When the waters retire from under the tangled arcades of the mangroves, the black mud, which forms the congenial soil of these plants, appears teeming with a boundless variety of life. It absolutely swarms with the lower marine animals, with myriads of holothurias, annelides, sea-urchins, entomostraca, paguri, and crabs, whose often brilliantly coloured carapaces form a strong contrast to the black ooze in which they are seen to crawl about. Life clings even to the roots and branches bathed by the rising floods; for they are found covered with muscles, barnacles, and oysters, which thus have the appearance of growing upon trees, and pass one-half of their existence under water, the other in the sultry atmosphere of a tropical shore.
The close-eyed Gudgeon (Periophthalmus), or ‘Jumping Johnny,’ as he is more familiarly named by the sailors, plays a conspicuous part in the animal world of the mangrove swamps, where the uncouth form of this strange amphibious fish may be seen jumping about in the mud like a frog, or sliding awkwardly along on its belly with a gliding motion. By means of its pectoral fins, it is even enabled to climb with great facility among the roots of the mangroves, where it finds a goodly harvest of minute crustaceans. It must, however, not be supposed that ‘Johnny’ has all the swamp to himself; for though he manages to swallow many a victim, he is not seldom doomed to become the prey of creatures more wily or stronger than himself. A large and powerful crab of the Grapsus family may often be observed stealing, with an almost imperceptible motion, and in a cautious, sidelong manner, towards a gudgeon basking on the shore, and, before the fish has time to plunge into the sea, the pincer of the crab secures it in a vice-like gripe, from which it is perfectly hopeless to escape.
‘Johnny’ is a pugnacious little fellow, and rather prolonged fights may be observed between him and his brethren. At the mouth of the Zambesi, Dr. Livingstone saw one which, in fleeing from an apparent danger, jumped into a pool a foot square, which another evidently regarded as his by prior discovery. In a twinkling the owner, with eyes flashing fury, and with dorsal fin bristling up in a rage, dashed at the intruding foe. The fight waxed furious. No tempest in a teapot ever equalled the storm of that miniature sea. The warriors were now in the water and anon out of it, for the battle raged on sea and shore. They struck hard, they bit each other, until becoming exhausted, they seized each other by the jaws like two bull-dogs. They paused for breath, and were at it again as fiercely as before, until the combat ended by the precipitate retreat of the invader.
The vast multitude of marine animals which peoples the mangrove swamps naturally attracts a great number of strand, lacustrine, and sea birds; for it would be strange, indeed, if guests were wanting where the table is so prodigally supplied. The red ibis, the snow-white egrette, the rosy spoonbill, the tall flamingo, and an abundance of herons and other water-fowl, love to frequent the mangrove thickets, enhancing by their magnificent plumage the beauty of the scene. For, however repulsive may be the swampy ground on which these strange trees delight, yet their bright green foliage, growing in radiated tufts at the ends of the branches, and frequently bespangled with large gaily-coloured flowers, affords a most pleasing spectacle. Many an interesting discovery would here, no doubt, reward the naturalist’s attention; but the mangroves know well how to keep their secrets, and to repel the curiosity of man. Should he attempt to invade their domains, clouds of bloodthirsty insects would instantly make him repent of his temerity; for the plague of the mosquitoes is nowhere more dreadful than in the thickets of the semi-aquatic Rhizophoræ. And supposing his scientific zeal intense enough to bid defiance to the torture of their stings, and to scorn the attacks of every other visible foe—insect or serpent, crocodile or beast of prey—that may be lurking among the mangroves, yet the reflection may well bid him pause, that poisonous vapours, pregnant with cholera or yellow fever, are constantly rising from that muddy soil. Even in the temperate regions of Europe the emanations from marshy grounds are pregnant with disease, but the malaria ascending from the sultry morasses of the torrid zone is absolutely deadly.
Thus there cannot possibly be a better natural bulwark for a land than to be belted with mangroves; and if Borneo, Madagascar, Celebes, and many other tropical islands and coasts, have to the present day remained free from the European yoke, they are principally indebted for their independence to the miasms and tangles of a Rhizophora girdle, bidding defiance alike to the sharp edge of the axe or the destructive agency of fire.
As the mangroves are found in places suited to their growth throughout the whole torrid zone, it is not surprising that there are many species, some rising to the height of stately trees, while others are content with a shrub-like growth. Some are peculiar to America, others to the Old World; some grow near the sea, others prefer a brackish water and the low swampy banks of rivers.
The Jriarteas and Screw-pines are as singular as the mangroves in the formation of their roots; but those of the Lum, a large tree which Kittlitz found growing on the island of Ualan, are perhaps without a parallel in the vegetable world. Each of the roots, running above-ground for a considerable distance, is surmounted by a perfectly vertical crest, gradually diminishing in size as the root recedes from the trunk, but often three, or even four, feet high near its base. These crests, which are very thin but perfectly smooth, regularly follow all the sinuosities of the root, and thus form, for a considerable distance round the tree, a labyrinth of the strangest appearance. Large spaces of swampy ground are often covered with their windings, and it is no easy matter to walk on the sharp edges of these vertical bands, whose interstices are generally filled with deep mud. On being struck, the larger crests emit a deep sonorous sound, like that of a kettledrum.
The thorns and spines with which many European plants are armed, give but a faint idea of the size which these defensive weapons attain in the tropical zone. The cactuses, the acacias, and many of the palm-trees, bristle with sharp-pointed shafts, affording ample protection against the attacks of hungry animals, and might appropriately be called vegetable hedge-hogs, or porcupines. The Toddalia aculeata, a climbing plant, very common in the hill-jungles of Ceylon, is thickly studded with knobs, about half an inch high, and from the extremity of each a thorn protrudes, as large and sharp as the bill of a sparrow-hawk.
The black twigs of the buffalo-thorn (Acacia latronùm), a low shrub abounding in northern Ceylon, are beset at every joint by a pair of thorns set opposite each other, like the horns of an ox, as sharp as a needle, from two to three inches in length, and thicker at the base than the stem they grow on; and the Acacia tomentosa, another member of the same numerous genus, has thorns so large as to be called the jungle-nail by Europeans, and the elephant-thorn by the natives. In some of these thorny plants, the spines grow, not singly, but in branching clusters, each point presenting a spike as sharp as a lancet; and where these shrubs abound, they render the forest absolutely impassable, even to animals of the greatest size and strength.
The formidable thorny plants of the torrid zone, which are often made use of by man to protect his fields and plantations against wild beasts and robbers, have sometimes even been made to serve as a bulwark against hostile invasions. Thus Sir Emerson Tennent informs us, that, during the existence of the Kandyan kingdom, before its conquest by the British, the frontier forests were so thickened and defended by dense plantations of thorny plants as to form a natural fortification impregnable to the feeble tribes on the other side; and at each pass which led to the level country, movable gates, formed of the same thorny beams, were suspended as an ample security against the incursions of the naked and timid lowlanders.