Tardigrada: India
Contents |
Faunal Diversity In India: Tardigrada
This is an extract from FAUNAL DIVERSITY IN INDIA Edited by J. R. B. Alfred A. K. Das A. K. Sanyal. ENVIS Centre, Zoological Survey of India, Calcutta. 1998 ( J. R. B. Alfred was Director, Zoological Survey of India) |
Introduction
Tardigrades are commonly called as 'Water Bears' since they are strictly aquatic animals. They are minute in size (about 1 mm long) and have characteristic external and internal structures. The body is flattened or slightly arched and carries ventrally four pairs of stubby legs bearing claws. This gives the animal the appearance of a miniature bear and hence they are referred to as water bears. The body surface is covered with varied ornamented plates which sometimes bear spines or hairs. Many species have eyes. The mouth is terminal or ventroterminal. Sexes are separate and the females are oviparous. Development is direct, the cuticle being moulted. Tardigrades are capable of withstanding very adverse conditions by passing into a state of very low metabolic activity. This phenomenon is referred to as anabiosis (Cryptobiosis).
Tardigrades are not planktonic organisms, but they are collected from aquatic mosses and algae, rooted aquatic weeds, or mud and debris of puddles, ponds and lakes. Active tardigrades are found only in droplets and film of water on terrestrial wet masses, liverworts and certain angiospherrns with a rosette growth form. They also occur as miofauna in the sandy beaches up to 2 or 3 m from water's edge of sea coast.
There is some confusion about whether the tardigrades are to be placed in the phylum Arthropoda or Annelida. They are placed near mites in the phylum Arthropoda, especially because of their indistinct segmentation, four pairs of legs and piercing stylets. Some authors consider them crustaceans, and a further suggestion places them near the Onychophora. Ramazzotti (1962) considered the Tardigrada as a phylum and separated it from both Arthropoda and onychophora. Several workers place this phylum after the Chaetognatha and Bryozoa in the phylogenetic series. In this report they will be regarded as Phylum Tardigrada until more definite evidence is forth coming.
Status Of The Taxon
Global and Indian Status
Global Tardigrada species diversity is around 600. From United States alone 70 species have been reported and this number includes marine, moss and freshwater fonns (Pennak, 1978). Europe has 40-50 species of freshwater tardigrades (Edmondsen, 1959). These numbers do not represent anywhere near the actual number of existing species. The third edition of "II Phylum Tardigrada", to be published, by Ramazzotti includes 514 species of tardigrades from the globe. From the monographs, it is found that only eight species are recorded from India including some of the stray records. So far 30+ species of tardigrades have been reported from India (Table -1). However, it is likely that Indian species of Tardigrada will be at least 70 considering the wide range of aquatic habitats and the cooler temperatures in the hills.
Distribution
Tardigrades are commoner at lower temperatures. Their favourite habitat is moss and they are also often found in the littoral region of lakes and among vegetation at the edge of ponds. Few marine species are also recorded in India. Majority of the species have been recorded from wet moss.
Biological Diversity And Its Special Features
The structure of buccal apparatus, claws of the legs and armour sculpturing (trunk plates) are largely taken into consideration for the classification of tardigrades. There are three main orders, the Heterotardigrada, Mesotardigrada and Eutardigrada, based on the morphology of pharynx. The heterotardigrada is again divided into 2 suborders based on the structure of claws, the Arthrotardigrada and Echiniscoidea. The Arthrotardigrada has three families, the Halechiniscidae with nine genera (16 species), the Batillipedidae with 2 genera (12 species) and the Stygarctidae with 2 genera (2 species). The suborder Echiniscoidea has two families, the Oreellidae with three genera (5 species) and•Echiniscidae with four genera (179 species). The other two orders have five families, with 13 genera (299 species). Among the eleven families of the phylum Tardigrada only 3 families 5 genera and 30 species are known from India (Table -1). Research on Tradigrada has received \'1.'1)' little attention in India and there is no single worker engaged in the study of this group. This may be due to their microscopic size and the difficulty in separation from the habitat. However, many areas on the Indian continent still remain unexplored. At present, the taxonomy of this group is being studied in the Zoologic~1 Survey of India based on the stray collections made during the general faunistic surveys. Table• 1 Number of family, genera, species of tardigrades recorded so far, showing their habitat Order Number in Number in Habitat World India Family Genera Species Family Genera Species
VALUE Tardigrades are capable of withstanding very adverse climatic conditions by passing into a state of Cryptobiosis or very low metabolic activity. The mechanism of this remarkable property is the gradual removal of surrounding moisture, which reduces the size and configuration of the water bear to a protective state called the tun. In this state they are able to withstand extremes in temperature, radiation, oxygen and water concentrations. Tardigrades, thus provide a unique experimental living system for use in the study of basic biological phenomena including the role of water in the maintenance of macromolecular and membrane structure, the characteristics of life itself and the events which result in sensescence (IDCN, 1984).
Threats, Conservation And Future Studies
Tradigrades occur in high altitude mountains, freshwater, brackishwater habitats and intertidal zone of coast line area. At present most of the areas of wetlands, coast line and high altitude mountains are under threat due to over exploitation and pollution and hence these little known forms may become extinct before their discovery. Due to agricultural activities, urbanisation, industrial development programmes and over exploitation of natural resources, most of natural habitats are altered and will become a death trap for the fauna depending on them. For example, a study in the U.S.A. and Hungary showed that the use of DDT, and other herbicides and pesticides was found to greatly reduce both the density and diversity of tardigrades. However, another study has shown tardigrades to be fairly resistant to insecticides. Substrate structure disturbance can also reduce tardigrade population and the disruption of soil structure by cultivation has a detrimental effect on tardigrade abundance.
The lack of detailed ecological and environmental studies allow only speculation in this area. Increased studies of the density and development of comparative faunal lists to monitor population changes are vital. Greater care must be directed to the supporting substrates as there are delicate microhabitats which have a direct role on tardigrades as well as on many other microorganisms. Thus, it is apparent that conservation of natural habitats will conserve the fauna depending on them.
Selected References
Bartos, E. (1967) Fauna CSSR Vol. 17 : Tardigrada, Paraha, 190 p. Edmondson, W.T. (Edit) (1959). Freshwater biology. John Wiley (ed.), N.Y. 1248, pp. LV.C.N. Invertebrate Red Data Book (1984). Ed. Wells. S. M., R. M. Pyle and N. M. Collins. IVCN, Gland, Switzerland, 632 pp. Pennak, R.W. (1978). Freshwater invertebrates of the United States. John Wiley (ed.), N.Y. 803 pp. Ramazotti, G. (1972). IL Phylum Tardigrada, Mem. Inst. Ital. Idrobiol., 28 :732 pp. Rao, G. C. 1971. On two species of marine interstitial Tardigrada from the east coast of India. Proc. Indian Acad. Sci., LXXIII (2) : 53-57. Rao, G. C. 1972. Occurrence of the interstitial Tardigrade Parastyogarctus higginsi Renaud-Debyser, in the intertidal sands on Andaman Islands. Curro Sci., 41 (23) : 845-846. Rao, G. C. 1991. Meiofauna, In the Fauna of Lakshadweep. Zoological Survey of India, Calcutta, 41-135. Rudescu, L. (1964). Fauna Republicii Populare Romine, Budapest, 4 : 1-400.
