NATURE NOTES.

Otago Witness, Issue 3205, 18 August 1915, Page 73

NATURE NOTES.

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

No. XXVI.—AQUARIA (Continued)

At first sight it seemed to me as if _ a few notes on the subject of aquaria, arising out of the things to be met with in sea coast rock-pools, where all I would make in the course of these papers, but as I got into the subject it grew to such an extent that it seemed advisable just to point out what a field of interest and beauty there lay in this direction. And having in a general Avay shown what can be done in the way of studying aquatic plants, fishes, mollnsca, and insects, it is advisable to complete the survey with a brief account of some other forms of freshwater life, and then go on to speak of salt-water aquaria. I do not know how many 7 species of water-spiders there are in New Zealand; but besides one that lives in sea-water in rock-pools, I know either one or two which live in fresh water. It is a most curious fact that air-breathing animals should take to living in water; but it is one of the marvels which we meet with in many widely 7 -separated groups of animals. Whales and seals belong to totally different orders of Mammalia, yet both are aquatic in their mode of living; but both must come to the surface to breathe air. So in the lower animals avb find species which have so modified their mode of life that they can Ha-c under Avater. In pools in the Water of Leith and other streams I haA’e seen diving-spiders, rather large, handsome, hairy 7 creatures, extremely actiA 7 e Avhen pursued, Avhich could instantly diA-e to the bottom of a pool when they were disturbed, and apparently go into hiding there. If carefully Avatched it Avill be seen that the whole under-surface of the abdomen on Avhich the breathing spiracles open is coA’ered by a large bubble of air, Avhich the spider carries to the bottom of the AA r ater, so that eA-en Avhen at the bottom of a pool it only breathes air. These animals are not very easy 7 to catch; but once caught and placed in the aquarium, and it Avould be interesting if their movements. It is well to remember, however, that they can run as Avell as they can diA-e, so they must be coA-ered up, just ag is the case Avith some Avaterbeetles. Of fresh-Avater Crustacea the largest and most familiar is the crayfish (Paranephrops). This animal is much more like the home lobster than our common marine crayfish is, and it has the characteristic development of the first pair of nipping claws. It is fairly easily kept in an equarium, and it Avould be interesting if anyone would keep one or more in order to study- their life-histories. We know their anatomical structure fairly well, and a good deal about their habits, but we do not know about their groAvth, times of moulting, extrusion of eggs, times of hatching, and subsequent development of the y 7 oung. They used to be extraordinarily abundant at one time; but trout have nearly 7 exterminated them in many 7 streams. It is not .advisable to put large crayfish into a small aquarium tank, nor too many of them, because big ones are apt to go for little ones, and, like all these lower animals in confinement, they never stop at cannibalism—indeed, they are best kept by themselves. Then there is an extremely interesting fresh-Avater shrimp, which also used to be very common at one time, but has also suffered seA’erely at the hands, I Avas going to stay 7, .of the trout. It has also suffered very much from the drying up of so many of the small streams which used to abound all 0A r er Otago, but which are hoav dried up in summer, and become muddy torrents after heavy rain. This shrimp is a clear, delicate little species lin or more in length. It has nassed under more than one name at the hands of species-makers, hut has now settled down to its oavh in the rather formidable name of Xiphocaris curvirostris. One most interesting fact about it is that it belongs to a family which has a very curious distribution. One species is found in the West Indies in several of the islands; another ranges from Japan down to the east coast of Australia, Avhere it is common, while the Ncav Zealand one is quite distinct from both the others. Tf one of these shrimps is examined Avith a good lens it Avill be seen that its carapace is produced forward into a beak, Avhich usually bends upAvards—hence its specific name, —and carries from nine to 15 teeth on its upper edge. Both crayfish and shrimps prefer running water; therefore the more steadily it can he changed the better. There are two species of Isopoda which occur in fresh Avater, and though they been gathered in only one or tAvo localities, it is probable that both are common. One of these (Idotea lacustris) is like an elongated Avood-lonse or slater. I originally found it at the mouth of Tomahawk Lagoon, near Dunedin, in brackish Avater, and later it Avas collected in the Straits of Magellan, in ' South America, but whether in salt or fresh Avater I cannot say. Then Dr Chilton found it at the mouth of a stream that flows into Otago Harbour at Sawyer’s Bay, and, following up the stream, he traced it right up to Mihiwaka, some 1500 ft or 1600 ft above sea-leA-el. This suggested that it was not uncommon; so one day I set my boy's on the search, and they 7 brought me specimens from a stream that rises on Swampy Hill at nearly 2000 ft elevation, and flows into the Silverstream. It is a very hardy species, and thrives well in confinement. The other species is Phreatoicus, to which I have already referred in these notes, and Avhich also is probably 7 common. It is a sluggish creature, like a thick, pale-colonred wood-louse, and creeps about among grass and weeds in pools. It hasbeen taken at Woodhangh, in Dunedin, at Mosgiel. on Ruapuke Island, in Foveaux Strait, and high up on the mountains to the Avest of the Waiau River. The very small crustaceans known as Entomostraca are A-cry abundant, especially Avhen the Aveather gets warmer, and are not only interesting in themseh’es. tmt are great scavengers, and furnish food

for the larger organisms. There are three orders which we need to consider, for the large species, known as Lepidurus viridis, a most remarkable and interesting form, is so erratic in its occurrence—appearing one summer in a district and tnen not seen again for years—that it can hardly be looked on as a tenant of the aquarium. The “ water-fleas,” of 'which there are seven fresh-water species, belong to the order Branchiopoda, because they haul their gills or branchiae conspicuously attached to their swimming legs. They get their popular name from the peculiar jerking way they swim or jump in the water, using their long, branched lower antennae as swimming organs. They have a bivalve shell, which encloses most of the body, except these large antennae. The common Daphnias are frequently found in enormous quantities, so that a bottle dipped in a pool will bring up hundreds. They are beautiful things in an aquarium; but, like all the Entomostraca, you need a magnifying glass to see their structure. Of Copepoda there are nearly a dozen fresh--water species, of which some are to be found in almost any pond or ditch where water-weeds grow. You can recognise Cyclops or its allies by the curious jerky way in which they progress through the water. The body is like that of a shrimp, tapering backwards to a long forked tail, usually furnished with large plumose antennae in front and, in the case of the females, bearing an elongated egg-sac on each side of the body, like a couple of saddle-bags. One of the early writers on the Copepoda, Mons. Jurine, of Geneva, kept Cyclops in aquaria, studied their life-history, egg-pro-duction, and rate of growth, and amused himself with a calculation as to their development. He found that if all the eggs produced by a single Cyclops were to hatch and the young to grow to maturity, and these in turn were to produce more eggs and more young, there would at the end of a year (supposing none died) be 4,442.189,120 individuals. The exact number does not matter very much. Only it helps one to understand not only how these little creatures become so abundant when the conditions are favourable,. but also how important they are as food material for the larger organisms living in the same water as themselves. The third order of Entomostraca, which is well represented in the fresh waters of New Zealand, is that of the Ostracoda, The animals of this group have a bivalve shell, usually more or less globose or kidney-shaped. The hinge is on the upper side,"and from the open edges of tbe valves below the eggs protrude, and are used either for swimming, or for walking along on the bottom or on the waterweeds. The commonest species is a little rounded green form, known as Cypridopsis viridis, which is found in ponds and ditches all over New Zealand during the summer months. It is an active little creature, and is very pretty in an aquarium tank. A great many of these Ostracodes live pretty near the bottom of the water, and when a pond or lagoon dries up either they themselves or their eggs are found in the dried mud A good many years ago Professor G. 0. Sars, of Christiania, wrote and asked me to send him dried mud from lagoon beds. I got solid blocks of such mud from three localities in New Zealand —viz., the Taieri Plain, West Oxford (in the Canterbury district), and Hokianga (in the far north of the North Island). These were placed in aquaria in Christiania, and hatched out in a greenhouse. In due time both Ostracoda and Copepoda appeared, and were examined, described, and named by Professor Sars. Of the first-named order he obtained four species of Cypris, two of Cypridopsis (including the green one already mentioned), two of Candonocypris, and five of Hyodromus —not a bad result from three blocks of mud. I think this case is veryinteresting, just as showing one of the numerous ways in which a collector here may add to the knowledge of the natural history of the country. The last fresh-water organisms which I am going to refer to here are the Hydne, animals somewhat like very miniature, elongated sea-anemones, which are found commonly on water-weeds. When y r ou collect the weeds you may not see any of them, but after they have been in the aquarium for a short time you will see them if there are any about. The best way is to place a little of the weed in tumblers in clear water, and let it stand for a short time in a well-lit window or greenhouse. The polyps, when disturbed, contract into a little shapeless lump of jelly, but after a period of quiescence they begin to stretch themselves" out. One end remains attached to the weed, while the body elongates into a slender stalk, bearing at the extremity a month surrounded by several tentacles. When fullyextended these tentacles are as long as the body, which may be £in in length or more, and as these wave about in the water thev come in contact with other animals, which serve as food. By- using a good lens you can see that these tentacles are covered with minute tubercles, from which can be ejected peculiar stinging hairs called “indicating hairs” or threads, which have the power of paralysing and ultimately killing the small organisms which the animal feeds on. For instance, if a small worm or an insect larva comes in contact with the tentacles of a hydra, it appears to he numbed so that it cannot resist the enemy-. Then the tentacles slowly twine round it, drag it towards the mouth or “oral aperture,” and. if it is not too big, it is devoured. This power of stinging the prev is not confined to hydra, but is possessed by 7 a great manv animals belonging to the Hydrozoa. Many bathers know the numbing effect produced by handling some kinds of jelly-fishes, which have numbers of indicating sells in their tentacles. According to Hutton, the common Hydra here is the green species, H. viridis. I am inclined to think a brown species. 11. fnsca, is the commonest. Headers will see that our fresh waters contain an immense number of animals suitable for an aquarium, and it is probable that by careful research and collecting the number might be easily doubled.