CHAPTER X.
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.