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.