Freshwater Sponges, Hydroids & Polyzoa.
by Nelson Annandale.
GENERAL INTRODUCTION TO THE VOLUME.
Although some zoologists have recently revived the old belief that the sponges and the coelenterates are closely allied, no one in recent times has suggested that there is any morphological relationship between either of these groups and the polyzoa. Personally I do not think that any one of the three groups is allied to any other so far as anatomy is concerned; but for biological reasons it is convenient to describe the freshwater representatives of the three groups in one volume of the "Fauna."
Indeed, I originally proposed to the Editor that this volume should include an account not only of the freshwater species, but of all those that have been found in stagnant water of any kind. It is often difficult to draw a line between the fauna of brackish ponds and marshes and that of pure fresh water or that of the sea, and this is particularly the case as regards the estuarine tracts of India and Burma.
Pelseneer[A] has expressed the opinion that the Black Sea and the South-east of Asia are the two districts in the world most favourable for the study of the origin of a freshwater fauna from a marine one. The transition in particular from the Bay of Bengal, which is much less salt than most seas, to the lower reaches of the Ganges or the Brahmaputra is peculiarly easy, and we find many molluscs and other animals of marine origin in the waters of these rivers far above tidal influence.
Conditions are unfavourable in the rivers themselves for the development and multiplication of organisms of many groups, chiefly because of the enormous amount of silt held in suspension in the water and constantly being deposited on the bottom, and a much richer fauna exists in ponds and lakes in the neighbourhood of the rivers and estuaries than in running water. I have only found three species of polyzoa and three of sponges in running water in India, and of these six species, five have also been found in ponds or lakes. I have, on the other hand, found three coelenterates in an estuary, and all three species are essentially marine forms, but two have established themselves in ponds of brackish water, one (the sea-anemone _Sagartia schilleriana_) undergoing in so doing modifications of a very peculiar and interesting nature. It is not uncommon for animals that have established themselves in pools of brackish water to be found occasionally in ponds of fresh water; but I have not been able to discover a single instance of an estuarine species that is found in the latter and not in the former.
[Footnote A: "L"origine des animaux d"eau douce," Bull. de l"Acad. roy. de Belgique (Cla.s.se des Sciences), No. 12, 1905, p. 724.]
For these reasons I intended, as I have said, to include in this volume descriptions of all the coelenterates and polyzoa known to occur in pools of brackish water in the estuary of the Ganges and elsewhere in India, but as my ma.n.u.script grew I began to realize that this would be impossible without including also an amount of general introductory matter not justified either by the scope of the volume or by special knowledge on the part of its author. I have, however, given in the introduction to each part a list of the species found in stagnant brackish water with a few notes and references to descriptions.
BIOLOGICAL PECULIARITIES OF THE SPONGES, COELENTERATES, AND POLYZOA OF FRESH WATER.
There is often an external resemblance between the representatives of the sponges, coelenterates, and polyzoa that causes them to be cla.s.sed together in popular phraseology as "zoophytes"; and this resemblance is not merely a superficial one, for it is based on a similarity in habits as well as of habitat, and is correlated with biological phenomena that lie deeper than what are ordinarily called habits. These phenomena are of peculiar interest with regard to difficult questions of nutrition and reproduction that perhaps can only be solved by a close study of animals living together in identical conditions and exhibiting, apparently in consequence of so living, similar but by no means identical tendencies, either anatomical or physiological, in certain directions.
One of the most important problems on which the study of the sponges, coelenterates, and polyzoa of stagnant water throws light is that of the production of resting buds and similar reproductive bodies adapted to withstand unfavourable conditions in a quiescent state and to respond to the renewal of favourable conditions by a renewed growth and activity.
Every autumn, in an English pond or lake, a crisis takes place in the affairs of the less highly organized inhabitants, and preparations are made to withstand the unfavourable conditions due directly or indirectly to the low winter temperature of the water: the individual must perish but the race may be preserved. At this season _Hydra_, which has been reproducing its kind by means of buds throughout the summer, develops eggs with a hard sh.e.l.l that will lie dormant in the mud until next spring; the phylactolaematous polyzoa produce statoblasts, the ctenostomatous polyzoa resting-buds ("hibernacula"), and the sponges gemmules. Statoblasts, hibernacula, and gemmules are alike produced as.e.xually, but they resemble the eggs of _Hydra_ in being provided with a hard, resistant sh.e.l.l, and in having the capacity to lie dormant until favourable conditions return.
In an Indian pond or lake a similar crisis takes place in the case of most species, but it does not take place at the same time of year in the case of all species. Unfortunately the phenomena of periodic physiological change have been little studied in the freshwater fauna of most parts of the country, and as yet we know very little indeed of the biology of the Himalayan lakes and tarns, the conditions in which resemble those to be found in similar ma.s.ses of water in Europe much more closely than they do those that occur in ponds and lakes in a tropical plain. In Bengal, however, I have been able to devote considerable attention to the subject, and can state definitely that some species flourish chiefly in winter and enter the quiescent stage at the beginning of the hot weather (that is to say about March), while others reach their maximum development during the "rains" (July to September) and as a rule die down during winter, which is the driest as well as the coolest time of year.
The following is a list of the forms that in Bengal are definitely known to produce hard-sh.e.l.led eggs, gemmules, resting-buds, or statoblasts only or most profusely at the approach of the hot weather and to flourish during winter:--
_Spongilla carteri._ _Sponging alba._ _Spongilla alba_ var. _bengalensis_.
_Spongilla cra.s.sissima._ _Hydra vulgaris._ _Victorella bengalensis._ _Plumatella fruticosa._ _Plumatella emarginata._ _Plumatella javanica._
The following forms flourish mainly during the "rains":--
_Spongilla lacustris_ subsp. _reticulata_.
_Trochospongilla latouchiana._ _Trochospongilla phillottiana._ _Stolella indica._
The following flourish throughout the year:--
_Spongilla proliferens._ _Hislopia lacustris._
It is particularly interesting to note that three of the species that flourish in the mild winter of Bengal, namely _Hydra vulgaris_, _Plumatella emarginata_, and _P. fruticosa_, are identical with species that in Europe perish in winter. There is evidence, moreover, that the statoblasts of the genus to which two of them belong burst more readily, and thus give rise to new colonies, after being subjected to a considerable amount of cold. In Bengal they only burst after being subjected to the heat of the hot weather. Does extreme heat have a similar effect on aquatic organisms as extreme cold? There is some evidence that it has.
The species that flourish in India during the rains are all forms which habitually live near the surface or the edge of ponds or puddles, and are therefore liable to undergo desiccation as soon as the rains cease and the cold weather supervenes.
The two species that flourish all the year round do not, properly speaking, belong to one category, for whereas _Hislopia lacustris_ produces no form of resting reproductive body but bears eggs and spermatozoa at all seasons, _Spongilla proliferens_ is a short-lived organism that undergoes a biological crisis every few weeks; that is to say, it begins to develop gemmules as soon as it is fully formed, and apparently dies down as soon as the gemmules have attained maturity. The gemmules apparently lie dormant for some little time, but incessant reproduction is carried on by means of external buds, a very rare method of reproduction among the freshwater sponges.
The facts just stated prove that considerable specific idiosyncrasy exists as regards the biology of the sponges, hydroids, and polyzoa of stagnant water in Bengal; but an even more striking instance of this phenomenon is afforded by the sponges _Spongilla bombayensis_ and _Corvospongilla lapidosa_ in Bombay. These two sponges resemble one another considerably as regards their mode of growth, and are found together on the lower surface of stones. In the month of November, however, _C. lapidosa_ is in full vegetative vigour, while _C.
bombayensis_, in absolutely identical conditions, is already reduced to a ma.s.s of gemmules, having flourished during the "rains." It is thus clear that the effect of environment is not identical in different species. This is more evident as regards the groups of animals under consideration in India (and therefore probably in other tropical countries) than it is in Europe. The subject is one well worthy of study elsewhere than in India, for it is significant that specimens of _S.
bombayensis_ taken in November in S. Africa were in a state of activity, thus contrasting strongly with specimens taken at the same time of year (though not at the same season from a climatic point of view) in the Bombay Presidency.
GEOGRAPHICAL DISTRIBUTION OF THE INDIAN SPECIES.
The geographical distribution of the lower invertebrates of fresh and of stagnant water is often an extremely wide one, probably because the individual of many species exists at certain seasons or in certain circ.u.mstances in a form that is not only resistant to unfavourable environment, but also eminently capable of being transported by wind or currents. We therefore find that some genera and even species are practically cosmopolitan in their range, while others, so far as our knowledge goes, appear to have an extraordinarily discontinuous distribution. The latter phenomenon may be due solely to our ignorance of the occurrence of obscure genera or species in localities in which they have not been properly sought for, or it may have some real significance as indicating that certain forms cannot always increase and multiply even in those localities that appear most suitable for them. As an example of universally distributed species we may take the European polyzoa of the genus _Plumatella_ that occur in India, while of species whose range is apparently discontinuous better examples could not be found than the sponges _Trochospongilla pennsylvanica_ and _Spongilla crateriformis_, both of which are only known from N. America, the British Isles, and India.
My geographical list of the species of sponges, coelenterates, and polyzoa as yet found in fresh water in India is modelled on Col.
Alc.o.c.k"s recently published list of the freshwater crabs (Potamonidae) of the Indian Empire[B]. I follow him in accepting, with slight modifications of my own, Blanford"s physiographical rather than his zoogeographical regions, not because I think that the latter have been or ought to be superseded so far as the vertebrates are concerned, but rather because the limits of the geographical distribution of aquatic invertebrates appear to depend on different factors from those that affect terrestrial animals or even aquatic vertebrates.
[Footnote B: Cat. Ind. Dec. Crust. Coll. Ind. Mus., part i, fasc. ii (Potamonidae), 1910.]
"Varieties" are ignored in this list, because they are not considered to have a geographical significance. The parts of India that are least known as regards the freshwater representatives of the groups under consideration are the valley of the Indus, the lakes of Kashmir and other parts of the Himalayas, the centre of the Peninsula, and the basin of the Brahmaputra. Those that are best known are the districts round Bombay, Calcutta, Madras and Bangalore, Travancore and Northern Tena.s.serim. Little is known as regards Ceylon, and almost nothing as regards the countries that surround the Indian Empire, a few species only having been recorded from Yunnan and the Malay Peninsula, none from Persia, Afghanistan, or Eastern Turkestan, and only one from Tibet.
Professor Max Weber"s researches have, however, taught us something as regards Sumatra and Java, while the results of various expeditions to Tropical Africa are beginning to cast light on the lower invertebrates of the great lakes in the centre of that continent and of the basin of the Nile.
It is not known to what alt.i.tude the three groups range in the Himalayas and the hills of Southern India. No sponge has been found in Indian territory at an alt.i.tude higher than that of Bhim Tal in k.u.maon (4,500 feet), and _Hydra_ is only known from the plains; but a variety of _H.
oligactis_ was taken by Capt. F. H. Stewart in Tibet at an alt.i.tude of about 15,000 feet. _Plumatella diffusa_ flourishes at Gangtok in Sikhim (6,100 feet), and I have found statoblasts of _P. fruticosa_ in the neighbourhood of Simla on the surface of a pond situated at an alt.i.tude of about 8,000 feet; Mr. R. Kirkpatrick obtained specimens of the genus in the Botanical Gardens at Darjiling (6,900 feet), and two species have been found at Kurseong (4,500-5,000 feet) in the same district.
GEOGRAPHICAL LIST OF THE FRESHWATER SPONGES, HYDROIDS, AND POLYZOA OF INDIA, BURMA, AND CEYLON.
[A * indicates that a species or subspecies has only been found in one physiographical region or subregion so far as the Indian Empire is concerned; a ! that the species has also been found in Europe, a $ in North America, a + in Africa, and a @ in the Malay Archipelago.]
1. Western Frontier Territory[C].
(Baluchistan, the Punjab, and the N.W. Frontier Province.)
[Footnote C: I include Baluchistan in this territory largely for climatic reasons.]
SPONGES:-- 1. _Spongilla_ (_Eunapius_) [email protected] (Lah.o.r.e).
HYDROIDS:-- 1. _Hydra oligactis_!$ (Lah.o.r.e).
POLYZOA:-- 1. _Plumatella fruticosa_!$ (Lah.o.r.e).
2. _Plumatella diffusa_!$ (Lah.o.r.e).
2. Western Himalayan Territory.
(Himalayas from Hazara eastwards as far as Nepal.)
SPONGES:-- 1. _Spongilla_ (_Eunapius_) [email protected] (Bhim Tal).
2. _Ephydatia [email protected] (Bhim Tal).
HYDROIDS:--None known (_Hydra oligactis_ recorded from Tibet).
POLYZOA:-- 1. _Plumatella allmani_! (Bhim Tal).
2. _Plumatella fruticosa_!$ (Simla).