The name “plankton” comes from a Greek word meaning “drifting”. It refers to the many kinds of living organisms which float at all depths in almost any natural body of water, whether pond, river, lake, or sea, and which are either very small or very weak so that they cannot travel any distance without the aid of water currents. Stronger swimmers such as fish and squids are known as “nekton”, while plankton and nekton together are “pelagic”, as compared with the “benthic” organisms living on the bottom.
As many, but not all, of the plankton are small or even microscopic, only a few of the larger forms are well known and have common names. They may be either plants (phytoplankton), animals (zoo-plankton), or bacteria. The plants are all microscopic and single celled, although in many species the cells, as they reproduce by dividing in two, remain connected in chains. The best known are the diatoms and dinoflagellates, which have hard cases and can be easily seen under a low-powered microscope; but perhaps the greatest bulk of plant life is composed of even smaller and more fragile cells, known collectively as nannoplankton. The zooplankton are extremely varied, and represent nearly every major group of the animal kingdom. Many belong to the crustacea and are thus related to the crabs and fish. The commonest of all plankton are tiny crustacea, the copepods, which may well be called the “insects” of the sea because they are just as ubiquitous and as numerous in species as the insects on land. Other crustacea may be up to several inches long, some of them closely resembling the common tide-pool shrimp, and they include many of the species known as “whale-feed”.
Among the most familiar, because of their larger size, are the various forms of jellyfish, which really include two entirely distinct kinds of animals: the jellyfish proper, or coelenterates, a primitive group related to the sea anemones and corals; and the tunicates which are related to the “sea-squirts” found under rocks around the shore, a group very much higher in the evolutionary scale. Examples of the coelenterates are the common jellyfish, the Portuguese man-of-war, and the sea gooseberry, while the tunicates include the chain-jelly and the fire salp.
Less often seen, though visible to the naked eye, are various bizarre kinds of worm, some related to the common earthworm, while others, such as the arrow worms, are an obscure group on their own, quite unrelated to any other kind of worm. Even some shellfish are found in the plankton, including a group related to the sea slugs, the pteropods (literally “winged-feet”) or “sea butterflies” which appear to fly in the water like a butterfly in slow motion. Less active planktonic forms are the violet snails, which float at the surface on little rafts of bubbles, and sometimes attach themselves to the “by-the-wind sailors” on which they feed.
The temporary plankton, which spend only their young stages as plankton, represent a number of other groups in the animal kingdom. The bony fish, such as the snapper and tarakihi, release thousands of transparent spherical eggs about 1 mm in diameter into the water. These are fertilised and hatch, sometimes in as little as two days, into tiny larvae recognisable as fish but quite unlike their parents. Most of these will be eaten by larger animals, but a few survive to adulthood. Many of the invertebrate larvae are so totally unlike their parents that they were once thought to be separate species, and were given their own names, such as phyllosoma (crayfish), zoaea and megalopa (crabs), and pluteus (sea urchin). Sometimes very different kinds of parents produce almost identical larvae. It is difficult, for example, to tell the difference between the nauplius larva of a copepod and that of the common barnacle.
In addition to the more complex zooplankton, there are many single-celled forms such as the protozoa and the bacteria. Protozoa include the foraminifera, the radiolaria, and the tintinids, all of which often have beautiful and complex shells. The foraminifera are particularly interesting in the fossil form and play an important part in oil geology. One species, Globigerina, is so abundant in warmer waters that it forms huge deposits of “Globigerina ooze” on the bottom. The least obvious of all are the bacteria, most of which are known more from their behaviour than their appearance. They play a very similar role to that of soil bacteria in the regeneration of inorganic materials such as nitrogen, phosphorus, and sulphur, after they have been utilised by other organisms. Possibly they also play an important part in the building up of organic material, and in producing special organic compounds such as vitamins which are necessary for the growth of other organisms.
Plankton are very easily collected from the water by means of a cone-shaped net made of fine-meshed cloth. The base of the cone is left open, and is attached to a metal ring with three rope bridles spaced around its circumference, while the apex of the cone opens into a wide-necked jar. If the net is towed slowly through the water by a rope attached to the bridles, it will often take a surprising number of organisms, many visible to the unaided eye, though a microscope is usually necessary to examine the details. The best plankton catches are taken at night, because many of the animals are repelled by the light and sink to greater depths in the daytime. For some reason, not yet fully understood, they seem to “know” at what time to start their vertical migration; for example, about midnight they start moving downward, even though there is no vestige of light in the sky. Some people think they have interval “clocks” which tell them what to do regardless of the light, but this is unlikely because in the perpetual daylight of the Arctic or Antarctic summer, there is no vertical migration at all.