The Phosphorescent Sea

A singular and highly remarkable case of diffused marine phosphorescence was observed by Nordenskiöld during his voyage to Greenland in 1883. One dark night, when the weather was calm and the sea smooth, his vessel was steaming across a narrow inlet called the Igaliko Fjord, when the sea was suddenly observed to be illumined in the rear of the vessel by a broad but sharply-defined band of light, which had a uniform, somewhat golden sheen, quite unlike the ordinary bluish-green phosphorescence of the sea. The latter kind of light was distinctly visible at the same time in the wake of the vessel. Though the steamer was going at the rate of from five to six miles an hour, the remarkable sheet of light got nearer and nearer. When quite close, it appeared as if the vessel were sailing in a sea of fire or molten metal. In the course of an hour the light passed on ahead, and ultimately it disappeared in the remote horizon. The nature of this phenomenon Nordenskiöld is unable to explain; and unfortunately he had not the opportunity of examining it with the spectroscope.

If we come now to consider the more partial phosphorescence of the sea, we find that it is due to animals belonging to almost every group of marine forms—to Echinoderms, or creatures of the sea-urchin and star-fish type, to Annelid worm, to Medusidæ, or jelly-fish, as they are popularly called, including the "great domes" and the "silvery disks" of the [pg 233] passage above quoted from Professor Martin Duncan, to Tunicates, among which is the Pyrosoma, to Mollusks, Crustaceans, and in very many cases to Actinozoa, or forms belonging to the type of the sea anemone and the coral polyp.

Of these we will single out only a few for more special notice.

Many of the Medusidæ, or jelly-fish, possess the character of which we are speaking. In some cases the phosphorescence is spontaneous among them, but in others it is not so; the creature requires to be irritated or stimulated in some way before it will emit the light. It is spontaneous, for example, in the Pelagia phosphorea, but not in the allied Pelagia noctiluca, a very common form in the Mediterranean.

In both of the jelly-fishes just mentioned the phosphorescence, when displayed at all, is on the surface of the swimming disk, and this is most commonly the case with the whole group. Sometimes, however, the phosphorescence is specially localized. In some forms, as in Thaumantius pilosella and other members of the same genus, it is seen in buds at the base of tentacles given off from the margin of the swimming-bell. In other cases it is situated in certain internal organs, as in the canals which radiate from the centre to the margin of the bell, or in the ovaries. It is from this latter seat that the phosphorescence proceeds in Oceania pilata, the form which gives out such a light that Ehrenberg compared it to a lamp-globe lighted by a flame.

The property of emitting a phosphorescent light, sometimes spontaneously and sometimes on being stimulated, is likewise exemplified in the Ctenophora, a [pg 234] group resembling the Medusidge in the jelly-like character of their bodies, but more closely allied in structure to the Actinozoa. But we will pass over these cases in order to dwell more particularly on the remarkable tunicate known as Pyrosoma, a name indicative of its phosphorescent property, being derived from two Greek words signifying fire-body. As shown in the illustration Pyrosoma is not a single creature, but is composed of a whole colony of individuals, each of which is represented by one of the projections on the surface of the tube, closed at one end, which they all combine to form. The free end on the exterior contains the mouth, while there is another opening in each individual toward the interior of the tube. Such colonies, which swim about by the alternate contraction and dilatation of the individuals composing them, are pretty common in the Mediterranean, where they may attain the length of perhaps fourteen inches, with a breadth of about three inches. In the ocean they may reach a much greater size. Mr. Moseley, in his "Notes of a Naturalist on the Challenger," mentions a giant specimen which he once caught in the deep-sea trawl, a specimen four feet in length and ten inches in diameter, with "walls of jelly about an inch in thickness."

A. PYROSOMA. B. PONITON. (Magnified.)

The same naturalist states that the light emitted by [pg 235] this compound form is the most beautiful of all kinds of phosphorescence. When stimulated by a touch, or shake, or swirl of the water, it "gives out a globe of bluish light, which lasts for several seconds, as the animal drifts past several feet beneath the surface, and then suddenly goes out." He adds that on the giant specimen just referred to be wrote his name with his finger as it lay on the deck in a tub at night, and in a few seconds he had the gratification of seeing his name come out in "letters of fire."

Among mollusks, the best known instance of phosphorescence is in the rock-boring Pholas, the luminosity of which after death is mentioned by Pliny. But it is not merely after death that Pholas becomes luminous—a phenomenon perfectly familiar even in the case of many fish, especially the herring and mackerel. It was long before the luminosity of the living animal was known, but this is now a well-ascertained fact; and Panceri, an Italian naturalist, recently dead, has been able to discover in this, as in several other marine phosphorescent forms, the precise seat of the light-giving bodies, which he has dissected out again and again for the sake of making experiments in connection with this subject.

A more beautiful example of a phosphorescent mollusk is presented by a sea-slug called Phyllirhoë bucephala. This is a creature of from one and a half to two inches in length, without a shell in the adult stage, and without even gills. It breathes only by the general surface of the body. It is common enough in the Mediterranean, but is not easy to see, as it is almost perfectly transparent, so that it cannot be distinguished [pg 236] without difficulty, by day at least, from the medium in which it swims. By night, however, it is more easily discerned, in consequence of its property of emitting light. When disturbed or stimulated in any way, it exhibits a number of luminous spots of different sizes irregularly distributed all over it, but most thickly aggregated on the upper and under parts. These phosphorescent spots, it is found, are not on the surface, but for the most part represent so many large cells which form the terminations of nerves, and are situated underneath the transparent cuticle. The spots shine with exceptional brilliancy when the animal is withdrawn from the water and stimulated by a drop of ammonia.

Among the Annelid worms a species of Nereis, or sea-centipedes, has earned by its phosphorescent property the specific name of noctiluca (night-shining), and the same property is very beautifully shown in Polynoë, a near ally of the familiar sea-mouse. M. de Quatrefages speaks with enthusiasm of the beauty of the spectacle presented by this latter form when examined under a microscope magnifying to the extent of a hundred diameters. He then found, as he did in the great majority of cases which he studied, that the phosphorescence was confined to the motor muscles, and was manifested solely when these were in the act of contracting, manifested, too, not in continuous lines along the course of the muscles, but in rows of brilliant points.

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