According to Mr. Bennett, "Whaling Voyage round the Globe," a species of shark first discovered by himself is distinguished by an uncommonly strong emission of light. When the specimen, taken at night, was removed into a dark apartment, it afforded a very interesting spectacle. The entire inferior surface of the body and head emitted a vivid and greenish phosphorescent gleam, imparting to the creature by its own light a truly ghastly and terrific appearance. The luminous effect was constant, and not perceptibly increased by agitation or friction. When the shark expired, (which was not until it had been out of the water more than three hours,) the luminous appearance faded entirely from the abdomen, and more gradually from other parts; lingering longest around the jaws and on the fins.
The only part of the under surface of the animal which was free from luminosity was the black collar round the throat; and while the inferior surface of the pectoral, anal, and caudal fins shone with splendour, their superior surface (including the upper lobe of the tail fin) was in darkness, as were also the dorsal fins, and the back and summit of the head.
Mr. Bennett is inclined to believe that the luminous power of this shark resides in a peculiar secretion from the skin. It was his first impression that the fish had accidentally contracted some phosphorescent matter from the sea, or from the net in which it was captured; but the most rigid investigation did not confirm this suspicion, while the uniformity with which the luminous gleam occupied certain portions of the body and fins, its permanence during life, and decline and cessation upon the approach and occurrence of death, did not leave a doubt in his mind but that it was a vital principle essential to the economy of the animal. The small size of the fins would appear to denote that this fish is not active in swimming; and, since it is highly predaceous and evidently of nocturnal habits, we may perhaps indulge in the hypothesis, that the phosphorescent power it possesses is of use to attract its prey, upon the same principle as the Polynesian islanders and others employ torches in night-fishing.
Some of the lower sea-plants also appear to be luminous. Thus, over a space of more than 600 miles (between lat. 8° N. and 2° S.), Meyen saw the ocean covered with phosphorescent Oscillatoria, grouped together into small balls or globules, from the size of a poppy-seed to that of a lentil.
But if the luminosity of the ocean generally proceeds from living creatures, it sometimes also arises from putrefying organic fibres and membranes, resulting from the decomposition of those living light-bearers. "Sometimes," says Humboldt, "even a high magnifying power is unable to discover any animals in the phosphorescent water, and yet light gleams forth wherever a wave strikes against a hard body and dissolves in foam. The cause of this phenomenon lies then most likely in the putrefying fibres of dead mollusks, which are mixed with the waters in countless numbers."
Summing up the foregoing in a few words, it is thus an indisputable fact, that the phosphorescence of the sea is by no means an electrical or magnetic property of the water, but exclusively bound to organic matter, living or dead. But although thus much has been ascertained, we have as yet only advanced one step towards the unravelling of the mystery, and its proximate cause remains an open question. Unfortunately, science is still unable to give a positive answer, and we are obliged to be contented with a more or less plausible hypothesis. When we consider that the phosphorescence most commonly resides only in the outward mucous covering of the body, in which a number of particles cast off by the skin are continually undergoing decomposition, the phenomenon seems to be a simple chemical process, during which more or less phosphorus may be disengaged, which by agitation or friction gives rise to the emission of light. It is more difficult to explain those cases in which the entire mass of the body is luminous (as in Pholas), or the muscular substance (as in some Annelides), or the vibratory cilia (as in the Beroës); and here we do better to confess our entire ignorance, than to resort to the hypothesis of electrical discharges, extremely improbable in an element which is so excellent an electrical conductor, and particularly when we consider that no emission of light takes place in the few and powerful electrical fishes we are acquainted with.
We know as little of what utility marine phosphorescence may be. Why do the countless myriads of Mammariæ gleam and sparkle along our coasts? Is it to signify their presence to other animals, and direct them to the spot where they may find abundance of food? So much is certain, that so grand and wide-spread a phenomenon must necessarily serve some end equally grand and important.
As the phosphorescence of the sea is owing to living creatures, it must naturally show itself in its greatest brilliancy when the ocean is at rest; for during the daytime we find the surface of the waters most peopled with various animals when only a slight zephyr glides over the sea. In stormy weather, the fragile or gelatinous world of the lower marine creatures generally seeks a greater depth, until the elementary strife has ceased, when it again loves to sport in the warmer or more cheerful superficial waters.
In the tropical zone, Humboldt saw the sea most brilliantly luminous before a storm, when the air was sultry, and the sky covered with clouds. In the North Sea we observe the phenomenon most commonly during fine tranquil autumnal nights; but it may be seen at every season of the year, even when the cold is most intense. Its appearance is, however, extremely capricious; for, under seemingly unaltered circumstances, the sea may one night be very luminous, and the next quite dark. Often months, or even years, pass by without witnessing it in full perfection. Does this result from a peculiar state of the atmosphere, or do the little animals love to migrate from one part of the coast to another?
It is remarkable that the ancients should have taken so little notice of oceanic phosphorescence. The "Periplus" of Hanno contains perhaps the only passage in which the phenomenon is described. To the south of Cerne the Carthaginian navigator saw the sea burn, as it were, with streams of fire. Pliny, in whom the miracle (miraculum, as he calls it) of the date-shell excited so lively an admiration, and who must often have seen the sea gleam with phosphoric light, as the passage proves where he mentions in a few dry words the luminous gurnard (lucerna) stretching out a fiery tongue, has no exclamation of delight for one of the most beautiful sights in nature. Homer also, who has given us so many charming descriptions of the sea in its ever-changing aspects, and who so often leads us with long-suffering Ulysses through the nocturnal floods, never once makes them blaze or sparkle in his immortal hexameters.