Mesozoa: India
Contents |
Faunal Diversity in India: Mesozoa
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 mesozoa is a small group of interesting but little known, lowly organised group of endoparasites found in the nephridia of squids and octopuses and in the body spaces and tissues of various other invertebrates, such as, nemertines, annelids and brittle stars. Structurally, they are simple, minute, worm-like organisms composed of an outer layer of ciliated cells or syncytium, the somatoderm and one or more inner axial cells from which the reproductive cells are formed. These layers are not comparable to epidermis of other metazoans. The life-cycle is, however, complex with an alternation of sexual and asexual phases.
Investigations on mesozoans date back to 1787, when Cavolini first described these organisms from the kidneys of Octopus later confirmed by Krohn (1839) and Erdl (1843). Erdl (op. cit.) noticed two types of embryos that alternate with each other in the life cycle of Mesozoa. Denoting this as an important feature, Von Kolliker (1849) suggested the name 'Dicyemida' to these organisms. Later, Van Beneden (1876) while describing their structure, morphology, life-cycle and phylogeny proposed the name 'mesozoa' to indicate their transitory position between protozoa and metazoa.
Status Of The Taxon
Though discovered in the early 19th century, the mesozoans still constitute a taxonomic puzzle and were not included in any major phyla because of their structural peculiarites. Some earlier investigators believed that they are structurally primitive and intermediate between protozoa and metazoa. During later years, several investigators considered them as degenerate flat worms since they had true gametogony. More recently Hanelt et al. (1996) and Powlowski et al. (1996), on the basis of 18S rONA and rRNA sequence analysis, showed that the mesozoans branched out early in animal evolution close to nematodes and myxozoans. Further, the studies also indicated that the 2 classes, Dicyemida and Orthonectida probably had separate origins.
Orthonectids, according to Hanelt et al. (op. cit), are more closely aligned with triploblastic metazoan taxa rather than platyhelminthes suggesting a need for the revision of the phylum. Originally phylum Mesozoa included a large number of heterogenous organisms of uncertain, unrelated affinities, such as Trichoplax, Treptoplax, Salinella, living inside radiolarians (Protozoa), Neresheimeria in the gonads of tunicates, all of which are now shifted to different phyla, thus making Mesozoa a better defined group. At present, phylum Mesozoa is comprised of 2 classes, namely Orthonectida and Dicyemida (Melhorn, 1988).
Global Status
Altogether 71 species of dicyemids represented by 4 genera namely, Dicyema, Dicyemennea ,Conocyema and Dodecadicyema were described from 59 species of sepioids and octopuses belonging to 18 genera. On occassions, each cephalopod may harbour a complex of assemblages of dicyemid mesozoans belonging to different genera. For instance, Dicyema balamuthi, Dicymennea obscita and Conocyema adminicula comhabit the kidneys of Octopus rebescens; Dicyema australis, D. platycephalum and Conocyema marplatensis in Octopus telll/elchus; Dicyema benthoctopi, Dicyemennea littlei in Benthoctopus magallansicus and Dieyemennea gracile, Dicyema tumeatum and Conocyema vespa in Sepia officianalis. Among orthonectids, the genus Rhopalura is widely distributed and represented by several species whereas genera Stoecharthrum and Pelmatosphaera have only one species each.
Indian Status
In the Indian context, studies on mesozoa are sporadic and scanty. So far, only 10 species of dicyemids represented by 3 genera namely, DiClJema, Dicymennea and Dodecadicyema, all from Bay of Bengal, have beeen recorded. There is no record of any orthonectid from this region.
Distribution
Class Orthonectida include a heterogenous group of rare parasites of coelomic spaces and tissues of a number of marine invertebrates, such as, turbellarians, polychaetes, molluscs, echinoderms and ascidians.
Mesozoans belonging to Class Dicyemida are widely distributed. These are most common and characteristic parasites of excretary organs and renal appendages of cephalopod molluscs, particularly the benthic and epibenthic forms. Hochberg (1983) noticed that in temperate and polar regions prevalence of infection of these mesozoans could be as high as 100% where as it is always less in subtropics (10-20%) and absent all together in and off oceanic islands. Nevertheless, Nouvel (1934) and Kalavati et al. (1978) recorded dicyemids from truely tropical locations, viz., West Africa and East Coast of India. Indian mesozoans have so far been collected from East Coast only from the hosts belonging to Sepia, Loligo and Octopus.
Biological Diversity And Its Special Features
The Orthonectid mesozoans are identified by the formation of agametes from multinucleate plasmodia belonging to both sexes. Somatoderm of sexual individuals bears transverse grooves. Sexes are separate. Egg cells are fertilised while inside the female and give rise to ciliated larva which later develops into amoeboid plasmodia.
There is an alternation of generations, a short lived, free swimming sexual phase alternating with a period of asexual reproduction and parasitic existence. The parasitic asexual stage is a multinucleate syncytical amoeboid mass, reproducing by simple schizogony in host tissues producing agametes. The agametes develop into male and female adults which escape from the host tissues and lead a short free-swimming life. Both male and female individuals are minute in size, with an outer ciliated somatoderm of definite number of cells arranged in concentric circles. The inner cells may gives rise to either spermatagonia or ovocytes which in tum give rise to sperms and eggs. Fertilisation is internal. Zygote develops inside the female into a ciliated larval stage, which eventually released to reinfect a new host.
The dicyemid mesozoans exhibit truely unique characters without definite affinities to any other group. Their life-cycle is still incompletely known. Despite reports of frequent occurrence of the parasites, their mode of entry into a new host is not known. It is generally believed that initial infections normally occur in very young cephalopods. Infection takes place after hatching in forms with demersal juveniles or after settling at the bottom in froms with planktonic larval stages. However, according to Lapen and Morowitz (1972) the germinal cells from the urn of the infusoriform could directly infect the circulatory system of the host and then penetrate the kidneys. The earliest known stage in the juvenile cephalopod is termed as stem nematogen. Subsequently, they develop into ciliated vermiform embryos with one axial cell and specific number of somatic cells known as primary nematogens. The stage of the dicyemid life cycle depends on the maturity of the host. Immature host harbour populations of nematogens. The adult nematogens are long, slender and vermiculate measuring 0.05-10.0 mm in size. They attach themselves to the renal appendage with the calotte embedded in the inter cellular spaces. The remaining part of the body floats freely in the coelomic fluid. The calotte is covered with thigmotactic cUlia which interdigitate with the brush border of the renal epithelial cells.
It consists of 8-10 cells arranged in 2 tires. The first tier of cells is known as propolars, while the second as metapolars. Body is covered with specific number of somatic cells, the diapolars which encircle the central axial cell. The central axial cell contains axoblasts and accessory nuclei. Vermiform embryos develop asexually from axoblasts and resemble the parent nematogen at the time of release. There is always a constant proliferation of daughter nematogens.
As the cycle progress, all nematogens are eventually transfonned into rhombogens. In sexually mature hosts, the shift in phase is more evident. Many investigators attribute this to honnonal flux and host maturity while other assume that crowding triggers the transfonnation (Lapen and Morowitz, 1975). The axial cell of this stage fonns gamete producing infusorigens and infusorifonn larvae. Each infusorigen consists of a spherical axial cell and all the stages leading to the maturation of spennatozoa which are enveloped by a layer of oogonia or oocytes. Amoeboid spennatozoa emerge from the central axial cell, penetrate the peripheral oocytes resulting in the formation of a zygote which eventually develops into a infusorifonn larva. These ciliated infusorifonn larvae escape from the renal fluid and their life cycle outside the host is still not known. According to Nouvel (1947), McConnaughey (1951) and Stunkard (1954), they infect a secondary benthic host before they reenter the cephalopod host. However, Lapan and Morowitz (1975) showed experimentally that sepia reared in the laboratory were infected when exposed to infusoriform larva indicating a possible direct life-cycle.
Host-Parasite Relationship
A spectrum of conditions ranging from a purely commensal to a true parasitic life is exhibited by mesozoa. Most dicyemid species adhere lightly to the renal tissue of the host by the ciliature of the calotte or the adhesive tuft ( Nouvel, 1931). However, presence of many dicyemid species appears to have no influence on the host tissue. On the contrary, orthonectids parasitize gonadal tissues of their host and destroy the cells. Host castration by the parasite is evident in the geneus Rllopalura.
Very little infonnation is available on the physiology of these parasites. According to Nouvel (1933), most dicyemids derive their nourishment from cephalopod urine. Carbohydrate reserves are mainly in the fonn glycogen. Faunal Diversity in India
Endemicity
As mentioned earlier, 10 species of mesozoans belonging to the genera Dicyema (5 spp.), Dicyemellllea (4 spp.) and Dodecadicyema (1 sp.) have been recorded so far from India, that too from Bay of Bengal. Insterestigly, all of them are endemic to this country.
Future Studies
Mesozoa being one of the least explored animal groups in India need to be thoroughly surveyed. Foregoing discussion reveal that mesozoans have so fare been collected from 4 few species of Cephalopod molluscs of Bay of Bengal off the East Coast India. Therefore, besides Cephalopod molluscs other marine invertebrates., viz., polychaetes, echinoderms, ascidians, etc., are to be examined for mesozoan diversity from both East and West Coast as well as Andaman and Nicobar and Lakshadweep Islands. Further, expertise needs to be developed to identify these animal groups.
Selected References
Bogolepova-Dobrokhotova, I. I. 1963. The current classification of the dicyernides. Parasit. Sbor., 21 : 259-271.
Hanlet, B. D., Van Schyndel. Adame C. M., Lais, L. A. and Locker, E. S. 1996. The Phylogenetic position of Rlropalura ophiocoma, Orthonectida based 185 ribosomal DNA sequence analysis. Mol. Bioi. and Evolution, 13 (9) : 1187-1191.
Hartmann, M. 1925. Mesozoa. In : Handbuch der Zoologie (Eds. Kukenthal, W. and Krumbach, T.), Berlin, Gryter, 1 : 996-1014. Hyman, L. H. 1940. Phylum Mesozoa. In : The Invertebrates, I (Protozoa through Ctenophora), McGraw Hill, New York, 726pp. Kalavati, C. and Narasimhamurit, C. C. 1980. A new dicyemid mesozoan, DodecadiClJema loligoi n. gen., n. sp. from the renal appendages of Loligo sp. Proc. Ind. Acad. Sci., 89 : 287-292.
Kalavati, c., Narasimhamurti, C. C. and Suseela, T. 1978. A new species of Dicyemenea, D. coramandalensis n. sp. from Sepia e/liptica Hoyle. Proc. Ind. Acad. Sci. (B), 87 (6) : 161-167.
Kalavati, c., Narasimhamurti, C. C. and Suseela, T. 1984. Four new species of mesozoan parasites (Mesozoa: Dicyemidae) from cephalopods of Bay of Bengal. Proc. Ind. Acad. Sci (B), 93 (7) : 639-654.
McConnaughey, B. H. 1949. Mesozoa of the family Dicyemidae from California. Univ. Calif. Publ. Zool., 55 : 1-34. McConnaughey, B. H. 1951. The life cycle of the dicyemid Mesozoa. Univ. Calif. Publ. Zool., 55 : 292-336
