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Molluscans are the environment as well as bio-indicators  and  they  play  a  very  important  role  in  maintaining  aquatic  ecosystem  by  recycling  nutrients  and  surviving  as  nutrition  for  certain  aquatic  organisms.

Freshwater molluscs play a significant role in aquatic ecosystem, and some of them are edible. Also, they serve an important source of food for other animals i.e. fishes, birds and mammals even for human being. Wagh (2019). The taxonomic study of Indian fresh water molluscs has been done by Zoological Survey of India, Subba Rao (1989), Also in Maharashtra, freshwater Mollusca reported by Rao (1925), Tonapi and Mulherkar (1963), Tonapi ((1971), Magare et al. (2016), Kambale, (2018), Kumar et al. (2019).
Benthic organisms constitute an important component in the food web of a freshwater ecosystem. A detailed and complete knowledge of the bottom fauna is an essential pre-requisite not only for the determination of productivity but also for an assessment of the dermersal fisheries of any area. (Shibu et al., 2006). However, species diversity of molluscan has not yet been investigated in Indian tropical lentic waters of semi arid region of north Karnataka. In recent years the literature on molluscan distribution, occurrence, ecology and population density is rare. Earlier surveys on the ecology of benthos, though several investigations were made on the occurrence and distribution of benthic communities of most of the freshwater bodies. Fresh water molluscs are known to play significant role in the public and veterinary health and therefore needs to explore their diversity. (Magare et al., 2016). The current investigation deals with the comparison of the distribution, abundance and relationship of two of the reservoirs. The study also provides data on the molluscan fauna in Khaji Kotnoor reservoir a topic that has received little attention as yet.

MATERIALS AND METHODS

The Karanja reservoir is major perennial reservoir of the Bidar district and located at Byalhalli village, which is 100 kms away from the Gulbarga University campus which falls under 170-55’ N latitude and 770-32’ E longitude. The Khaja Kotnoor reservoir is also one of the major perennial reservoir of the district and located at about 20 km away from Gulbarga University campus which falls under 17º-22' N latitude and 76 º -59' E longitude.
Water samples were collected from October 2001 to September. 2003 on monthly basis. Collections were made on specific dates of every month. Surface samples were collected using a clean plastic container for the study of various physico-chemical and biological parameters. Water samples were colleted from eight stations in Karanja reservoir and Kahaji Kotnoor reservoir. Water samples collected were subjected to analysis by using Standards Methods for Examination of Water and Waste Waters (20thEdition (APHA., AWWA, 1998).
Collection of Molluscan fauna
Molluscans both benthic and peripheral forms were collected from the reservoir with the help of dipnet or dredges and live ones and shells were collected by hand. The live ones were cleaned and preserved carefully in 4% formalin and the collected shells were thoroughly washed with methyl alcohol and water before they were subjected to identification. The molluscans were separated and enumerated group wise. The specimens are identified as per Subba Rao (1989).

RESULT AND DISCUSSION

The seasonal occurrence and species diversity of molluscans in Karanja and Khaji Kotnoor reservoirs are summarized in Table. No. 1 and 2. Average values of molluscans collected at different regions and different seasons of the reservoir are presented in this paper. In both reservoirs two groups of mollusca viz., gastropoda and bivalve were identified during the course of the study. Total eight species were in gastropoda and three species were in bivalves identified. Gastropoda of mollusca contributed the main bulk of the fauna. About 75.43% of gastropoda and 24.57% bivalves were represented. Comparatively second year represented more gastropoda population. The bivalve population restricted to shallow zones of the reservoir where macrophytic vegetation was dense. This could be attributed to availability of food and substratum for attachment. In the present study, Lymnea luteola, L. acuminata, Melania (Palitia scabra) and Melania scabra var elegans were the more dominant species in Karanja reservoir. Melania tuberculata and Viviparous variata were the second dominant species. Similarly in Khaji Kotnoor reservoir Melania scabra var elegans, Digoniostoma pulchella, Lymnea luteola, L. acuminate were more dominanat, widely distributed and also observed in all the seasons during the study period. However, Melana scabra, M. tuberculata, Faunus ater and Planorbis exustus were second dominant species during the study period. The total gastropoda population was more during northeast monsoon season and southwest monsoon season. In both reservoirs an inseparable association between gastropods and macrophytes in four different ponds have been observed by Dudani et al. (1987). Hence, the quality and quantity of macrophyte vegetation appear to play a vital role in determining the variations in gastropoda (Soszkal, 1975). Molluscans were found throughout the year indicating their tolerance to varied environmental conditions. Abundance of gastropods might be due to dense vegetation and shallow zone coupled with plenty of dissolved oxygen optimum range of pH and alkalinity. Almost all the bivalves are aquatic and great majority of freshwater forms occurs in shallow banks of reservoirs and rivers. Bivalves representing three genera were found throughout the study period. Parreysia corrugate var nagvorensis, Lamellidens corrianus and Indonia coeruler were the dominant species of bivalves in Karanja reservoir. Similarly, in Khaji Kotnoor reservoir Parreysia corrugate and Lamellidens corrianus were more dominant and these species are more abundant during northeast monsoon season and summer season of the two years of the study period. Indonia coeruler were also observed in the present study but they are relatively low when compared to other two species.
The maximum and minimum concentration of different physico-chemical parameters of water samples are presented in Table No.3 and 4. From the results of analysis, it has been observed that the atmospheric and water temperature was between 31.5 to 39.91 and 26.13 to 34.16 respectively in Karanja reservoir. Similarly, 30.6 to 41.0 and 24.82 to 33.9 in Khaji Kotnoor reservoir. The pH values were recorded between 6.9 to 8.5 in Karanja reservoir and 7.2 to 8.6 in Khaji Kotnoor. The DO values varied between 2.6 to 10.7 in Karanja reservoir and 2.4 to 9.0 in Khaji Kotnoor reservoir. The total alkalinity values were recorded between 70.7 to 249.3 in Karanja Reservoir and 75.2 to 260.8 in Khaji Kotnoor reservoir. The total hardness, calcium hardness and magnesium hardness varies from 50.5 to 162.0, 33.25 to 131.33 and 6.04 to 70.93 respectively in Karanja reservoir. Likewise, 72.0 to 161.3, 34.92 to 96.8 and 23.32 to 64.6 in Khaji Kotnoor reservoir indicating the hardness. The chloride content was from 67.69 to 126.86 in Karanja reservoir and 17.78 to 80.84 in Khaji Kotnoor reservoir.
In the present study, the occurrence of molluscans directly correlated with calcium and total hardness, similar correlation was noticed by Sitaramaih (1966) and Sampath et al. (1981). In general based on the molluscan population recorded for the period of two years in Karanja and Khaji Kotnoor reservoirs cannot be compared to other reservoirs located in different parts of India, because the reservoirs are moderately productive. However, from the present findings it is obvious that molluscan diversity and density are in close association with microphytes, water and sediment types for their normal life.

In the hitherto study we have subjected the data, for some diversity indices. The similarity index between Karanja and Khaji Kotnoor reservoirs is 0.68. Similarly the values of each species of Shannon Weiner diversity index is given in Table No. 5 and 6. In Nutshell, Karanja reservoir has shown the almost similar diversity index in all season. The values are 2.942 (NEM), 2.977 (Summer) and 2.483 (SWM). While, Khaja Kotnoor shown 2.927(NEM), 2.907 (Summer) and 2.920 (SWM). The result indicates that NEM and Summer seasons of Karanja showed slightly higher diversity than the Khaji Kotnoor. While, in summer season of Karanja shown slightly lesser diversity than the Khaji Kotnoor reservoir.

Table 1: Seasonal variation of Molluscan species in Karanja Reservoir

Table 2: Seasonal variation of Molluscan species in Khaji Kotnoor Reservoir

Table 3: Average values of Physico-chemical parameters of Karanja reservoir