Platyhelminthes: India
Contents |
Faunal Diversity in India: Platyhelminthes
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
Platyhelrnnithes could well be defined as a group of extremely variable, mostly parasitic animals with body dorsoventrally flattened having an excretary or osmoregulatory system composed of flame cells, tubules and excretory vesicles. Majority of them are either ecto or endo parasites and hermaphrodite in nature. The phylum Platyhelminthes comprise four classes Turbellaria, Monogenea, Trematoda and Cestoda. Among Platyhelminthes, Turbellaria are mostly freeliving though a few of them are parasitic or commensals. The remaining groups of Platyhelminthes are parasitic and contain many important pathogenic forms from veterinary and medical point of view.
They cause various disease among all wild and domesticated animals including man. Some of these parasites were known from almost prehistoric time, such as liver fluke, Fasciola hepatica and broad tape worm of the genus Taenia. Zoologically they are very interesting in the sense that they have modified their body organs and physiology to a very great extent for successful parasitic existance. Some of them altogether discarded digestive, respiratory and circulatory systems and developed the mechanism to live either in anaerobic condition or with very little oxygen. Most of the endoparasitic forms develop enormous capacity to reproduce. A single Fasciola hepatica may produce in its life time about 1/2 million ova. This ova after being released from vertebrate host could reproduce itself asexually in molluscan host to rediae, daughter rediae and finally cercariae, the freeliving infective stage. The total comes out to 4.2 x 108 offspring from a single ova. Egg production in trematode is probably the highest among animal kingdom. Fortunately most of them die due to nonavailability of suitable host both in larval and infective stages.
Stoll (1947) estimated that 72 and 148 million human being are infected with cestodes and trematodes respectively out of 2200 million of global population at that time. Parasitic diseases caused by helminth parasites are almost equally important as those of bacterial or viral diseases. Bacterial diseases are devastating but not of regular or frequent occurrences. They attack suddenly and cause considerable loss for a specified period of time, in a limited geographical area. But parasites are present almost always in animals throughout the globe and cause damage every day and year after year.
Status Of The Taxon
Global and Indian Status
Number of species, genera and families known India and percentage of species diversity in relation to world fauna in different classes of platyhelminthes is shown in Table 1. Table -1 Class Indian World % families genera species species Turbellaria 8 15 47 4000 1.17 Monogenea 28 110 295 2500 11.80 Digenea 47 250 750 6500 11.54 Cestoda 33 156 530 4500 11.78 Total 116 531 1622 17,500 9.27
Distribution
Platyhelminthes are found mostly as ecto or endo parasites; so their distribution is related with those of their hosts. Tropical countries provide most optimum condition for survival and propagation of these parasites; as such, they are more abundant both in number and variety in this part of the globe. Distribution of Platyhelminthes is of course in some cases confined within a limited geographical area because of the limited distribution of intermediate hosts which are mostly molluscs and arthropods.
Biological Diversity And Its Special Features
The parasitic platyhelminthes are vey much host specific. A particular species is capable of infecting only a definite species of host or taxonomically and ecologically closely related host species. In larval stages also they are very host specific. Not only the hosts but even site of location within the host organ is also very much fixed for a particular species. A negligible record of freeliving turbellarian species is available in literature.
The phylum is broadly divided into four classes, Turbellaria, Monogenia, Digenea (fulkes) and Cestoided (flatworms). Most of the Tuberllaria are small and freeliving, some are commensal or parasitic; large forms have also been recorded. Their size ranges from less than 2 mm to50mm. Survey of available turbellarian species is far from complete. Only about 4000 species have been recorded all over the globe from marine, freshwater and terestrial habitats. Study of this group remained very much neglected in India. In India, studies mostly remained confined to terrestrial fauna in high altitude Himalayas, Khasi, Jayantia and Nilgiri hill region. Considering the vast area lying unexplored and diversified habitats being present in India, large number of species are expected to be available. Species diversity in Himalayan region with high humidity and evergreen forests, expected to be most interesting, both in number and variety as far as terrestrial species are concerned.
Morphologically, most of the species are Monogenea bear close similarity with some turbellarians of the order Rhabdocoela. They are mostly small, almost all are ectoparasitic. Life cycle is simple with direct development from egg. Larva is ciliated, with a short freeswimming period before it gets anchored in final host. Final hosts are almost exclusively fishes, though a few species have been reported from mouth and urinalry bladder of amphibia and reptiles. Monogeneans are mostly host specific. Very little work has been done on this group in India. About 2500 species of freshwater and marine fishes are available in India and only 7% so far been screened for monogenea.
The adult trematodes could broadly be divided morphologically into the two broad groups or subclasses, Aspidogastrea and Digenea. Species of this class in their sexually mature stage live as parasites in vertebrates. They have been recorded from almost all body organs. Size and shape are extremely variable, from less than one millimeter to a few centimeters. Most of them are characterised by having one or two muscular sucker and a digestive system. Aspidogastrea and Monogenea have direct one host life cycles, Digenea have complex life cycle, involving freeliving and parasitic stages in different invertebrates, mostly molluscs and adult in vertebrate hosts. The larval stages can also multiply asexually. This is an unique feature in this group.
Members of Cestoidea are almost exclusively parasitic in vertebrate intestine. In their adult stages some species, of course, inhabit bile ducts of liver. Body shape and size are variable-2 mm to several meters and with a single segment to many hundred. The most interesting feature in all cestodes is the complete absence of digestive organ. Life cycle involves more than one host which may be either vertebrate or invertebrate. The class Cestoidea are broadly divided in two subclasses Cestoderia and Encestoda.
Endemicity
Present knowledge does not throw much light on endemicity of different species of Platyhelminthes because of pausity of such studies and large number of host species are still to be examined for platyhelminth parasites. In Table -3 a very rough extimate of number of endemic species in India is given based on the presently available data.
Value
Stoll (1949) estimated an outstanding figure that humanhelminth infection in the world stood at 2 billion. Peters (1978) observed that 4-5 billion people are suffering from helptinth infection. It is indeed a challenging medical problem, considering many more further millions of infection in domestic and wild animals, belonging to vertebrate and invertebrate fauna. Helminths are well represented in Animal kingdom and almost all animals are directly involved as definitive, intermediate, paratenic or accidental hosts. No animal, great or small is immune to the ravage of helminth parasites and all the species so far screened for parasites have yielded some species or other.
THREAT, CONSERVATION STRATEGIES AND FUTURE STUDIES Wild animals are all the more vulnerable to diseases caused by parasites because no effective prophylactic measure could be introduced. The faecal content which normally contains the egg and larvae are capable of spreading the parasites to other animals. Uncooked food and drinking water from ponds and streams may contain the larval forms of parasites which get transmitted into suitable hosts. Their bodies with hair and fur provide very suitable habitat for lice, mites and ticks which in tum serve as intermediate hosts to many tapeworms. Al these factors explain the reason of heavy load of parasites in wild animals as well as, in livestock animals. The concentration of wild animals in National Park, Zoo Garden and reservoirs also provide an unique opportunity for the parasites to propagate and complete their life history. Due to shrinkage of grazing field and home ranges the eggs, larvae and other infective stages of parasites get easy access to suitable host which was rather difficult when the home range and forest cover were much wider than at present.
Control of known pathogenic parasites in wild animals sometimes becomes more complex. As liver fluke Fasciola sp., blood parasite, Schistosoma sp., or lung parasite, Paragonimis sp., could be controlled by eradicating mollusc shell from the home range of wild animals. Similarly, some soil mites are responsible for the spread of Anoplocephalid cestodes, which are major intestinal tape worms of grazing animals, such as, deer, antelope, wild buffalo, elephant, etc., and are causing heavy mortality particularly in younger stages. But complete eradication of mollusc shell or soil mites is neither possible nor desirable, as they will disturb the food chain, ultimately causing harm to the animal which is intended to be saved. Other prophylactic measures, such as, improvement of sanitary condition and change of food habit can not be applied effectively in case of wild animals. Well controlled experiments to obtain accurate information on the effect of a particular parasite on wild animal have not been undertaken. Studies and record of parasites of wild animals in captivity do not reflect the true picture to be encountered in natural condition. Therefore, it is very difficult to differential pathogenic symptoms cause by parasite from nutritional disorder or by other factors.
Moreover, some worms may cause damage when present in a small numbers while others, though present in large numbers, may not cause any or produce only negligible problems. At the present state of our knowledge, no effective control method for the control of platyhelminth parasites could be suggested until parasitological survey of wild animals is undertaken and parasites thus obtained are identified.
Selected References
Hyman. L. 1951. The Invertebrates: Platyhelminthes and Rhynchocoela. Vol. II, WcGraw Hill Book Co. Inc. New York. Mehra. H. R. 1980. Fauna of India and the adjacent countries, Platyhelminthes vol. Trematoda: Digenea. Zool. SIIrv. India, Calclltta. Skrjabin, K. 1. and Spasalkii, A. D. 1951. Essentials of Cestodology, Vol. 1, Israel Programme of Scientific Translation, Jerusalum : 1-783. Southwell, T. 1930. Fallna of British India inclllding CelJlon and Burma, 1-391. Srivastava, C. B. 1992. The Fauna of India and adjacent cOllntries platyhelminthes, Vol. 1, (Supplements) Trematode: Digenea 1-163. Zool 5ltrv India, Calcutta. Wardle and Mcleoad, J. A. 1952. The Zoology of tapeworms: 1-780.
Minneapolis. Whitehouse, R. H. 1918, Indian Land Planarians, Rec. Indian, Mus, 14 : 1-187. Yamaguti, S. 1959. 5ystema helmint/III11-II, Interscience Publishers, New York, 1-860. Yamagati, S. 1971. Synopsis of digentic trematodes of vertebrates, Vol. I & II, Keigaku Publishing Co., Tokyo: 1-1074.
Platyhelminthes
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Introduction
The Platyhelminthes or flat worms constitute one of the major phyla of Animal Kingdom. These animals exhibit a great diversity of form and diverse bionomic relations.
They are included in a single phylum due to certain common features, viz. bilateral symmetry, dorsoventral flattening of body, ciliated epiderims in free living forms and at some stage in parasitic species, absence of coelome or body cavity, an incomplete digestive system, excretory system formed of protonephridia or flame-cells. One or more of these characters may be vestigeal or lacking in parasitic forms. They also have certain common negative features such as absence of metamerism, skeletal, circulatory and respiratory systems. Hyman (1951) defined them as "A coelom ate Bilatoria with a definite anus". Fonnerly they were divided into three classes viz. Turbellaria, Trematoda and Cestoda. Some authors include Mesozoa and Nemertea also under this phylum, while majority consider them separate phyla. In recent times class Tre~atoda has been divided into two separate classes viz. Monogenea and Trematoda.
Most of the members 9f the phylum are parasitic and this phenomenon has greatly influenced the character of the group~ Within the phylum there are groups illustrating gradual transition from a free-living mode of life to extreme dependence. The organs useful for parasitic mode of life have been proliferated while those structures which are essential for free living existence have been reduced or discarded. Thus we find absence of locomotary and sensory organs. The absorption of nutrients by the body surface has resulted in progressive regression of digestive system, so much so that in cestodes it is entirely absent.
The metabolic rate is probably not affected and efficient excretory system has developed for discharging nitrogenous waste. Their location inside the host body with limited supply of free oxygen led them to adapt to araerobic respiratfon. Nonnally the parasite remains within and perishes with the host. For perpetuation of the species the progeny must escape from the first host and find a new host. The larval stages are minute with little capacity to take food and limited locomotive capacity. To cope up with this hazards of parasitism these groups have developed enormous reproductive capacity and interpolated alternative hosts.
The phylum Platyhelminthes comprises four classes: Turbcllaria, Monogenea, Trematoda, and Cestoda.
Turbellaria
Turbellarians are generally small, free living carnivorous worms. Mostly they are less than 5 mm in length but the larger forms also occur in the orders Tricladia and Polycladida; some of the terrestrial planarians attain a length upto 50 ems. They are oval to elongate, flattened ventrally and somewhat convex dorsally. The body may be tuberculate or papillate; members of the Temnocephalida have anterior tentacles and a ventral muscular adhesive organ. The smaller species are white or translucent and present shades of gray or brown depending on the ingested food. The larger species are often brilliantly coloured due to presence of pigments in or under the epidermis, and have stripes, bars, or blotched patterns in grecn, red, yellow, orange and black colours. The anterior cnd bears the sensory organ, contains the principal ganglionic mass or brain of nervous system and precedes in locomotion.
The body is covered by a cellular or syncytscal epidermis beset with cilia or minute rhabdoral spiCUles. They arc primarily hermaphroditic; with often complex reproductive system. Thc fertilization is internal and life cycle simple. The cggs usually contain large amount of food material in the form of vistellinc cells, but the Acocla and Polycladida lack vitelline glands and in some of the polyclads a free swirnlning larva is produced. Many of the Turbellaria are either commensals or parasites : the rhabdocoels in echinoderms and molluscs; the allocoels in Crustaceans; the triclads in crustaceans, echinoderms, molluscs, chelicerate arthropods and selachian fishes.
They are said to be the precursors of different classes of the phylum Platyhelminthes. The classification of the group is based on the details of digestive or reproductive systems by different workers. The most accepted classification is based on the fonn of intestine and comprises the following five orders:
Acoela Minute,. marine forms, mouth ventral, no distinct intestine, gonad in parenchyma, without reproductive or excretory ducts~ at times coloured green or brown due to symbiotic algae.
Rhabdocoela Small, marine, freshwater or terrestrial forms, free living, commensal or parasitic worms, pharynx simple, intestine saccate, gonads compact, testes few, reproductive ducts and simple protonephridial excretory system.
Allococla Usually larger than rhabdcoels, mostly marine, a few freshwater fonns, pharynx of variable type, intestine lobate, testes numerous, prostonephridial system complex.
Tricladida Large, elongate marine fresh. Water or terrestrial worms. Pharynx plicate, intestine triclad, Gonopore single. Excretory system complicated with numerous nephridiopores.
Polycladida Large, broad, typically marine forms of shrinking shape and colouration. Pharyrx plicate, intestine much branched, gonads numerous, vitellaria absent; gonadapore cOlnmon or separate for male and female ducts.
Historical Resume
The foundation for a sound knowledgc of Indian Turbellarians was laid in the rust quarter of the 20th century by workcrs such as White-house, Mcxiner. Muth. Kaburaki, DUlta and others. Unfortunately the trend has not been maintained in recent years. Except for a few occasional papers the group appears to have received vcry lilllc attention.
Mexiner and Muth (1912) reported on a collection of aquatic turbellarians, made in Tibet by Capt. F. H. Stewart, during the year 1907. They dealt with the description of certain species of the families Catcnulidae, Dalydcllidae, Typhloplanidae under the Rhabdocoela, and Sorocelis under the Tricladida
Whitehouse (1913) described two new species of planarians, viz. Planaria aborensis and P. kempi from the Abor Hills in the North-east India. Again in 1914, he described eight new species of land turbcllarians of the genera Bipalium. Placocephalus. Pelmatoplana and Cotyloplana. Further, Whitehouse (1918) gave a detailed systematic account of Indian land turbellarians in the possession of Indian Museum.
The collections were earlier made from several parts of former Travancore and Cochin states; Nilgiri Hills; Coimbatorc in south India; Kumaon Hills in the Western Himalayas; Kurseong, Cherrapunji (Meghalaya), in thc Eastern Himalayas; and some from Sri Lanka. In this interesting paper he dealt with 11 species of Bipalium of which six were described as, new to science. Further, new species of COlyloplana from Nilgiris and four new species of Pelmatoplana from Kumaon, Kurseong, Pallipola (Sri Lanka) and Madathoray (fonner Travancore state) were described at length.
Based on a very small collection of freshwater and land planarians from Andaman Islands, Kabursaki (1925) dealt with new species, viz. Planaria andamanensis and Bipalium vinosum. Besides, Kabusami (1918, 1920) also contributed to our knowledge on the freshwater triclad fauna of adjacent countries such as Inle Lake in Burma and Siam in Thailand. Arora (1944) recorded freshwater turbcllaria Croln Kashmir and Kapadia (1947) the occurrence of Bipalium sp. in Junagadh (Kathiawar, Gujrat State). Ramakrishna (1953) reported for the first lime occurr~nce of land
Platyhelminthes
Planarian, Dolichoplanafeildeni from India. Saxena (1957) gave an account of a new species of Bipaluim viz. B. keshavi, from Nepal. Chauhan and Ramakrishna (1958) dealt with description of anew turbellarian Bipalium roonwali. Kawakatsu (1969) described Dugesia indica from Jabalpur. Ramakrishna Rao (1987) studied the systematic and the biochemical contents of polyclad worms from Waltair coast
Very little attention seems to have been paid to other aspects of study, viz., anatomy, bionomics, physiology etc. Dutta (1925) dealt with the anatomy of a rhabdocoelid turbellaria, Mesostoma gangelica and he (1926) described the structure and bionomics of a freshwater turbellaria belonging to Acoela. Ghoshal (1988) gave an account of regeneration in a freshwater turbellarian. Much remains to be done in this group and there is a need to develop expertise in the country.
Estimation of Taxa
C. B. Srivastava, Zoological Survey of India, 234/4, AJ.C. Bose Raod, Calcutta -700 020.
Elsewhere
G. Ramakrishna -Calcuua
B. S. Chauhan -Narsinghpur
V. Ramakrishna Rao .. Regional Centre, Ministry of Environment &Forests Bhubaneswar.
Anthony Basil -Madurai
Abroad
M. Kawakatsu Biology Depu., Fuji Womens' College, Kiatku, Kita -16 Nishi Sapporo (Hokkaido) 00 1, Japan.
Jerzy Kolasa, Agrobiology Depu., Institue of Ecology, Suierczewskiego, 19-60, Poznan, Poland. Lcigth Winsor, Zoology Depu., James Cook University, Todnsville, Queensland 4811, Australia. T. K. Rucbush, Osborn Zoology Lab, New Haven, Connecticut, U.S.A.
Selected References
Hyman, L. 1951. The Invertebrates: Plalyheitninlhes and Rlzynchocoela Vol. II. McGraw Hill Book Co. Inc. New York. Whitehouse, R. H. 1918. Indian Land Planarians. Rec. Indian Mus.• 14 : 187, 16 : 29.
