General Character of Populations of the Primary Biotic Divisions
Under the previous headings we have dealt with the classification of the marine environment. For purposes of future discussion it is desirable at this point to outline briefly a broad, highly practical classification of the marine population inhabiting the above primary biotic divisions, a classification based not on natural phylogenetic or taxonomic relationships, as given on p. 282, but rather on an artificial basis, grouping heterogeneous assortments of organisms depending upon common habits of locomotion and mode of life and upon common ecological distribution.
On these grounds the population of the sea may be divided into three large groups—namely, the benthos, nekton, and plankton, the first belonging to the benthic region and the other two to the pelagic region.
In the benthos (Gr., deep or deep-sea) are included the sessile, creeping, and burrowing organisms found on the bottom of the sea. Representatives of the group extend from the high-tide level down into the abyssal depths. The benthos comprises (1) sessile animals, such as the sponges, barnacles, mussels, oysters, crinoids, corals, hydroids, bryozoa, some of the worms, all of the seaweeds and eel grasses, and many of the diatoms, (2) creeping forms, such as crabs, lobsters, certain copepods, amphipods, and many other crustacea, many protozoa, snails, and some bivalves and fishes, and (3) burrowing forms, including most of the clams and worms, some crustacea, and echinoderms.
The nekton (Gr., swimming) is composed of swimming animals found in the pelagic division. In this group are included most of the adult squids, fishes, and whales—namely, all of the marine animals that are able to migrate freely over considerable distances. Obviously, there are no plants in this general group.
In the plankton (Gr., wanderer) is included all of the floating or drifting life of the pelagic division of the sea. The organisms, both plant and animal, of this division are usually microscopic or relatively small; they float more or less passively with the currents and are therefore at the mercy of prevailing water movements. Many of the animals are able to make some progress in swimming, although their organs of locomotion are relatively weak and ineffective. The plankton is divided into two main divisions, the phytoplankton and the zooplankton. The former comprises all of the floating plants, such as diatoms, dinoflagellates, coccolithophores, and sargassum weeds. In the zooplankton are included (1) myriads of animals that live permanently in a floating state, and (2) countless numbers of helpless larvae and eggs of the animal benthos and nekton. Since the plankton and nekton occupy the same biotic realm and are part of the same community, it is necessary always to remember that the distinction is one based primarily on relative size and speed of swimming, and does not signify a divergence of ecological relationship.
Each of these three ecological groups will be more fully discussed in later chapters.