NATURE NOTES.

Otago Witness, Issue 3197, 23 June 1915, Page 64

NATURE NOTES.

By G. M. Thomson, F.L.S

No. XIX.—ROCK-POOLS AND THEIR CONTENTS.

Reefs of rocks above high water-mark which run out seawards nearly always contain hollows and cracks, in which are left beautiful pools when the tide is out. The character of the pools varies very much with the nature, lie, and dip of the rocks forming the reefs, and also with the position of the reefs themselves, those exposed to the full force of the ocean being very different from, and containing a somewhat different type of life from, those which occur in sheltered boys.

A well-stocked rock-pool is a microcosm, so great is the number and variety of living organisms which it contains. The vegetable life is entirely formed of various algae or seaweeds, belonging to tw T o or three distinct types. The most beautiful, and at the same time the most highlydeveloped, of these are the red and purple forms, sometimes consisting of flattened fronds, but more often exquisitely cut into delicate divisions and fringed like line lacework, which form the great group of the Floridem. In these, as in all highlycoloured seaweeds, the green colour is masked by a red or other hued pigment; but the chlorophyll is probably there all the time, which enables them to decompose carbon dioxide in solution in the water, and to give off oxygen. It must be remembered, however, that the water in which they live, especially in the surffilled pools, is already oxygenated to saturation point by the tossing and splashing to which it is subjected. One interesting section of the Floridem which seems specially fitted for withstanding the force of the surf consists of somewhat hard seaweeds, in which the fronds become encrusted by a deposit of carbonate of lime, giving tlie plants the externa] form and appearance of miniature corals. The amount of this white lime-salt the colour considerably, so that the corallines, os they are called, are usually pink or lilac-coloured, pale brown, yellowish, or dirty white. Green seaweeds, belonging to various families, nearly always occur in shallow water, while the brown seaweeds occur at all depths to which vegetable life extends. The brown seaweeds also belong to several families, and all those_ which grow at any depth are furnished with airbladders, the function of which is-to float the fronds up to the surface of the water in which they grow. In this rich country of New Zealand, where Nature is so lavish with her supplies that we can afford to pick and choose the things which wo care to use, we neglect materials which in other more densely-peopled countries are utilised and valued. All Avho have been brought up near the sea coast in the Old Country will recall the seaweed which is still largely eaten, and is known as “ dnlce ” or “ dulse.” I don’t know which is the correct spelling. Certain seaweeds—varieties of which are common enough on our own coasts —are known as carageen. If the salt is thoroughly washed out of them they yield under the action of hot water a gelatinous materia] some what like the pectine of some fruits—e.g., red currants, and this is used to gelatinise milk. Seaweeds don’t produce starch under the influence of sunlight, but many of them produce this gelatinous, and others a mucilaginous, substance of somewhat the same composition as starch. Many r.f the large brown seaweeds and “ tangles ” belonging to the Laminariacese when dried and slowly burned vield “kelp,” which is the chief source of iodine. The name kelp has somewhat incorrectly come to be applied to all large floating seaweeds, whereas ib really belongs only to the im-perfectly-burned ash of these plants. Other seaweeds when burned give an ash called “ barilla,” consisting mainly of carbonate of soda, and this was formerly used in soap-making. But all the substances called barilla soap to-day are made from the ordinary alkali as produced from common salt, in spite of the untrue statements made by soap-makers to the contrary. Of course, most barilla was not made from seaweeds, but from seashore (littoral) plants, like Saponaria and others; but the fact remains that they are not used now for soap-making. The average maker of fancy soap is just about as adept a trade-liar as is to .be met with, whether he calls his material a barilla, soap or—that contradiction in terms —a glycerine soap. This is, of course, a digression from seaweeds and rock-pools, but it is a useful digression. Before getting back to the rock-pools irom which we started, it is interesting to note that some of the larger fucoids or bro mi seaweeds are the longest, though not the largest, plants, known. Hooker, in the “ Handbook of New Zealand Flora,” a work now-procurable only with difficulty, states that macrocystis grows from 300 ft to 1000 ft in length. The authoritv on seaweeds in New Zealand is Mr R. M, Laing, of the Boys’ High School, Christchurch, who has been assisted in bis work by Mr J. Crosby Smith, of Invercargill. If anyone wishes to study this group of organisms, he should put himself in communication with one or other of these gentlemen, who will start him on the right lines. One cannot do much without a fairly good microscope, so far, at least, as classification is concerned.

Tho little seaweeds in the rock-pools, especially the hard corallines and the stiff brown fucoids, form the shelter in which many animals take refuge. It is one of the wonders of a pool on a surf-beaten roast to see ho>v many kinds of fish can live in tho swish and turmoil of a surffilled pool. Some, as the sucker-fishes (Dinlocrenis) are only to bo met with In such positions. The underside of the head is practically converted into a peculiar flattened disc or sucker, by means of which the fish adheres to the rocks, and the rough and tumble of the waves fails to displace it. When we consider the gigantic force with which waves break on

the rocks, and the weight and rapidity of the water which dashes for hours unceasingly through the pools, we can form some faint idea of the strength of the mechanism which enables these little fish to hold on with such security. Other fishes found normally in rock-pools belong to the genus Tripterygion. They are all popularly known as '‘cockabullies,” which is a corruption of the Maori name “kokopuru.” One of the interesting features posessed by some of them is their protective colouring. They do not depend entirely on this for safety, for they are all very quick in their movements, but they avail themselves of the fact to a very considerable extent. The dark-coloured ones mostly hide in brown or purple or other dark-coloured weeds, but the lighter grey or brown species frequently rest on sand at the bottom of the pools, and are thus as completely hidden as if they were under cover. The rapidity with which these fishes can change colour has never been investigated, but it would be interesting to record it. It is probable that they can change their hue very distinctly in a very few minutes. At the Portobello Fish Hatchery, where young turbot have been under close observation for over two years, it is noticeable that the change of colour is nearly as rapid and pronounced as that of the classic chameleon. If the fish are on the bottom of a concrete tank they have a uniform dull greyish colour. When a layer of white sand is thrown in they become distinctly paler. Then, if ; few handfuls of coarser black sand or fine gravel are placed in the tank, in a few minutes the fish have developed dark spots and rings. Those in a black wooden tank are mostly beautifully marked with dark grey and nearly black markings, so that they look as if they were a different species from the fish in the white tanks.

In some of the larger rock-pools, bullheads and Maori chiefs are occasionally left by the tide, and in calm weather, when many fishes swim near the surface, other kinds are occasionally left behind by the receding tide. I have counted altogether some 12 or 13 species, but a daily visit would probably have revealed very many more.

Star-fishes, and more occasionally seaurchins, are met with in many x’ock-pools. I need not trouble my readers with technical names of these —anyone desirous of finding them out can do so easily enough, —but I may indicate the commoner kinds. The commonest star-fish is a thick fiverayed star, red or pinkish above and pain beneath. A very large one may reach 6in in spread of its rays, but they are mostly smaller. They are most destructive to shell-fish, particularly oysters, which they attack by inserting the end of an arm into the slightly-opened shell. Once they gain an entrance) the mollusc seems to have no chance, and they soon eat the body. The oyster beds in Foveaux Straits are" greatly infested with these five-rayed star-fishes, which are brought in vast numbers in the dredges. In these star-fishes the central disc is very large, and is extended out between the bases of the five thick arms, so that the animal is somewhat pentagonal in form. Another star-fish common on the coast is much larger, sometimes 12in in diameter; the disc is relatively small, and the fleshy arms are from 10 to 15 in number. Another very intersting modification of the type is found in the brittle-stars, of which there are two or three fairly common specie:. The most abundant one is dark red or reddish-biown in colour. The disc is nearly circular, and in the largest-sized specimens rarely exceeds lin in diameter. From this proceed five slender, very sinuous rays, which, in the living animal, can twist and writhe about like worms. If caught and handled roughly', these arms or rays are snapped off; hence the popular names. One species which I have commonly taken in the dredge in Stewart Bland is difficult to preserve on account of the brittleness of its limbs. If dropped into spirits or formalin it will divest itself of its arms in five minutes. I have found the best way to preserve it is to kill it in sea-water by the gradual addition of fresh water, aiid then to place it in weak spirits, and transfer it by degres to the full preserving strength. A star-fish is an easily-handled creature, and if you take one out of the pool and place it in a jar or dish (a pudding dish or soup-plate does quite well) of seawater, vou can study its movements easily. A pocket lens will' greatly help you to understand its structure and mode of hie. You will observe that the whole upper surface is covered with little tubercles or nodules, evidently of somewhat calcareous matter. This latter fact you can easily prove if you find ci dried st«ai-nsili and place a drop of hydrochloric acid on it, when bubbles of carbonic acid gas will come off these nodules. One of these nodules is much larger than any of the others, and is called the madreponc tubercle. It is not in the centre of the upper surface, but is not far from it. Its function appears to he to act as a sort of filter to the sea-water, which largely fills the internal canal system of the animal. Among the tubercles on the upper surface are a number of small stalked pincers or forceps. These are known as pedicollarira. If you turn the star-fish on its back, so that its under surface Is exposed, you will find in the centre a mouth, and radiating from it are fine grooves or furrows which run out to the extremities of the fine rays. On each side of these orooves are numerous little tubes all movmg about actively, and which are pushed out and drawn back by being filled and emptied with fluid. These are called the ambulacra! feet and, as most of them can be converted into suckers by the withdrawal of their contained fluid, the animal creeps along by catching on to the rocks or seaweed by these sucking discs, it it wants to creep up a perpendicular or inclined surface covered with waving seaweeds, the little pedicellarise are also brought into use, and the little forceps help "to support the animal and enable it to climb. In Brittle stars the central disc _is small, but the limbs are produced like

little snakes or eels. They are very flexible and muscular, and can catch hold of objects by twisting round them. So we find in them that the ambulacral feet are reduced to rudiments, for they would be of no use to such an actively-moving animal. A common five-rayed star-fish when moving along can cover about 2in per minute; a Brittle star will get over 6ft in the same time.