the deep sea, there are sparse, but diverse, populations of specialized benthos. It is possible that some epifauna (e.g. spiriferide brachiopods) developed especially efficient feeding systems to cope with a limited food supply in these deep bottom environments.
Suspension-feeding epifauna usually require a stable anchorage, and are thus more common (at least since the Mesozoic) on stable sands and silts rather than on muds. Moreover, a large mud supply would tend to choke the filter-feeding structures of many bivalves. It has recently been suggested (Steele-Petrovic, 1975) that some brachiopods can tolerate a high mud flow through their feeding structures. This may account for the fact that in the Palaeozoic, when brachiopods were a major part of these epifaunal communities, there appears to have been a much lower correlation between an epifaunal community and the sediment on which it rested than there is at present time (Ziegler, 1965).
Rocky bottom communities consist mainly of suspension feeders and grazers; they are a mixture of epifauna and of infaunal boring animals. In the geological record they are restricted to beds immediately above depositional breaks and to hardgrounds. As far as we understand them at present, the animals inhabiting rocky bottoms have always formed quite distinct communities from those on areas of unconsolidated sediments.
Carnivores and scavengers feed on other animals. They can thus never be as abundant as suspension feeders and deposit feeders. Today, certain worms, echinoderms, gastropods and fish are the most important carnivores on the sea floor; similar forms are also present as far back as the Mesozoic. The direct evidence for early carnivores is very meagre; before fish became common in the sea and before gastropods developed radulae, soft-bodied worms, arthropods, medusoids and sea-anemones appear to have been the most probable carnivores and scavengers. If a carnivore species becomes specialized in its choice of food, it will naturally become a member of the animal community in the habitat where its food is situated. Unspecialized carnivores may, on the other hand, feed on members of several communities and thus extend over broader areas than any one species of the suspension feeders or deposit feeders on which they prey.
It can be concluded from the above discussion that Mesozoic, Tertiary and modern, bottom-dwelling communities are more intimately linked with sediment type than those in the Palaeozoic (except for some early Palaeozoic communities which were dominated by the deposit-feeding trilobites). All communities appear to be influenced to some extent by depth of water, but this influence has nothing to do with hydrostatic pressure. There is no question of depth control, but depth is correlated with greater food supply in shallow environments, and with uniform conditions on deeper sea floors. It will be observed that in this book the community names reflect this change with time. Those from the Ordovician to the Carboniferous are often named after a characteristic genus or species, while those in the Cambrian and from the Jurassic onwards are normally named after the habitat in which they occur.
Plankton and nekton may occur in all depths of water. The chief controls which govern their distribution are temperature, salinity and water currents. Their abundance can vary greatly with the distribution of dissolved nutrients in the water. Many migrate long distances during their life span, and seasonal changes in distribution are common in many groups. From the palaeontological point of view, plankton and nekton are not always very reliable indicators of the environment in which they are found fossil; many may drift for long distances after death. For example, Nautilus shells have been recovered from Madagascar and Japan, though living forms are only known between Australia, the Philippines and the Fiji Islands.
Salinity is largely independent of water depth or sediment. In most open seas the salinity is nearly constant at about 35 parts of dissolved salts to a thousand parts of water. But in isolated or semi-isolated seas and lagoons this may change a great deal: much of the Baltic Sea, for example, has a salinity of less than 10 parts per thousand, while in areas subject to high evaporation rates the salinity may increase to over 40 parts per thousand or much more.
When there is a change in "salinity (either upwards or downwards) only the more tolerant (euryhaline) species survive. Most marine species are not very tolerant of changes in salinity (they are stenohaline), so that the effect of any marked change is reflected by a reduction in the number of species present. But this does not necessarily mean that there are fewer animals; many brackish water regions contain an abundance of individuals of a very few species. It is often possible to determine salinity tolerance in extinct species. If a sedimentary formation changes from normal marine deposits into brackish water deposits, a series of fossil collections may then show a progressive reduction in the number of species present. In the Middle Jurassic of England, for instance, it is possible to deduce that ammonites are among the most stenohaline animals, and that certain oysters are among the most euryhaline.
In addition to the marine faunas, there are some aquatic organisms which have become adapted exclusively to fresh water. These are usually stenohaline, that is, they are restricted to fresh water; but some species are euryhaline and can extend their range into brackish environments, where they may occur with euryhaline marine species.
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