ROTIFER COMMUNITIES (ROTIFERA : EUROTATORIA) OF RICE-FIELD ECOSYSTEMS OF MEGHALA Y A : COMPOSITION AND DISTRIBUTION

Rice fields exhibit a rich mosaic of ecotones, changing in time and space seasonally and rapidly and are characterized by complex limnology not mirrored by any natural aquatic habitat (Fernando, 1995), In addition, they combine unique features of aquatic and semi-aquatic environs during different growth phases of paddy cultivation and are known to be biologically diverse biotopes colonized by a wide spectrum of aquatic organisms (Fernando et al., 1979; Fernando, 1993) in general and micro-invertebrate communities in particular. The later, in turn, are invariably characterized by greater rotifer richness.


INTRODUCTION
Rice fields exhibit a rich mosaic of ecotones, changing in time and space seasonally and rapidly and are characterized by complex limnology not mirrored by any natural aquatic habitat (Fernando, 1995), In addition, they combine unique features of aquatic and semi-aquatic environs during different growth phases of paddy cultivation and are known to be biologically diverse biotopes colonized by a wide spectrum of aquatic organisms (Fernando et al., 1979; Fernando, 1993) in general and micro-invertebrate communities in particular.The later, in turn, are invariably characterized by greater rotifer richness.
A review of the Indian literature reveals general lack of attention on micro-faunal diversity of Phylum Rotifera in rice-fields of this country (Sharma, 1998a), Sharma andShanna (1999, 2005), however, remarked on occurrence of rotifers in these ecosystems and commented on importance of extensive faunal investigations in these interesting environs.The present study, an attempt to fill up the stated lacuna, provides a detailed account of the rotifer taxocoenosis of the ricc-field ecosystems of Meghalaya based on collections examined from different parts of this state.Seventyfour species of Eurotatoria are documented in this account.Various rare and interesting species arc briefly diagnosed and illustrated.Remarks are made on nature and composition of the examined rotifer fauna and on distribution of interesting elements.

MATERIALS AND METHODS
The present observations are based on plankton samples collected, during 200 1-2003 and also those collected earlier (during 1988-1990) for the "State Fauna of Meghalaya : Zooplankton Survey" from rice-fields located in different districts of Meghalaya state (Fig. 1, a-b).The material examined for this systematic survey included the samples deposited in Freshwater Biology Laboratory, Department of Zoology, North-Eastern Hill University, Shillong and those in the holdings of the Eastern Regional Station, Zoological Survey of India, Shillong.
The examined samples are collected, during the period of paddy-cultivation (May/June-October), with a nylobolt plankton net (No. 25) and preserved in 50/0 formalin.Various samples are screened, the rotifer taxa are isolated, and permanently mounted individually in Polyvinyl alcohol-lectophenol mixture.The drawings are made with a Leitz-Dialux phase contrast stereoscopic microscope using a drawing tube attachment and measurements are indicated in micrometers (Jlm).

SYSTEMATIC NOTES ON RARE AND INTERESTING TAXA
The examined collections revealed a number of rare and interesting elements.These taxa are briefly diagnosed below, with notes on their distribution: (Fig. 2) Diagnosis: Lorica finn, stippled, dorso-ventrally compressed and with maximum width in its posterior region.Anterior occipital margin with distinct median spines of variable length.Posterior spines moderately long and divergent.
Distribution : Meghalaya : reported earlier by Shanna and Sharma (1999).India: Meghalaya, Tripura and Assam.Elsewhere: Australasian, with records from Australia and Thailand.

Lecane jaintiaensis
Distribution: Meghalaya : described (Sharma, 1987) from a rice field at Nartiang, Jaintia Hills district and known so far only from its type locality.India: an endemic species.Shanna, 1978 (Figs. 17 & 18) Diagnosis : Lorica ovate, anterior dorsal margin concave and anterior ventral margin with a shallow sinus flanked by undulating sides.Dorsal plate smaller than ventral p1ate.Ventral plate with postero-Iateral extensions.Toes long, paral1el-sided along 314 of their lengths, then tapering and terminating into stout claws; each claw with one basal spicule.
Lecane mOllostyla (Daday, 1897) (Fig. 19) Characters: Lorica small, oval, with straight and coincident anterior margins; dorsal plate with characteristic lateral spine-like processes, ventral plate with very small spines at anterior external angles.Toe parallel-sided for % of its length and then tapering to an acute point.Testudinella parva (Ternetz, 1892) (Fig. 22) Characters : Lorica pear-shaped and with maximum width In the posterior region.Lateral antennae located behind the middle region of lorica.Foot-opening elliptical and located at a short distance from posterior margin of lorica.
Distribution: Meghalaya : recorded earlier by Sharma and Shanna (1999) Testudinella tridentata Smirnov, 1931 (Fig. 23) Diagnosis: Lorica vase-shaped, compressed, with maximum width behind its middle and then tapering gradually to a posterior lobate projection.Anterior dorsal margin with long median spine.
Foot-opening elongated, located at posterior ventral end of lorica.

REMARKS
Seventy-four species of Eurotatoria documented presently from rice fields of Meghalaya indicate fairly rich and diversified taxocoenosis.This salient feature reflects a greater environmental heterogeneity of the sampled habitats which, in turn, is attributed to their unique and complex limnological nature (Fernando, 1995), water level fluctuations, plantation and growth of paddy, and growth of aquatic weeds.Interestingly, the recorded richness represents the highest qualitative diversity of freshwater rotifers known till date from rice fields of any part of the Indian subcontinent and of the Oriental region and is followed by earlier reports (Fernando, 1980(Fernando, , 1995) ) of 7] and 5 J species from rice fields of Sri Lanka and Thailand, respectively.The rich nature of the rotifer taxocoenosis is also supported by greater higher diversity of this phylum (30 genera and 17 families) which, in turn, certainly compares well with the reports of 39 genera and 20 families from N. E. India (Sharma and Sharma, 2005).Qualitative dominance of Rotifera over other groups of microinvertebrates noticed in all the collections examined from rice fields of Meghalaya concurred with the trends noted in these biotopes from other parts of the world (Fernando, 1995).
Biogeographically interesting elements constitute a notable fraction (8.1 0/0) of the listed species; these include the endemic Lecane jaintiaensis, the Australasian Brachionus dichotomus reductus and four palaeotropical species namely Lepadella discoidea, Lecane lateralis, L. unguitata and Trichocerca cylindrica.Among these, the endemic lecanid deserves special mention because of its rare occurrence in the Indian fauna and is so far known exclusively from its Htype-Iocality" i.e., a paddy field located in Nartiang, laintia Hills district of Meghalaya state.The second species represents one of the four Australasian elements recorded from India and is so far documented only from N. E. region (Sharma and Sharma, 2005).Further, this brachionid serves an interesting example to assign special affinity of the rotifer faunas of Meghalaya and N. E. India with that of S. E. Asia and tropical Australia (Sanoamuang et al., 1995;Sanoamuang, 1998;Sharma, 2004Sharma, , 2005)).Segers (2001) commented on occurrence of reductus vicariant of B. dichotonzus outside Australia, hypothesized recent expansion of these populations to Southeast Asia and hinted at a possible Australian origin of this taxon.The restricted occurrence of this brachionid in N. E. region (Sharma, 2005) lends additional support to Segers's hypothesis.Among the members of the last category, Lepadella discoidea has been described (Segers, 1993) from Africa while Lecane latera/is, originally described froln West Bengal (Sharma, 1978), is now known to be apparently widely distributed in sub(tropica1) parts of the Old and New world.
The rotifer conllnunities of rice fields of Meghalaya exhibit predominance of cosmopolitan species (71.6%); their overall contribution is higher than noticed earlier in the faunas of Meghalaya (Sharma and Sharma, 1999) and N. E. India (Sharma and Sharma, 2005), respectively.On the other hand, Tropicopolitan (14.9%) species are well represented while the present study indicates relatively fewer Pantropica] elements (5.40/0).The stated pattern, however, deviates than the observed richness of Pantropical > Tropicopolitan species in the faunas of Meghalaya and N. E. region.Lecanidae (23 species) > Lepadel1idae (15 species) > Brachionidae (9 species) together constitute a significant component (63.3%) of the documented species.This stated pattern differs notably from that of Meghalaya and N. E. region Rotifera because of occurrence of distinctly fewer species of the last family in the examined collections.
Highest richness of up to 35 species/sample in sUbtropical rice-fields of Khasi (East and West) and laintia Hills districts of Meghalaya observed currently, with predominance of Lecane > Lepadella endorsed earlier results of Sharma (1987) and Sharma andSharma (1987, 2005).On the contrary, the samples from Garo Hills region indicated relatively lower richness with maxilnurn up to 22-25 species/sample.Though the present study did not make any systematic attempt to trace temporal changes in species richness during the whole paddy season (May/June-October), the analyzed collections indicate fewer pioneer Lecane species fol1owing ploughing and application of organic fertilizer (cow dung) during May/early June and the community heterogeneity increased with paddy plantation and its subsequent growth and peak richness was invariably observed in the samples collected during August-September.Such a generalization also concuned with the richness pattern reported by Fernando (1995).
The shallow nature of the sampled rice fields and the growth of paddy and some aquatic weeds apparently resulted in predominance of periphytic species in the studied collections, showed occurrence of very few facultative planktonic taxa and complete lack of any typical planktonic element.Plationus patulus, Trichotria tetractis, Lepadella patella, L. ovalis, Lecane bulla, L. leon tina, L. luna, L. lunaris, L. quadridentata and Testudinella patina exhibited relatively common occurrence.The list of presently documented taxa, though fairly exhaustive as per the examined materials, may still provide scope for its up-dating based particularly on regular fortnightly/monthly collections from these interesting ecotones.

SUMMARY
Seventy-four species of Eurotatoria, belonging to 30 genera and 17 families, documented presently from sub-tropical rice fields of Meghalaya represent the richest species and higher diversity of Rotifera known till date from these habitats from any part of the Indian subcontinent.Lepadella eurysterna represents a new record from N. E. India while ten species are new records ;to the rotifer fauna of Meghalaya.Biogeographically interesting elements constitute a notable fraction (8.1 0/0) of the listed species~ these include the endemic Lecane jaintiaensis, one Australasian el'ement and four palaeotropical species.Besides, this study indicates several elements of regional distributional importance including three species known to be so far restricted to N. E. India, two to Meghalaya and five recorded only from Meghalaya and Assam.Cosmopolitan species (71.60/(1) distinctly dominate Tropicopolitan (14.9%) > Pantropical (5.4%) species.The rotifcr taxococnosis depicts general tropical nature characterized by richness of 'tropic-centered' genus Lecune, qualitative importance of Lecanidae > Lepadel1idae, predominance of the littoral-periphytic clel11cnts and lack of typical planktonic taxa, paucity of Brachionus spp., and records highest richness of upto 35 species/sample.