BY CRAB-LIGHT.
In drifting over the calm waters of the ocean as night comes on,
we notice in the depths below luminous forms of infinite variety.
These are _medusæ_, as we have seen, moving here and there like
veritable comets. They approach so near the unruffled surface, at
times, as to expose the gleaming disk. The nets of the fishermen come
up entangled in their golden trains, and along shore processions and
columns of these wondrous shapes pass and repass.
As the night grows apace, and the wind rises, they sink into the
deeper waters; yet the foam and crest of the waves still give out the
curious light, though now from another source. Much of this is due
to Crustaceans, minute creatures often almost invisible to the naked
eye, yet possessing this wonderful gift of phosphorescence to a marked
degree.
Some species of the little _Gammarus_ are remarkable for their
clear silvery light. They are familiarly known as water-fleas,
attracting attention from their leaping powers, and are often found
under seaweed above high-water mark, darting here and there in
incredible numbers when their home is disturbed. These forms are
extremely valuable as scavengers.
That these interesting animals were light-givers, has long been
known; Viviani observing it in a number of species in the beginning
of the present century.[39] There is one peculiarity about many of
these small animals; that is, the light has a more decided red tint
than that of any other group of animals. This is especially true of
many of the water-fleas, or _Entomostracans_, and the extremely
transparent, ten-footed kinds. The light is often intense, but fitful
and shortlived. It seems to start from the locality where the legs join
the body, and rapidly spreads beneath the skin until the entire body
appears to be suffused with light, and the little animal consumed with
an internal fire. Yet if a bushelful of these gleaming living lights
were confined, and a thermometer placed among them, it would not show
the slightest variation or evidence of heat. The little Cyclops is very
common in our freshwater ponds, and forms a beautiful object under the
microscope.
Along our sea-shores we may often see, under the rocks, clinging
to the eel-grass, or among the thickly growing stems of _Coralina
officinalis_, in some pool left by the tide, gleaming spots that move
about in an erratic manner; now many collecting together, then breaking
up into small patches of light, which in turn separate again. They are
curious Crustaceans, known scientifically as the _Idotea phosphorea_.
By day we shall find that they are usually spotted or entirely a bright
yellow; at night emitting fitful gleams, perhaps as signals or as means
of communication to their fellows.
In the Arctic regions beautiful lights have often been seen, due
to a minute crustacean. Lieut. Bellot first observed it in the
North-American polar regions, and Nordenskiöld refers to it in his
“Voyage of the Vega.” The most brilliant displays have been seen at
Mussel Bay. Nordenskiöld says, “If during winter one walks along the
beach on the snow, which at ebb is dry, but at flood-tide is more or
less drenched through with sea-water, there rises at every step an
exceedingly intense beautiful bluish-white flash of light, which in
the spectroscope gives a one colored labrador-blue spectrum. This
beautiful flash of light arises from the snow, that shows no luminosity
before it is stepped upon. The flash lasts only a few moments, but is
so intense that it appears as if a sea of fire would open at every
step a man takes. It produces, indeed, a peculiar impression on dark
and stormy winter days. The temperature of the air is sometimes in
the neighborhood of freezing of mercury. It is certainly a strange
experience to walk along in this mixture of snow and flame, which at
every step one takes splashes about in all directions, shining with a
light so intense that one is ready to fear that his shoes or clothes
will take fire. If carefully examined, the cause of this phenomenon is
found to be a little crustacean, _Metridea armata_, that somewhat
resembles the Cyclops. The great changes of temperature to which it is
subjected in the snow-sludge seem not to affect it.”
Few phosphorescent animals exhibit their glories during the day; but
_Sapphirina_ (Plate X., Fig. 6) is an exception. It is one of the
largest of the _Entomostracans_, about a quarter of an inch in length,
broad and flat, without the beauty of form which characterizes Cyclops,
Calanus, and others; but what it lacks in this respect is more than
compensated by its marvellous powers of light production, few animals
of any kind equalling it. So vivid is the phosphorescence, that it can
be distinctly seen by day; and, peering down into the depths where it
abounds, flashes of color--blue, gold, sapphire, purple, green, and
other hues--appear in bewildering frequency, ranging from the softest
to the most intense and vivid lights, marking this living sapphire as
one of the true gems of the sea.
[Illustration:
PLATE XIV.
SPIDER CRAB.
(_Colossendeis._)]
Giglioli mentions an Isopod crab, brilliant with gold and purple,
gorgeous with iridescence, and possessed also of the additional
charm of phosphorescence. The light-emitting organs in the
_Entomostracans_ observed by him were in the anterior portion of
the thorax.
The young (Zoëa) of the graceful little opossum shrimp _Mysis
stenolepis_ is phosphorescent. The adult forms are extremely
interesting objects for study, the eggs and young being carried in a
little pouch beneath the thorax. Allied to this little sea-opossum is
_Lucifer_, that is to the crustaceans what the walking-stick is to the
insect world; a veritable incongruity, resembling a branch of weed,
and doubtless finding some protection in the mimicry. Some specimens,
according to Giglioli, are luminous; the gift perhaps forming a signal
language, a code understood in this world under the sea. The position,
or seat, of the luminosity in crustaceans differs as widely as the
intensity and color of the light; and in the little _Stomatopod_,
formerly considered as an adult, and described as _Squillerichthus_, we
find the culmination of wonders, as, in a specimen of this genus found
in the Atlantic, the seat of the brilliant intermittent yellowish-green
light is in the eye-stalk; so that the eyes themselves may be said to
be veritable lanterns.
The phosphorescence of crabs was probably observed for the first
time by Sir Joseph Banks, on his voyage from Madeira to Rio Janeiro;
a small crab, named _Cancer fulgens_, being captured, which was
remarkably luminous. Sir Joseph does not state whether the light
came from the entire body or was confined to certain localities.
MM. Eydoux and Souleyet, naturalists of the French exploring ship
“La Bonite,” noticed a small luminous crustacean, and succeeded in
separating the phosphorescent secretion from the animal. They describe
it as yellowish, viscous, and soluble in water, and found that its
luminous properties soon disappeared. It was their opinion that certain
crustaceans secreted the luminous matter, and that they differed
much in their method of producing it. Certain small crabs, they
believed, could display a certain amount of light when irritated; the
phosphorescence at these times appearing in jets, forming a cloud or
halo of light in which the animal seems to disappear.
In the abyssal depths of the ocean, where probably no ray of sunlight
reaches, the crabs are possibly all luminous. Many of these deep-sea
forms have a wide geographical distribution. Thus the Lithodes
are found from the shallow waters of the north and south poles to
the tropics, in the latter living in a region over which rests
three-quarters of a mile of water. Many other crustaceans live in
depths vastly more inaccessible than this, and under a much greater
pressure. Thus _Colossendeis titan_, a strange creature, whose
stomach is prolonged to the ends of the feet, is found living at a
depth of about two miles and a half. These creatures, a species of
which is shown in Plate XIV., are the spiders of the sea, resembling
their not distant allies of the land, at least in appearance.
The different depths affect the inhabitants to a more or less extent.
In some, the eyes seem to have lost their proper functions; and an
instance is thus described by the Rev. A. M. Norman, naturalist of
the “Porcupine,” the crustacean being _Ethusa granulata_: “The
examples at one hundred and ten to three hundred and seventy fathoms
in the more southern habitat have the carapace furnished in front with
a spinose rostrum of considerable length. The animal is apparently
blind, but has two remarkable spiny eye-stalks, with a smooth rounded
termination where the eye itself is ordinarily situated. In the
specimens, however, from the north, which live in five hundred and
forty-two and seven hundred and five fathoms, the eye-stalks are
no longer movable. They have become firmly fixed in their sockets,
and their character is quite changed. They are of much larger size,
approach nearer to each other at their base; and, instead of being
rounded at their apices, they terminate in a strong rostrate point.
No longer used as eyes, they now assume the functions of a rostrum;
while the true rostrum, so conspicuous in the southern specimens, has,
marvellous to state, become absorbed. Had there been only a single
example of this form procured, we should at once have concluded that we
had found a monstrosity; but there is no room for such an hypothesis
by which to escape from this most strange instance of modification
of structure under altered conditions of life. Three specimens were
procured, on two different occasions, and they are in all respects
similar.”
Specimens of these crabs found in _shallow_ water had perfect eyes;
but, beyond one hundred and ten fathoms, they had changed as above
stated. As Darwin has said, the stand for the telescope is there,
though the telescope with its glasses has been lost.
Probably many of the deep-sea forms are luminous in some way.[40]
_Aristeus_ and allied forms are known to have phosphorescent eyes.
Others have phosphorescent organs in various parts of the body. In one,
the legs bear luminous bands that sparkle and gleam as the animal moves
along in its dismal home. In others there are certain globular luminous
organs beneath the thorax, and between the abdominal swimmerets that
have been described as eyes. The light emitted by the several organs is
of different degrees of brilliancy.
Vaughn Thompson is opposed to the theory that the objects on the side
of the trunk, and along the ventral face of the tail, of these little
creatures are eyes. “A re-examination,” he says, “proves that they are
not visual organs at all, but constitute rather a highly complicated
luminous apparatus together; the lenticular body of the organs acting
as a condenser, which, in connection with the great mobility of the
globules, enables the animal to produce at will a very bright flash of
light in a given direction. The great majority of species possess these
organs, generally arranged in a perfectly similar manner; but in a
large, deep-sea, non-pellucid _Euphausia_, V. Willemoes Suhm could
not detect these globules in their usual place.
[Illustration:
PLATE XV.
LUMINOUS CRUSTACEANS.
_Nematocarcinus gracilipes._
_Cyclops_ (magnified).]
“The phosphorescent light emitted by the species of the _Euphausiidæ_
was frequently under observation. One taken by forceps exhibited a pair
of bright, phosphorescent spots directly behind the eyes; two other
pairs were on the trunk, and four other spots were situated along the
median line of the tail,--all quite visible to the naked eye. The light
of these is a bluish white. After a brilliant flash has been emitted
from the organs, they glow for some time with a dull light. The light
is given out at will by the animal, and usually, but not always, when
irritated. The most brilliant flashes occur when freshly taken from
the sea. Under the microscope these phosphorescent organs appear as
pale-red spots, with a central, clear, lenticular body. The light comes
from the red pigment surrounding the lenticular space.” Mr. Murray
observed at night, on the surface of the sea in the Faeroe Channel,
large patches and long streaks of apparently milky-white water. The
tow-nets caught in these immense numbers of _Nyctiphanes novejica_, and
the peculiar appearance of the water seemed to be due to the diffused
light emitted from the phosphorescent organs of this species.
Many of the deep-sea shrimps are remarkable for their brilliant
coloring. _Aristes_ is a bright red, with _antennæ_ five or six
times as long as its body. Equally strange is the long-legged
_Nematocarcinus_ (Plate XV., Fig. 1), and the _Oplophori_ and
_Notostomi_, curious little creatures, that have no common names, are
of an intense red hue, while others are brown, rose, or spotted with
red; showing that Nature decorates her own even in the uttermost depths
of the sea.