CHAPTER XI.
SEAS OF FLAME.
In the summer months in tropical and semi-tropical waters, often during several days in succession, the ocean presents a surface almost unruffled. The fin of some roving shark, the splash of the flying-fish, or, if near shore, the plunge of the pelican or gull, are the only objects that disturb the sea of glass. At such times, after the sun had gone down, we have lain in our boat, with faces as near the surface as possible, and watched the wondrous panorama of the submarine world. Here great globes of light seemed to shoot through the watery space: every fish left a train of light; while the dolphin, or other great forms, gliding by, appeared converted into fiery monsters; and, as they rose to the surface, fountains of phosphorescence burst from the sea.
The forms which tend to produce this remarkable appearance in the ocean depths are many; but, in the warm waters of the tropics, the most noticeable are those belonging to the class known scientifically as _Tunicata_. Aside from their luminous properties, the Tunicates are extremely interesting, from the fact that they are now supposed to represent, with perhaps one exception, the lowest form of backboned life,--being what are called degenerate forms. In the larval stage of some species a noto-cord is present, which is supposed to represent the backbone of higher vertebrates. In some, when the animals assume the adult form, the little spinal cord is absorbed; but in others, as the _Appendicularia_ (Plate XVI., Fig. 3), the noto-cord and neural cord persist throughout the entire life of the animal. The life-history of these forms is of extreme interest; but, as it can be found in any text-book, we will pass to the feature that has rendered some of the class most conspicuous.
In exploring the depths of southern seas, among others we shall see a columnar form, the _Pyrosoma_, or “fire-body” (Plate XVII.), the giant of the Tunicates. It is an aggregation of individuals, forming a hollow cylinder closed at one end, and from two inches to four feet in length.[41]
The _Pyrosomæ_ are richly tinted during the day; but at night, as their name implies, they resemble incandescent bodies. Humboldt refers to the spectacle he enjoyed when passing through a zone of them in the Gulf Stream, distinguishing by their light the forms of fishes, that, bathed by their gleams, stood out in bold relief far below the surface.
The light is extremely beautiful. That of the Atlantic forms is said to be polychroic, or an intense green; while in the very large species it is azure. So brilliant and striking is the light, that the impression is gained that it proceeds from the entire surface of the animal; but this is not the case, according to Panceri.[42] When the _Pyrosoma_ is moving along in its curious fashion,--which calls to mind the old stern-wheel steamers,--and is undisturbed, the light is intermittent, now flashing from one cell, and now from another; the vast number of gleams giving it the appearance at times of constant light over the entire surface.
Panceri found that the luminous bodies produced an albuminoid substance that may become diffused by handling, and retain its luminosity for some time. Curiously enough, fresh water increases the intensity of the light, and causes it to continue for a longer period. The intensity of the light may be realized, when we learn from Figuier that Bibra, a Brazilian navigator, employed six _Pyrosomæ_ to illuminate his cabin, which was thus rendered so bright that he could read to a friend the description he had written of these living lanterns.
Mr. Bennett, the naturalist, thus describes his experience with these beautiful creatures: “On the 8th of June, being then in latitude 30° south, and 27° 5′ west longitude, having fine weather and a fresh south-easterly trade-wind, and the thermometer ranging from 78° to 84°, late at night the mate of the watch called me to witness a very unusual appearance in the water. This was a broad and expansive sheet of phosphorescence, extending from east to west as far as the eye could reach. I immediately cast the towing-net over the stern of the ship, which soon cleaved through the brilliant mass, the disturbance causing strong flashes of light to be emitted; and the shoal, judging from the time the vessel took in passing through the mass, may have been a mile in length. On taking in the towing-net, it was found half filled with _Pyrosoma atlanticum_, which shone with a beautiful pale-greenish light. After the mass had been passed through by the ship, the light was still seen astern, until it became invisible in the distance, and the ocean became hidden in the darkness as before this took place.
“The second occasion of my meeting these creatures was in a high latitude, and during the winter season; the weather dark and gloomy, with light breezes from northnorth-east, in latitude 40° 30′ south, and 138° 3′ east longitude, at the western entrance to Bass’s Straits, and about eight o’clock P.M., when the ship’s wake was perceived to be luminous, while scintillations of the same light were abundant all around. To ascertain the cause, I threw the towing-net overboard, and in twenty minutes succeeded in capturing several _Pyrosomæ_, which gave out their usual pale-green light; and it was, no doubt, detached groups of these animals which occasioned the light in question. The beautiful light given out by these molluscans[A] soon ceased to be seen; but, by moving them about, it could be reproduced for some length of time after. The luminosity of the water gradually decreased during the night, and toward morning was no longer seen.”
[A] When this account was written, the Tunicates were supposed to be mollusks.--NOTE BY THE AUTHOR.
M. Peron, says Figuier, observed the beauties of the _Pyrosoma atlanticum_ on his voyage to the Isle of France. The wind was blowing with great violence, the night was dark, and the vessel was making rapid way, when what appeared to be a vast sheet of phosphorus presented itself, floating on the waves, and occupying a great space ahead of the ship. The vessel having passed through this fiery mass, it was discovered that the light was occasioned by animalcules swimming about in the sea, at various depths, round the ship. Those which were deepest in the water looked like red-hot balls, while those on the surface resembled cylinders of red-hot iron. Some of the latter were caught; they were found to vary in size from three to seven inches. All the exterior of the creatures bristled with long, thick tubercles, shining like so many diamonds; and these seemed to be the principal seat of their luminosity. Inside, also, there appeared to be a multitude of oblong, narrow glands, exhibiting a high degree of phosphoric power. The color of these animals, when in repose, is an opal yellow, mixed with green; but, on the slightest movement, the animal exhibits a spontaneous contractile power, and assumes a luminous brilliancy, passing through various shades of deep red, orange green, and azure blue.
Professor Moseley captured a _Pyrosoma_ four feet long, ten inches in diameter, with walls an inch in thickness. It was placed upon the deck of the vessel, and, when the naturalist wrote his name upon the animal with his finger, it came out in letters of fire: each letter seeming to increase in size, until the entire name was lost in a blaze of light, that radiated rapidly and soon suffused the entire animal; presenting a marvellous spectacle, and showing, in a striking manner, how intimately the animals are connected. In Plate XVII. a _Pyrosoma_ of the largest size is shown in comparison with a native diver.
Sir Wyville Thompson observed the _Pyrosomæ_ off the Cape Verde Islands, and refers to the “blaze of phosphorescence and train of intense brightness that followed the ship;” and, while he did not experiment with the animals in his cabin, as did Bibra, he says, “It was an easy matter to read the smallest print, sitting at the after port in my cabin; and the bows shed on either side rapidly widening spaces of radiance, so vivid as to throw the sails and rigging into distinct lights and shadows. The first night or two after leaving San Iago, the phosphorescence seemed chiefly due to large _Pyrosomæ_, of which we took many specimens in the tow-net, and which glowed in the water with a white light like that from molten iron.”
Not the least wonderful feature of this animal is the variety of tints; white, green, various shades of deep red, orange green, and azure blue having been ascribed to it by different observers,--a fact that must stamp it as the most wonderful of all light-givers, a veritable living diamond.
One of the most remarkable exhibitions of phosphorescence was observed in January, 1880, by Commander R. E. Harris of the steamship “Shahjehan,”--a display so unusual that I quote Capt. Harris’s letter in full; and, while he is inclined to consider the exhibition as possibly electric, it would seem that the luminous objects referred to were phosphorescent animals of some kind, and possibly may have had some connection with the phenomenon.
“The most remarkable phenomenon,” says Capt. Harris, “that I have ever seen at sea was seen by myself and officers on the 5th instant, between Oyster Reef and Pigeon Islands (Malabar coast). At ten P.M. we were steaming along very comfortably. There was a perfect calm, the water was without a ripple upon it, the sky was cloudless, and, there being no moon, the stars shone brightly. The atmosphere was beautifully clear, and the night was one of great quietude. At the above-named hour I went on deck, and at once observed a streak of white matter on the horizon bearing south-south-west. I then went on the bridge, and drew the third officer’s attention to it. In a few minutes it had assumed the shape of a segment of a circle, measuring about forty-five degrees in length, and several degrees in altitude about its centre. At this time it shone with a peculiar but beautiful milky whiteness, and resembled (only in a huge mass, and greater luminous intensity) the _nebulæ_ sometimes seen in the heavens. We were steaming to the southward; and, as the bank of light extended, one of its arms crossed our path. The whole thing appeared so foreign to any thing I had ever seen, and so wonderful, that I stopped the ship just on its outskirts, so that I might try to form a true and just conception of what it really was. By this time all the officers and engineers had assembled on deck to witness the scene, and were all equally astonished and interested. Some little time before the first body of light reached the ship, I was enabled, with my night glasses, to resolve in a measure what appeared to the unassisted eye a huge mass of nebulous matter. I distinctly saw spaces between what again appeared to be waves of light of great lustre. These came rolling on with ever-increasing rapidity till they reached the ship; and in a short time the ship was completely surrounded with one great body of undulating light, which soon extended to the horizon on all sides. On looking into the water, it was seen to be studded with patches of faint, luminous, inanimate matter, measuring about two feet in diameter. Although these emitted a certain amount of light, it was most insignificant when compared with the great waves of light that were floating on the surface of the water, and which were at this time converging upon the ship. The waves stood many degrees above the water, like a highly luminous mist, and obscured by their intensity the distant horizon; and, as wave succeeded wave in rapid succession, one of the most grand and brilliant, yet solemn, spectacles that one could ever think of was here witnessed. In speaking of waves of light, I do not wish to convey the idea that they were mere ripplings, which are sometimes caused by fish passing through a phosphorescent sea; but waves of great length and breadth, or, in other words, great bodies of light. If the sea could be converted into a huge mirror, and thousands of powerful electric lights were made to throw their rays across it, it would convey no adequate idea of this strange yet grand phenomenon.
“As the waves of light converged upon the ship from all sides, they appeared higher than her hull, and looked as if they were about to envelop her; and, as they impinged upon her, her sides seemed to collapse and expand.
“Whilst this was going on, the ship was perfectly at rest, and the water was like a millpond.
“After about half an hour had elapsed, the brilliancy of the light somewhat abated, and there was a great paucity of the faint, lustrous patches which I have before referred to; but still the body of light was great, and, if emanating from these patches, was out of all proportion to their number.
“This light I do not think could have been produced without the agency of electro-magnetic currents exercising their exciting influence upon some organic animal or vegetable substance. And one thing I wish to point out is, that, whilst the ship was stopped and the light yet some distance away, nothing was discernible in the water; but, so soon as the light reached the ship, a number of luminous patches presented themselves: and, as these were equally as motionless as the ship at the time, it is only natural to assume that they existed, and were actually in our vicinity, before the light reached us, only they were not made visible till they became the transmitting media for the electro-magnetic currents. This hypothesis is borne out by the fact that each wave of light in its passage was distinctly seen to pass over them in succession; and, as the light gradually became less brilliant, they also became less distinct, and had actually disappeared so soon as the waves of light ceased to exist.”
A little Ascidian called the _Salpa_ is quite famous for its luminous properties. Like the previously mentioned form, it is a free swimmer, two kinds of individuals being recognized. One is known as solitary; while the others are termed chain zoöids, being many joined together, forming long chains, the links represented by the individual animals.
The _Salpa spinosa_, a familiar form upon our coasts, is quite cylindrical, often a little flattened above and below, and seemingly moulded in glass, so beautiful is its structure upon examination. As small and common as they are, they have created much discussion. Some observers deem their development one of the most remarkable instances of the alternation of generations. Chamisso, the German poet-naturalist, explains the relationship as follows: “A _Salpa_ mother is not like its daughter or its own mother, but resembles its sister, its granddaughter, and its grandmother.” Dr. W. K. Brooks has given much attention to these forms in this country; and, from his point of view, the alternation of generations would be impossible.
The _Salpæ_ give little signs of animation. “The only conspicuous vital action,” says Professor Owen, “is the rhythmical contraction and expansion of the mantle, in which the elasticity of the outer tunic antagonizes the contraction of the inner one. During expansion, the sea-water enters by the posterior aperture, and is expelled, in contraction, by the anterior one; its exit by the opposite end being prevented by a valve. The re-action of the jet, which is commonly forced out of a contracted tube, occasions a retrograde movement of the animal.” As they move along, on dark nights, they present the appearance of fiery serpents or luminous ribbons (Plate XVI., Fig. 1), winding their way over the sea,--a most striking spectacle.
[Illustration:
PLATE XVI.
CHAIN OF SALPS
_Salpa spinosa._ _Appendicularia._]
The light of _Salpæ_ observed by Giglioli was confined to the so-called nucleus, but was not constant; indeed, some were luminous and some were not. This was particularly evident in the month of September, when the exploring-ship “Magenta” passed through a bed of these little creatures, fifteen miles in extent. Some observed in the South Atlantic had the nucleus tinged with a brilliant red light. Very similar to _Salpa_ is _Doliolum_, which seems to burn with a vivid green light scattered over the entire body, and is one of the emeralds of the sea. In the very lowest order (_Copelatæ_) of the Tunicates, we find an interesting, indeed remarkable, light-giver, the _Appendicularia_ (Plate XVI., Fig. 3). It resembles a tadpole with quite a long tail, retaining in its adult life features that only characterize the _larvæ_ of others of the group. Professor Agassiz has noticed two specimens on the New-England coast, and they are very common in both tropical and temperate waters of various regions.
Some of the species are veritable house-builders, forming a gelatinous protection covering called a test. This habitation, if so we may term it, is formed or secreted with considerable rapidity, and is quite an elaborate affair; having two front chambers and a middle one large enough for the tail to move with ease. Curious to relate, this transparent residence is, according to Filhol, only used a few hours, being then deserted and another formed; so that its life would seem to be spent in making houses and deserting them.
The light of certain _Appendiculariæ_ is almost as remarkable as that of the _Pyrosomæ_, in the variety of its coloring; one, according to Giglioli, appearing first red, then blue, and finally green. The seat of the luminosity, which appears in intense flashes, was the central axis of the tail, or caudal appendage. Between Montevideo and Batavia in the South Atlantic, this naturalist observed many of these little creatures, nearly all of which showed these tri-colored favors; and in the Indian Ocean some were seen emitting white, blue, and green lights, marking them as among the most striking of all the light-givers.
Charles William Peach, an English naturalist, has observed the tadpole form of _Cynthia_ to emit light. _Cynthia pyriformis_ resembles a peach in form, size, and even bloom; its tests having rich reddish tints. It is a familiar form in deep water from Cape Cod to Greenland and across to Scandinavia. It is one of the most common objects on our New-England beaches after storms. The heavy seas throw it up from its hiding-places. To the student or interested visitor it is a beautiful object.