Aurora Australis/Life under Difficulties
LIFE UNDER DIFFICULTIES.
It is not intended in these notes, as the title might lead one to expect, to make any reference to the difficulties which we experience in camping during the long polar night in this latitude of somewhere between 77° and 78° south. Attention is invited rather to some of our very humble fellow-creatures, animals quite microscopic in size, which are able to live under conditions which seem to us extremely unfavourable.
Some of these deserve our interest as being, in the absence of Penguins and Skuas, the only land animals at present living in this region, perhaps the only living things besides ourselves on the whole Antarctic Continent.
The instances of Life under Difficulties are all selected from the class of the Rotifers. The animals of this class, though so small, are comparatively very highly organised and sensitive, yet they share with the simplest animals, (the Protozoa) the power of surviving all kinds of climatic rigours, heat, cold, drought, etc..
Larger animals may protect themselves from heat and cold in various ways, or they may migrate to avoid them. Emperor Penguins and other animals which winter in polar regions, keep up their heat by means of thick layers of fat and warm coats of fur or feathers. No such protection can serve our microscopic animals. A thin-skinned creature, measuring when contracted no more than one hundredth part of an inch in diameter, can hardly have a coating which will keep out cold and heat, and we can only suppose that they are able to live although they do become very hot and very cold when subjected to these conditions.
Too Small To Hurt.
A heavy swell is rolling in from the Atlantic and breaking on the rocks of a rugged little western seaport. On the cement wall of the pier the waves are rushing and climbing high up, till they are thrown back shattered into clouds of spray. Amid all this turmoil what of the little fragile creatures which are known to swarm everywhere in the water of the sea? Do they retire to calmer depths? If not, how will they fare as the water which is their home is shattered into dust? Surely they must be crushed to death, and perish in multitudes! Let us see!
A net is repeatedly thrown into the foaming crests of the waves as they tumble back, and a large quantity of spray allowed to strain through it. When the contents of the net are transferred to a little clean sea-water, and a drop of this is examined under a microscope, a busy and interesting scene meets the eye.
The water is alive with beautiful little cone-shaped animals of crystal transparency, with a ruby red eye in the middle of the large head. They swim powerfully by means of rapidly vibrating cilia on two projections at the sides of the head.
The animals are Rotifers, Synchæta by name, one of the comparatively few kinds which live in the sea. They dart about in every direction, pursuing some invisible prey: the scene is like a fair. But what of the numbers of maimed and dead which one would expect to find after their stormy experience of a few minutes ago? They do not exist. The water is pulsating with vigorous life, and the rotifers appear quite unconscious that anything unusual is toward.
These delicate animals must escape destruction by reason of their small size. When they have a drop of water to swim in they have a world. However small the drop of spray in which they may be enclosed, it will be covered by the elastic surface film, which will save the animals from jars. They are too small to hurt.
If, then, they cannot be hurt under these conditions, the conditions are not unfavourable, to Synchæta. They only seem so to us, since those breakers would kill us, and would destroy a strong ship. It is even so in all the other instances: conditions which would be quickly fatal to us do not really present any difficulty to animals which have become adapted to them.
Endurance Of Drought.
The leech-like creeping rotifers of the order Bdelloida supply the most remarkable instances of the capability to resist drought, as well as heat and cold. They are essentially aquatic animals, and can only remain active so long as they are surrounded by water. Yet many of them live in situations which are liable to become dry; streams and ponds go dry in summer, and moss, among which most of the kinds live, only receives occasional moisture from rain and dew and snow. If the rotifers could not cope with this difficulty they would perish in great numbers in dry weather, as rotifers of other orders do. If dried too quickly they are actually destroyed.
If dried more slowly, as when mixed up with grains of mud or sand, or when sheltered in the axils of moss leaves, they appear to have warning of the approaching crisis. They contract into little balls and the skin exudes a kind of varnish which dries and seems then to be quite impervious to air. In this condition they may remain for an indefinite time, and may be blown about as dust by the wind, and thus distributed to all regions of the earth.
Thus the sand of the desert, and the polar snows may receive these living dust particles, which may last have pursued an active existence in the woods or moors of temperate regions; and in either case, if they happen on moist places they may in a few hours resume their interrupted life.
It is a curious result of this faculty of resisting dessication that animals which would terminate their natural life, probably in a few hours, at the utmost in a few weeks or months, may live over a long period of years.
Whether an individual, after hibernating in this manner for many years, could again enter on a long hibernation, is not known, but groups of individuals have been revived again and again after shorter intervals, though in diminishing numbers, some dropping out after each resuscitation.
Endurance Of Heat.
The capacity to resist heat is intimately related to that of withstanding drought. In a state of nature, some Bdelloids when dessicated, must also suffer very severe scorching from the tropical sun; yet mosses from tropical regions are found to be as productive of Bdelloid Rotifers as any others, and they readily revive when conveyed to temperate regions and steeped in cold water.
Artificially some of these Bdelloids have been raised to very high temperatures. The actual figures given by Davis and others are not here available, but the temperature to which they were raised was certainly higher than anything to which they would be subjected under natural conditions anywhere on the surface of the earth, and many were revived after this treatment.
Endurance Of Cold.
The Rotifers which are able to endure cold should interest us especially in our present circumstances, as they are at the moment under observation in the lakes around us at Cape Royds, and we have some personal experience of the cold which they have to undergo.
Bdelloid Rotifers abound in the lakes of Cape Royds, and there are several species. The conditions to which they are submitted are extremely severe. They are frozen into the ice very early in the autumn, and must remain frozen solid for at least the greater part of the year. With the ice of the smaller lakes and the margins of the larger lakes they must take the lowest temperatures that occur in the district. We know by observation that they survive after experiencing a temperature of -30° Fahr.. They were found living in the Blue Lake under 15 feet of ice, there being some reason to believe that at this depth melting may only occur at intervals of years.
It has been generally assumed that animal life ceases at the temperature at which water freezes, and this is in the main true of animals which swim in water, but whether the death is due to cold or to mechanical causes is not known.
Those who have worked at the microscopic life of the Arctic Region know that it must survive extreme cold. somehow. The Arctic Region has a, genial summer climate of some months duration, with abundant water and a vegetation of higher plants. The winter might be passed by resting eggs, and a new generation produced each summer. Professor Richters revived Water Bears from Spitsbergen some years after they were collected, but it could not be known whether the adult animals would have survived the winter of their native land.
At Cape Royds there is no doubt that the adult animals survive through the winter. Some of the species lay eggs, but the eggs are not plentiful. One species (Adineta Grandis) produces living young, being an exception in this respect in the genus. The life both of parent and young may apparently be arrested at any stage. Animals bearing from one to seven young may be seen, some well developed, some at a very early stage. This species is further remarkable as living in water so saline as to be a sort of brine.
Whether the same species which endure great cold can also endure great heat, can only be settled by experiment. All the species found at Cape Royds have been brought quickly from -30° Fahr. to +60° Fahr., and have then been found actively feeding.
Some of the rotifers found at Cape Royds are supposed to be species widely distributed over the world. Others are peculiar, and unknown as yet any where else, and one is of a very peculiar form.
Portraits of some of these Cape Royds natives are shown on the plate, highly magnified.
From the instances given above of kinds which can resist heat and drought, it will appear that the Bdelloid Rotifer is one of the hardiest creatures in the world. It promises now to shed much light on the limits of temperature at which life is possible on the earth.
J. MURRAY.