Cnidaria: India
Contents |
Faunal Diversity in India: Cnidaria
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
The term Coelenterata which was popular till recently, originally included three groups, of which two have been separated into the phyla Porifera and Ctenophora. The currently accepted term for the third group of organisms is phylum Cnidari,a, their distinctive character being possession of cnidae, which are specialised cells of the ectoderm. The phylum Cnidaria includes radially symmetrical multicellular organisms. Their body wall is made of two layers of cells, the ectoderm and endoderm with an intervening acellular to rather cellular jelly-like mesoglea. The body is basically sac-like, enclosing a gastrovascular cavity, the coelenteron, which opens to the outside through a single opening, the m~uth. The mouth is surrounded by tentacles Which sometimes have the gastrovascular extensions. Some of the ectodermal cells are characteristically modified into specialised elements called cnidoblasts or nematocysts. These cells possessing poisonous secretions have a stringing and injecting apparatus to serve the dual functions of defense and prey-capture. A variety of physical and chemical stimuli trigger the activity of these cells.
Typically their life cycle includes two alternating stages-a sedentary asexual polyp and another free swimming reproductive medusa. However, there are forms with only one of the stages predominating and the other shortened or absent. Polyps may be solitary or colonial while medusae are always solitary. They are predominantly carnivorous, feeding on plankton and larger organisms such as fish. Yet, many others harbour symbiotic algae for nutrition or for growth promoting substances.
Status Of The Taxon
Global and Indian Status
The global estimates of cnidarian diversity vary between 9,000 and 12,000 speCies. According to Groombridge (1992), the current description rates RAO & SASTRY: C"idnrin estimated for various animal groups during the period from 1978 to 1987 show an annual growth rate of 0.63 per cent for Cnidaria with about 57 species per annum described as new. On the other hand, revisionary studies on certain groups of Cnidaria are bringing down the total figure by a larger percentage. On collating information from different sources, we estimate the cnidarian diversity to a modest 10,000 species. A comparison of the global and Indian cnidarian diversity as per available information is shown below. Phylum Families (N) Genera (N) Species (N) W I W I W
W =World; I = India; The Indian component of Cnidaria given above as per available information, forms a small portion of the world diversity (8.49%), but compares well with the percentages of other Indian faunal components. This percentage however should not be constructed as an accurate reflection of reality, since all the groups of Cnidaria have not received adequate and equal attention of Indian taxonomists. Excepting the pioneering works of Annandale, Leloup, Menon, Ganapti and Nagabhushanam recent studies are few and scattered. Comprehensive accounts are available only for siphonophores by Daniel and for scleractjnian corals by Pillai. Further, many of the groups need revision and an accurate inventory of valid species is yet to be attempted. A synonymized compilation of coral species of the Indian Ocean and adjacent seas by Sheppard (1987) has reduced 796 species to 439 species. Similar excercises in other groups may upset the species diversity presented above at global as well as Indian level.
Distribution
Cnidarians show a great diversity in habit and habitat. Their body size ranges from less than a millimetre in meiofaunal members, such as, Halammohydra to several metres in pelagic forms like Physalia commonly known as Portuguese man-o'-war, provided with very long tentacles. The reef building corals often attain a massive size with a large diameter and considerable height.
Though Cnidarians are predominantly marine, three families of the hydrozoan order Hydroida (Hydridae, Limnocnidae and Moersiidae) are exclusively freshwater or brackishwater inhabitants, while two others (Clavidae and Olindiidae) have freshwater or brackishwater representatives.
The polypoid stages lead a sedentary life attached to hard substrata like molluscan shells, rocks, corals, etc., or live buried or in tubes in the soft substrata. The medusae, with a few exceptions, are free swimming planktonic, pelagic or bathypelagic. Their bathymetric distribution extends to depths exceeding 4,000 metres. There are a large number of cnidarians which live symbiotically with a variety of organisms as commensals or mutualists, or as epiphytes or epizoans, with the sole exception of Polypodium hydriforme which is parasitic on freshwater fishes.
Biological Diversity And Its Special Features
There are some general accounts of medusae. These are found in the classes Scyphozoa, Cubozoa and Hydrozoa. Pelagic medusae have a wide geographic distribution in the world oceans. A total of 118 species of Siphonophora are reported from the Indian Ocean, of which 89 species are known from the coastal waters of India (Daniel, 1991). The other medusae are represented by 67 families, 272 genera and over 900 species in the world. The comprehensive account of Kramp on medusae of the world and some recent literature reported 47 families, 77 genera and 111 species of medusae in Indian waters. Kerala and Tamil Nadu coasts are rich with 54 and 51 species respectively, followed by Andhra Pradesh (45 species) and Andaman waters (35 species). Regarding the polyps, 24 species have been reported from Tamil Nadu, followed by Orissa (12 species). Pennatulids (15 species) and Gorgoneans (32 species) are abundant at the Andaman and Nicobar Islands.
Monotypic Genera And Endemicity
There are several monotypic genera in different families, of which near about 40 genera or more have been reported from Indian waters. The most RAO & SASTRY: Cnidaria common genera though not endemic to India are Pelagia, Solmundella, Liriope, Physalia, etc. The monotypic genera endemic to India are Coe1oseris (Scleractinia), Mena and Nevadne (Actiniaria) among others. Among the species endemic to India, though not of monotypic genera, mention may be made of Amphicaryon intermedia, Anactinia pelagia, Bythocellata cruciformis, Limmocnida indica, Moersia gangetica, Phialucium multitentaculatum, etc.
Value
With their varied feeding habits, cnidarians playa very important role in the food web of the environment. There are also carnivores preying on larger organisms such as fish. Many organisms such as sea anemones and hermatypic corals harbour and depend to some extent on zoochlorellae and zooxanthellae for nutrition as well as for growth promoting substances.
Some of these symbiotic algae are known to survive only with the association of their respective cnidarian hosts. Some freeliving planktonic forms are restricted to water masses of typical temperature-salinity characteristics and thus serve as indicators of water movement and circulatory processes in the marine environment. The hermatypic corals are the chief reef builders protecting the shoreline. In addition, they support a rich biodiversity ranging from protozoans to fish, with consequent complex community relationships. Coral skeletons are of ornamental and decorative value and as such are important in curio trade. The precious red coral, Corallillm rllbrllm of Mediterranean is highly valued in jewellery. The coral skeleton is composed of more than 98% of pure calcium carbonate and it is a rich source of calcium.
The antipatharians are commonly and commercially exploited for jewellery and it is believed that by wearing these health improves. The pharmacological importance of Cnidaria, particularly of sea anemones, soft corals and jelly fish is well-known. They contain a number of antibiotics, substances of physiological importance and substances useful in detecting and sometimes curing a variety of cellular and bone diseases. Their study in India is in the initial stages and is showing great prospects.
In spite of having the poisonous nematocysts in cnidariaus, a number of organisms such as crabs, molluscs and fishes enjoy a symbiotic relationship with many of them sometimes for protection from enemies and sometimes for mutual advantage. However, cubozoans and some siphonophores can be considered venomous as they cause great misery to even human beings with fatal consequences. Though an open ocean form Physalia, for example, occasionally drifts into coastal waters and unwary bathers on the coast get entangled in their long tentacles measuring several metres. Their numerous nematocysts cause unbearable pain and swelling of the body to the victims.
Threats And Conservation
Natural calamities and disturbances in the high seas cause drifting• of oceanic forms to coastal waters resulting in large scale mortality. But they never proved a threat to any species though they may damage coral and intertidal fauna in restricted areas. Discharge of toxic effluents and untreated sewage cause great harm to pelagic as well as benthic cnidarians of coastal regions. In addition, damage to the natural habitat and wanton collection by unguided individuals may totally destroy the intertidal fauna of restricted areas.
The most threatened and vulnerable cnidarians are the reef building corals and their inhabitants. In addition to protecting the shore line, they support a rich gene pool. Unintelligent and greedy exploitation of corals for domestic and industrial purposes and damage to the habitat by visitors with added exploitation of coral inhabiting fauna for curio trade evidently damaged the ecosystem in the Gulf of Kutch, Gulf of Mannar and Andaman & Nicobar RAO & SASTRY: Cllidnrin Islands. On one hand, exploitation of corals causes erosion of shore line and on the other such action destroys the coral associated fauna. Exploitation of coral associated fauna selectively for curio trade disturbs the natural balance in the community, resulting in population explosion of unwanted species threatening the corals and coral fauna.
Subsequent to control of exploitation and damage to the ecosystem and establishment of Marine National Park at Gulf of Mannar, the entire region has almost recouperated to normalcy. The same measures are being taken at Andaman and Nicobar Islands before considerable damage is done. The effect of establishment of National Marine Park and a Sancturay in Gulf of Kutch is yet to be felt probably due to the large scale damage already done and continued exploitation of the area. Developmental projects are to be well planned and control over their discharges and human interference effectively implemented so as to protect these fragile ecosystems.
Selected References
Daniel, R. 1991. Siphonophora. In : Animal Resources of India, pp. 35-40. zool. Surv. India, Calcutta. Dunn, D. F. 1982. Cnidaria. In : Synopsis and Classification of living organisms, pp. 669-706. Sybil P. Parker (Ed.), McGraw-Hill, N. Y. Groombridge, B. (Ed.) 1992. Global Biodiversity-Status of the Earth's Living Resources. World Conservation Monitoring Centre.
Sheppard, C. Rc. 1987. Coral species of the Indian Ocean and adjacent seas-A synonymised compilation and some regional distributional patterns. Atoll Res. Bull., No. 307 : 1-32.
