SCIENCE UP TO DATE

Evening Star, Issue 15478, 28 April 1914, Page 10

SCIENCE UP TO DATE

LIFE IN THE DEEP SEA. LXXIV. [By James Collieh.] (Special Rights Secured by the ‘ Stax.’) Even the 50 volumes describing the xrld-famous Challenger expedition and its jcientific outcome, though they were fresher and more novel at the time, are hardly richer in results than those already yielded by the Michael Sars cruise, or sure to be yet yielded by it. The depths and deposits of the ocean, physical oceanography, the flant and animal Ufa of the sea, its vertical and horizontal distribution, the invertebrate bottom fauna, and tbs general biology of the ocean have been brightly illuminated by the labors of the naturalists who accompanied that expedition.

—Coloring in Marine Animals.— The general laws of the colors of maxim uiimals have long been known. The fishes found in' tropical waters are of various tints of sky-blue. The herring has a black-isb-brwn back, but when it suddenly turns its sides emit a silvery flash. Deepsea animals are of dark shades—black or blackish, brown, violet, or red. These colors are all obviously protective. The sky-blua fish can hardly be distinguished from the waters in which it swims. The blackish-brown back of the herring is almost invisible from above, as fishermen well know. The deep-sea fishermen are equally familiar with the dark shades of deep-sea fish. —Light and Color.— There is manifestly a close correlation between the intensity of light and the colors of fishes and other animals. To realise how close it is we must first ascertain the nature of the action of light on the sea. Where the sun’s rays fall on the surface of the ocean, some of them are reflected, and the rest penetrate the water, though in a somewhat different direction. The various rays are refracted and absorbed, but in different degrees. The red rays are Jess refrangible, and are more speedily absorbed. The dark heat rays, imperceptible to the eye, are absorbed in the uppermost layers. The light rays penetrate deeper before they disappear. At a certain depth light has not the same composition as terrestrial light. In general, the further down we go the fewer red ray* we find and the more blue rays. A photometer, invented by Dr Helland-Han-sen, a member of the Michael Sars expedition, clearly showed that rays penetrated to 500 metres (about 1,600 ft), and had still a distinct direction ; at 1,000 metres they acted on photographic plates, but not at 1,700. —The Marine Day.— An apparatus constructed by Regnard to determine the length of the day at different depths was used at Madeira by the Prince of Monaco. At 20 metres the day was 11 hours long; at 50, five hours; it 40, only half an hour. The day grows shorter and the intensity of light less as we descend. At only 10 metres the Swiss naturalist Fol, going down in a diver’s dross, found the light suddenly disappear in the afternoon long before sunset. At 50 metres it was so dim that he had a difficulty in gathering animals on the floor , Bea - At 25 metres only shining objects in favorable positions could be seen. —Light and Life.— In all the seas there is a definite correlation between the colors of animals and their regional depth. In the upper layers of th©_ Sargasso Sea the Michael Sara expedition found transparent young fish. At 000 metres they got fish with silvery sides and brownish backs. At from 500 to 750 metres, according to the latitude, they trawled only black fishes and red crustaceans There is therefore a close correlation between the development of pigment Mid the distribution of fishes, etc-, in the sea. Above 150 metres fishes are almost transparent, and in the

surface layers most animals are colorless, even their blood being transparent and without hemogkumn. J r . the next lower layers—between 150 and 500 metres—marine animals are grey, mirror-like, and silvery, and this silvery sheen is often »disoent. with dark green, violet, and 2SBe tinges, and brown or black backs. 3unng the night, when fish that live fielow 500 metres in the davtirae come up Qom a greater depth, they are often of deep red colors, with blue ' patches. .Lastly, fish without blue pigment, but only red and yellow colors, are found below. 500 metres. _ These deeper layers contain a great variety of animals, and always •ith the same dark colors—dark brown id, dpfc violet, black or blackish violet! ill alike seem to bring with them evidence in favor of the old and new doctrine that the colors of fishes and other marine animals are the outcome of adaptation to the environment, whether by the direct action of that environment or by means of self-adaptation. —The Formation of Pigment. — Other theories of the formation of pigment have been proposed. According To Dofflein it is governed by light. A very feeble light-ray suffices to form red pigment. Again, by light, red may be converted into yellow or white pigment, which two latter may arise from the blending of pigments and lime salts in the body of a crustacean. Under the agency of fight the blue pigment may be derived from red, and by a similar chemical process it is destroyed. In the surface layers light creates and also destroys pigments, the two being thus kept in a moving equilibrium, though the construction of blue rxceeds its destruction. Temperature is another agent in the production of pigments. All of this appears to be imperfect consistency with the ordinary notions of adaptation. Light and heat are its agencies. —Light Organs.— It was at one time believed that the wonderful phenomenon of phosphorescence was due to electric sparks emitted by sea water, hut it is now known that the ? mission of light can arise only from living substances, and it is restricted to certain organs that have the emission of light for their peculiar function. Such organs are found in all stages of development, from simple membranes to complicated glands. These organs secrete a slimy, luminous substance, are furnished with a black layer that acts as a reflector, and often project the light through a transparent lens. —Their Habitat. — Such organs, glandular and localised, are found, chiefly in deep-sea animals, and these never in the cold northern seas, but mainly in warm oceanic waters, and in the upper 500 metres. Thev are not to be found in fishes inhabiting the deepest and darkest layers, nor did they exist as a means of illuminating the abyss. It appears to be beyond doubt that the light organ® have for their function the task of projecting light in certain directions. With what object? To avoid foes, to discern food, or to recognise their kind? '* —Their Uses.—

Brauer was the first to ascertain that the light organs are specific characters, found in all individuals of the same species, and he holds that the light organs serve the same purpose as specific color markings in terrestrial r-rimals, The light diffused was intended to be seen by an eye. Such organs are of vital importance to the animal, and are essential to the survival of the specie*!. A study of marine eyes may throw further light on them. —Piscine Eyes.— There is no apparent connection - between the size of the eves of marine animals and ' the intensity Of the light prevailing in the sea-layers where they dwell. In the abyssal regions, where we might have expected uniformity, some have small eyes and some have large. Clearly, Brauer notwithstanding, the surrounding conditions do not shape the form or organs of the denizens, which are various. Their eyes are hero simple or permutated ; there. In verv similar forms, they are highly differentiated. Dr Johan HJort, however, is able to show that the conclusion is hasty. Outward conditions are not so uniform as was imagined, or rather the diflernt species do cot really live under the same conditions. Some are bottomdwelfcrs, ethers are mare pelagic, and this difference initiates a law. Most dwellers

f ia the ebywna! depths hare large eyes. In certain species the diameter of the eyes equals one-fifth ithe length of the large head, and the eyes of these species are well devloped. This is the one conspicuous exception to the rule that fish eyes in tine upper layers are largo and complex, but decline in size, and apparently also in complexity with the greater depth, and the decreasing intensity of light. In these lowest depths, beyond 1,000 metres, imperfect organs of vision are typical. —Stalked and Telescope Eyes.—

The piscine and crustacean beyond 500 metres are not identical with human eyes, and they vary in structure. There are stalked eyes and telescopic eyes besides the ordinary fish eyes. The stalked eyes axe still somewhat of a mystery. They appear to belong to the larval stage, and they occur only in the uppermost layers. Telescope eyes are found only in depths less than 500 metres, and they belong to fishes that are bad swimmers—which rather float than swim. They usually point upwards, and they ax© thus best adapted to receive the faint light that pierces the dusky depths. —The Fish Eye.—

The structure of the fish eye varies with the depth of the water. In the surface layers it resembles the human ©ye in possessing a retina that has both “rods” and “cones,” while in deep-sea fishes the retina has only the rod cells. In this the pelagic fishes resemble nocturnal animals, and the resemblance is significant. Like nocturnal animals, the deep-sea fishes perceive varying intensities of light; they perceive no colors. In another point the deep-sea fishes are nocturnal animals, and are day-blind. The color substance or pigment of the retina has the peculiarities of. that of nocturnal terrestrial animals. Fishes, then, grow more and more blind as they descend. Beyond 1,500 metres, as the naturalists of the Michael Sars Expedition found, blind fish are common. But why have fishes living on the seabottom large eyes? Because they have no light organs. Fishes of the uppermost layers have both light organs and eyes, and both are well devloped ; in the lower layers both light organs and eyes decrease in size, and doubtless in complexity. In the lowest depths light organs disappear. Hence the enlarged ©yes to counterbalance the loss of the light organs.

—Evolution of the Eye.— The authors of these bulky volumes refuse to speculate on the origin and uses of light organs and eyes, and Dr Johan Hjort, who is the author of this particular section, contents himself with putting certain questions which, like jesting Pilate, he leaves unanswered. We need be less timid, and may step in where specialists fear to tread. It is rational to assume that the inferior organ was the first to be devloped. But the light organ cannot have as an organ, nor does it begin with fl.mrna.ls- Most marine plants, especially the predinese and certain flagellates, emit a brilliant phosphorescence. Most marine animals, beginning with bacteria, have the same power. It was doubtless at first a sensibility diffused over the epithelium, and the lowly organisms that exhibited it survived in virtue of possessing it. It was . recognition mark, and guided the members of a species in or to their habitat. Used as a recognition mark, it grew to be a light organ, diffusing light, and thus guiding a fish, crustacean, or other marine animal in search of food or enabling it to shun enemies. It may have resembled the policeman’s lantern, concealing the species, but revealing food or foe. We may believe that, originally, the marine animal had no other organ of sight. The light-organ graduated into the eye by a succession of stages. How gradual the transition was is shown by the fact that Sir John Murray and Professor Moseley both mistook light-organs for eyes in a species dredged up by the Challenger. One of the early phases may have been that of stalked eyes. Little is yet known about these organ o and Hjort does not venture to guess to wiiat species they belong. Yet it is suspected that they are peculiar to larval stages, and It is known that in certain cases they develop into normal fish eyes. Does it not naturally follow that they repeat in the larval stage the development gone through by previous mature species? Telescopic eyes, found in fishes living at about 500 metres, may be peculiar to those species, or they may be a stage intermediate between stalked eyes and normal fish eyes. Still another (the fourth) stage is that of the normal fish eye. Its more rudimentary form is that with “rods” in the retina, but not “ cones,” and this is found in deep-sea fishes at 300 metres in depth. As already "stated, the rod eyes resemble the lightorgans, in being sensitive only to the intensity of light, and in thus answering to nocturnal terrestrial animals, because a perception of color is useless to both. A fifth and last stage of fish-eye development is that of fishes belonging to the upper layers. These have cones as well as rods in the retina, and are perceptive of color as well as of intensity of light. A graduated scale in the pigment of the retina can be clearly traoed from the fishes of the surface to those of the bottom, and from surface larvae to the adult fishes of th deep sea. As the eye is perfected with the rise from bottom to surface the lightorgan disappears—that is, in boreal waters. In warm oceanic waters lightoigans are specially characteristic of upperlayer fishes. There the oceanic forms are protected in favorable environments, when they have_ ceased to exist in colder waters. P.S.—Sir John Murray, the oceanographer, was killed at Edinburgh, March 16, 1914.