After this general survey we may now consider the various kinds of organisms that occur in the plankton.
A great number of marine or ganisms "spawn" in the general sense—that is, they discharge their spores, eggs and larvae into the sea, where these things simply drift about while they undergo development. After the latter process is complete the organisms leave the plankton com munities, either to settle down on the sea-bottom or to roam about in the sea as freely-locomotory animals. Thus the eggs and larvae of innumerable marine species are transitorily plank tonic organisms, finally becoming benthic or nektic ones. Benthos includes all the organisms that live on or are attached to the sea bottom, while nekton includes those others that are actively locomotory in their habits. (See MARINE BIOLOGY.) Thus most of the fishes spawn, emitting their eggs and sperma tozoa into the sea water, where the processes of fertilization and embryonic development occur. When embryonic development ends the little fish is a larva, and as such it simply drifts about in the sea. Finally its development is completed by a meta morphosis and then it assumes the form and habits which are characteristic of its species. It leaves the plankton and becomes a nektic animal. Most of the marine Crustacea reproduce in the same way, that is they may directly emit eggs into the sea, or the parents may, for a time, carry these eggs attached to their bodies, but in most cases the embryos develop into larvae which are always small, practically non-migratory and which finish their development while drifting in the plankton. Many of the Mollusca lay their eggs (enclosed in cases, or cocoons) on the sea bottom, but others emit the eggs and sperms into the water. In all cases, however, the larvae drift about for a time. Just the same general life-history is displayed by most of the worms, the polyzoa and practically all the echinoderms, sponges and coelenterates. The rooted algae liberate their zoospores into the sea on the littoral and shallow-water zones and there these myriads of (even microscopically) tiny organisms drift about and perish.
The wealth of life that is so produced is incredibly great. Even
one large cod will spawn annually several millions of eggs, all of which exist for a short time in the plankton, and some of which become larvae and small baby cod. So also with almost every kind of marine animal: its eggs that are spawned every year must generally be counted by the thousands or hundreds of thousands. Yet, on the average, of all the eggs produced by any marine animal only one or two can undergo development and growth up to full reproductive maturity. The remainder, destined to perish in the struggle for existence, have their life-phase for a few weeks or months in the hordes of transitorily planktonic organisms.
In all parts of the seas and oceans the plankton always contains great numbers of organisms that have this habitat throughout their whole lives. There are many large, permanently planktonic organisms—some algae; many medusae (which are sometimes so numerous as to make a visible, coloured carpet on the surface of the sea) ; many other coelenterates (such as the Siphonophores and the Ctenophores); some worms ; some molluscs like the pearly nautilus, and the Heteropods and Pteropods (the latter may exist in great shoals and form the food of the whalebone whales). These large plank ton animals and plants are, however, few and unimportant when compared with the multitudes of the microscopically small ani mals and plants which abound even in water that appears, to the naked eye, to be perfectly clear and barren.
Among the Crustacea the little animals called Copepods, Schizopods, Ostracods, Cumacea, with some less abundant groups are always and everywhere present in the plankton and some times they are extraordinarily abundant. They live mainly in the upper layers of the sea, but many species are also found in very deep water. Their significance, even for human affairs, is very great for they are the food of such fishes as herrings, pilchards, sprats, hake and other kinds that are regularly caught by the fishermen. Many other marine animals, of course, eat them. They may be so very abundant that sufficiently large numbers to cook and eat may be taken by fine nets drawn through the sea.