The Biodiverse Rotifers (Rotifera: Eurotatoria) of Small Wetlands of the Brahmaputra River Floodplains of Lower and Upper Assam, Northeast India

Rotifera assemblages of small floodplain wetlands (dobas or dubies) of lower and upper regions of the Brahmaputra river basin of Assam state of northeast India (NEI) reveal 157 species, belonging to 34 genera and 18 families, and highlight notable speciose constellation of up to 50 species/sample. One species, each is new to the Oriental region and Assam, and species of global and regional biogeographic interest form notable fractions. The biodiverse rotifer fauna and various interesting species are hypothesized to habitat diversity of the sampled dobas or dubies, impact of `the Assam-gateway’an important biogeographic corridor of India, and location of the study area in the Indo-Burmese biodiversity hot-spot. Lecanidae and Lepadellidae are species-rich families, both of lower and upper Assam wetlands; upper Assam wetlands, in particular, are characterized by a distinct paucity of the Brachionidae and Brachionus spp., the relative paucity of Trichocerca, Keratella and Mytilina species, rare occurrence of Asplanchna and Filinia species, and lack of species of Conochilidae and Hexarthridae. Our results indicate the littoral-periphytic nature and tropical character of the rotifers. Overall, this study is an important contribution to Rotifera biodiversity of small lentic habitats of India, Asia and that of the tropics and subtropics.


Introduction
Small lentic ecosystems form over 90% of the standing water resources of our biosphere and ~ 30% of the global lentic biotopes by surface area and are thus suggested to be more explicitly considered in analyses of biodiversity, limnology and global processes (Céréghino et al. 2014). They are recognized as hotspots both in terms of species composition and biological traits (EPCN 2008) and are often considered as keystone systems for the conservation of biodiversity (Oertli et al. 2010;Céréghino et al. 2014;Vad et al. 2017;Oertli 2018). The management of these biotopes and that of their metazoan diversity is usually neglected (Céréghino et al. 2008;Oertli 2018) despite more vulnerability to severe threats of extinction and habitat degradation (Moss et al. 2011). The small lentic biotopes have attracted relatively more attention in hydro-biological works from India, but the relevant literature largely depicts paucity of studies on zooplankton diversity. A critical analysis attributes this lacuna to the limited sampling, ad-hoc reports by amateurs loaded with incomplete species lists, and lack of taxonomic expertise. Nevertheless, our studies from NEI (Sharma and Sharma 2015a; Sharma and Kensibo 2017;  hypothesized these water bodies to be one of the biodiverse habitats of the Indian sub-region vis-à-vis Rotifera -an important group of freshwater zooplankton, an integral link of aquatic food-webs, and food for larvae and adult fish (Tuna and Ustaoğlu 2016; Apaydın Yağcı et al. 2017).
The state of Assam of NEI, a part of the Indo-Burmese biodiversity hotspot, is notably known for the fluvial floodplains of the Brahmaputra river with characteristic small lentic wetlands (commonly known as dobas or dubies) forming an integral part of the rural landscape of the Brahmaputra valley. Sharma and Sharma (2014) first indicated ecological diversity importance of small wetlands in context of Rotifera of NEI and suggested their detailed assessment. The present study is thus an endeavor to analyze and compare species composition and richness of the rotifer assemblages of small wetlands of lower and upper reaches of the Brahmaputra river floodplains of Assam state. We provide an inventory of the documented taxa, illustrate interesting species, and comment on nature and composition of the rotifer fauna with reference to species richness, new records, important taxa, extraordinary high species consortium and elements of global and regional distribution interest. This study merits biodiversity and biogeographic interest for Rotifera of the floodplains of India, Asia as well as that of the rotifer heterogeneity of the small lentic ecosystems of the Indian sub-region.

Results
The plankton and semi-plankton collections examined from small wetlands of the Brahmaputra River floodplains of lower and upper Assam reveal 157 species belonging to 34 genera and 18 families. A detailed systematic list of the observed taxa is presented below:  is yet an un-determined species awaiting description pending examination of more specimens. Our collections indicate 23 species (marked as @) with their distribution in India known to be restricted to NEI.
We report 152 and 125 rotifer species from lower and upper Assam collections, respectively (Table 1) with consistent importance of Lecanidae (45 and 42 species), Lepadellidae (21 species each) and Trichocercidae (16 and 13 species). The comparison of Rotifera species composition of the two study areas records lower richness of the Brachionidae (14 species), Brachionus (6 species) and paucity of species of Keratella and Mytilina especially in our collections from upper Assam wetlands (Table 1).

Discussion
Our collections from dobas or dubies of the Brahmaputra river basin reveal 157 species, belonging to 34 genera and 18 families; the richness comprises ~65%, ~52% and ~27% of species of Rotifera known from Assam state, NEI and India (Sharma and Sharma 2017), respectively and thus affirms biodiverse nature of the rotifer assemblage. Besides, 152 and 125 species observed from lower and upper Assam, respectively depict species-rich rotifers of the two study areas individually, while higher community similarity (85.2% vide Sørensen's index) depicts overall homogeneity in species composition but with certain differences.
Lecane clara, a new record from the Oriental region, is diagnosed by its soft lorica and characteristic elongate toes. This lecanid is yet known (Segers 2007) from the African, Neotropical. Nearctic, Pacific, and Palaearctic regions, while our report extends its distribution to the Oriental region. Lepadella desmeti, described from Thailand , is known elsewhere from Neotropical and Pacific regions (Segers 2007). The only earlier record of this species from the Indian sub-region (Sharma and Sharma 2015b) relates to Loktak Lake (a Ramsar site), Manipur state; the present study further extends its distribution within NEI to Assam state. In addition, Lecane calcaria and L. stichoclysta deserve mention in view of the restricted reports from south Assam (Sharma and Sharma 2019a) and the eastern Himalayas (Sharma and Sharma 2019b).
Interestingly, Rotifera of dobas or dubies invariably record species consortia of maximum 30-35 species/sample while a few small wetlands from the Majuli River Island and the Dibru-Saikhowa Biosphere reserve of upper Assam indicate speciose constellations of up to 50 species/sample. We categorize these reports as `Rotifera paradox' following analogy to the classical `the paradox of the plankton` highlighted by Hutchinson (1961); the former, in turn, is hypothesized to the intriguing possibility of the co-existence of a number of species in `a relatively isotropic or unstructured environment of small wetlands`.
This study records biodiverse rotifers than the reports from dobas or dubies of the Majuli River Island michelangellii, Platyias leloupi, Testudinella parva, Trichocerca flagellata and Wolga spinifera depict regional distribution interest vis-à-vis the Indian Rotifera . We hypothesize overall biodiverse rotifer assemblage of dobas or dubies of the Brahmaputra river floodplains and occurrence of sizable fractions of specie of biogeographic interest to impact of `the Assam-gateway'-a vital biogeographic corridor of India that facilitated extensive interchanges between the Indian and Asian biota (Mani 1974 (Sharma andSharma 2008, 2014). The high richness of 'tropic centered' Lecane and that of Brachionus in lower Assam wetlands, large fraction of cosmopolitan species (~67%) and occurrence of several (~20%) pantropical and cosmotropical species imparts 'tropical character' to the rotifer assemblages of dobas or dubies following the reports on the tropical rotifer faunas (Fernando 1980;Segers 2008;Sharma and Sharma 2008, 2014, 2019a.
To sum up, this study is an important contribution to Rotifera biodiversity of small lentic habitats of India, Asia, and tropics and subtropics. The biodiverse rotifers affirm habitat and environmental heterogeneity of dobas or dubies of the Brahmaputra floodplains. `Rotifera paradox` hypothesizes niche diversification enabling coexistence of several species within an unstructured environment of small wetlands. The diverse species composition is attributed to the location the sampled study areas within the key biodiversity area of the Indo-Burma Hotspot as well as the historical influence of `the Assam-gateway' facilitating incursion of species from Asian and the Oriental faunas. Such studies need to be extended to other small lentic wetlands of NEI and elsewhere in India to explore ecosystem diversity value of small water bodies vs. biodiversity of Indian Rotifera.