The Plankton and Its Phosphorescence

The assemblage of plants and animals that together float or swim at or near the surface of the ocean (or other water), say within a layer of water one hundred fathoms thick, is scientifically called plankton  of the sea. In the open ocean, the pelagic plankton is much alike all round the world of waters, although it varies a little in composition, and still more in relative abundance, being denser in temperate than in either tropical or polar latitudes; but nowhere is it absent. The "waste of waters" teems with life. The plankton of the shallow waters near continental shores, however, presents a decidedly different assemblage from the pelagic plankton.

In the pelagic plankton, single-celled animals of the groups called foraminifers and radiolarians are exceedingly prominent, and play an enormous part in the economy of the sea, although almost or quite microscopic in size. They are incased in chambered shells of lime or flint; and over vast areas in warm latitudes the ocean floor is so thickly covered with the dead shells of one kind that the mud is called globigerina ooze. They are the eaters of the microscopic plants, and themselves are food for a wide variety of hydroids and jellyfish, large and small, whose silvery forms are often visible to the voyager, and which are mostly responsible for the pale stars of phosphorescence that shine about his prow and glorify his wake in dark nights. The queen of these far swimmers is the radiant Portuguese man-of-war. In the night a dragging fine-meshed net will capture more than by day of the plankton, because many little creatures that in daylight sink to considerable depths come to the surface at night.

Rising a step to the worms, we find them comparatively rare, but one kind of marine flatworm that abounds in midocean is rose-red and several inches long. Much more numerous is another flatworm, Sagitta, "which along with copepoda, salpæ, pteropoda and radiolaria, everywhere constitute the bulk of the small pelagic organisms" captured by towing nets. Like almost all of these usually defenseless creatures they are perfectly transparent, but some of them depart from the rule of pale blue in tint and shine in bright red. A longer step takes us to the Crustacea, represented in the pelagic plankton by queer little shrimplike forms that in countless hosts of individuals play a part in the ocean comparable to that of insects on land. The copepods are the most numerous probably—little things only a fraction of an inch in length, but amazingly abundant, and the principal users of plant food. Their relatives, the little ostracods, have similar habits, and are noted for their intense phosphorescence. Haeckel relates that on his way to Ceylon he saw the entire sea like a twinkling ocean of light, and his microscope showed him that it was made by throngs of ostracods, with some jellyfishes, salpæ and worms. Crustaceans of higher rank abound also. In northern waters species of Schizopoda, small, transparent prawns with red spots around the mouth and big, black eyes, swarm in enormous numbers, and are known to the fishermen as "kril."

An important part of the pelagic plankton consists of certain small mollusks; and "as regards abundance of individuals few groups of pelagic animals can compare with the winged snails, or Pteropoda." These are minute, rapidly swimming creatures with thin, glassy shells, and in some parts of the warmer oceans these discarded shells are so numerous on the bottom that they give the name pteropod ooze to the mud. One kind (Limacina), with a coiled shell about the size of a pinhead, which abounds in the north Atlantic, is much feared by the Norwegian fishermen because they very often spoil the herring that feed on them. Another kind (Clione), looking somewhat like a reddish butterfly an inch or so long, swims in shoals in the icy seas of the far North, and is known as "whales' food." Some larger mollusks, of which the beautiful purple Ianthina is most conspicuous, live among the vast patches of floating seaweed in the Sargasso Sea.

Great numbers and variety of tunicates or ascidians and their larvæ are taken in the surface nets of the sea naturalists, among them the salpæ—free-swimming, barrel-shaped, transparent animals well known to all seafaring people, and often seen crowding the surface of the ocean. One genus of them is Pyrosoma, which has from the earliest days excited the interest of mankind, mainly on account of the strong phosphorescent light emitted, the name, indeed, meaning "fire animal." These salpæ aggregate into colonies often several yards in length which glow like fiery serpents as they move sinuously on their way.

This property of luminosity, so widely possessed by marine animals, is one of the unsolved mysteries. It is called "phosphorescence," because it resembles the cold light given by phosphorus when undergoing slow oxidation, but phosphorus has nothing to do with the manifestation here, or in such insects as the firefly; nor is it owing to bacteria, as in the case of shining wood or decaying fish. What it really is no one knows, but it has, at least, been learned that in animals the power of emitting light is always attributable to certain structures of a glandular nature that secrete a slimy, luminous substance, or, rather, two substances, one luciferin and the other luciferase. When both together are exposed to seawater phosphorescent light results.

As a rule, the light organ is surrounded by a layer of black pigment that acts as a reflector, and often the light is projected through a transparent lens; and there is reason to believe that in the case of the higher animals, such as deep-sea fishes and squids, the rays may be thrown when and where the creature desires, as a man handles an electric flashlight. But for what purpose? Is it to illuminate the surrounding water so as to perceive, or to attract prey, or is it to avoid foes? A learned oceanographer replies that no one certainly knows. "At all events," he concludes, "the answers would probably tend to show that the many different kinds of light organs serve different purposes."