Keywords

Introduction

Molluscs are an important group for aquatic biodiversity and, where abundant, play an important role in ecosystem functioning (Vaughn et al. 2004). They form an important component of most biological monitoring programmes that rate water quality and status of aquatic systems based on invertebrate assemblages (Tonapi 1980; Ponder 1994; Seddon 1998; Strong et al. 2008). Bivalves in particular, as they accumulate toxic substances to a greater extent than other organisms, are used to monitor water quality (Salanki et al. 2003; Bogan 2008).

Freshwater molluscs are essential to the maintenance of wetland ecosystems, primarily due to their control of water quality and nutrient balance through filter-feeding and algal-grazing and, to a lesser degree, as a food source for predators including a number of fish species, and in some parts of the world, they compose a significant food resource, especially for the rural poor and tribal. In some regions, they are one of the most threatened groups of freshwater taxa (Kay 1995). The impact of developments such as dams, and siltation caused by deforestation and agricultural clearance, has not been adequately researched, and there is little awareness of the complex life histories of some groups such as unionid mussels that rely on the maintenance of migratory fish runs to carry their parasitic larvae to the river headwaters. Many species are also restricted to microhabitats, such as the riffles (areas of fast current velocity, shallow depth, and broken water surface) between pools and runs (areas of rapid nonturbulent flow).

Review of Literature

Studies of freshwater molluscs of India were at their peak during the late nineteenth and early twentieth centuries. Prashad (1928) and Preston (1915) contributed much to our knowledge on the taxonomy and distribution of molluscs in the region. Later, the Zoological Survey of India surveyed northeastern Indian states and published checklists of species occurring in different states (e.g. Fauna of West Bengal, Fauna of Meghalaya and the Fauna of Tripura). Recently, Nesseman et al. (2007) contributed to our knowledge of the freshwater molluscs of the Ganga river basin in India and Nepal. However, there are some taxonomic issues that need to be addressed urgently (e.g. the species recently described by Nesseman et al. (2007) needs further clarification). The taxonomic status of several species, including Lymnaea shanensis and Parreysia olivacea, collected from lakes in Myanmar need to be verified as these species have not been collected again since they were described. There has been little recent study of molluscs across large parts of the region, including Bhutan, parts of Nepal, and Myanmar, as well as several states in India.

We have attempted here to consolidate the freshwater Mollusca of India in brief including information about their origin, ecology, distribution, identification characters, habitat, edible form and parasites, medical importance, and conservation and recommendation.

Zoogeographical Origin of Freshwater Mollusca Fauna in India

The freshwater molluscs are distinguished into two natural groups, namely, primary freshwater and secondary freshwater (Subba Rao 1989). Nesemann et al. (2007) mentioned another two that is peripheral freshwater fauna of marine origin and peripheral freshwater fauna of terrestrial origin. The primary freshwater molluscs are confined exclusively to the freshwater habitats, i.e. which have their main evolution and distribution since the Paleozoic period in continental freshwater. The secondary freshwater are those which have their main evolution and distribution since the Paleozoic period in marine habitats. They are secondarily invading freshwater habitats since carbon period (Nesemann et al. 2007). There are many species which are distributed both in estuarine and freshwaters. Such genera as Neritina, Clithon, Septaria, Scaphula and Novaculina belong to this category. These genera are primarily freshwater with tolerance to saline waters. The family Stenothyridae and subfamily Iravadiinae include primarily estuarine or saline water species, occasionally occurring in freshwaters. Among bivalve the genera Villorita, Geloina, Batissa and Tanysiphon are primarily estuarine, rarely occurring in freshwaters. One species of the family Assiminedae is also an estuarine species occasionally found in freshwater also.

Habitat

Freshwater molluscs are common in ponds, lakes, quiet water pools, and flowing waters like perennial rivers, irrigation canals, etc. The malacofauna of the streams may be somewhat different from that of the stagnant waters. Most of the endemic freshwater molluscs are found in the Western Ghats and Nilgiri and are remarkable for their characteristics.

Gastropods are generally found attached to submerged vegetation, rocks, sticks, bricks, etc., but bivalves live partly buried in the sand or mud. Similar to the gastropods, the bivalves exhibit variations in shell depending upon the ecology of the species and may produce ecophenotypes (Subba Rao 1989).

Anhydrobiosis

Freshwater molluscs are usually faced with an annual dry season. Gastropods and bivalves show varying capacity to survive desiccation. Many snails can overcome this by aestivation either under dead vegetation, rocks, bricks or debris or actively burrowing at least the aperture of the shell in the mud. Some shells are capable of anaerobic respiration. With the advent of favourable conditions, the molluscs resume to their normal activities. The discontinuous and transient nature of freshwater bodies leads to isolation of snails or bivalves into small local populations. Nevertheless, much remains to be worked on the ecology of Indian freshwater molluscs.

Role of Freshwater Molluscs in the Ecosystem

Freshwater molluscs are one of the most diverse and threatened groups of animals (Vaughn et al. 2004; Lydeard et al. 2004). They are mostly unobtrusive and are not normally considered charismatic creatures, rarely attracting the attention of the popular media, unless in a negative light, as some species play a significant role (as a vector) in the transmission of human and livestock parasites and diseases (Sinha and Srivastava 1960). This is unfortunate, as they also play a key role in the provision of ecosystem services and are essential to the maintenance of wetlands, primarily due to their contribution to water quality and nutrient cycling through filter-feeding, algal-grazing, and as a food source to other animals (Strayer et al. 1999; Vaughn et al. 2004, 2008; Howard and Cuffey 2006). Some species are of high commercial value as food or ornaments (e.g. clams and some mussels and snails).

Freshwater Molluscs Used as Food

The freshwater molluscs are important as additional resources of protein for the increasing human population of our country. An increase in the shell production can augment our protein-rich food supply. It has been observed that at least 22 freshwater species are used as food and there is regular sell of shellfish in several markets of eastern (West Bengal) and northeastern states (Table 11.1). Shellfishes are harvested from the natural resources by different methods and brought to the markets. There are no regular markets in some places, but still people use the shellfish as food very frequently. It is very popular among poor and tribal people of India.

Table 11.1 Common use of freshwater mollusca as food and other purpose in various parts of India
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Freshwater Molluscs and Diseases

In India, millions of people and their livestock are infected with schistosomiasis, amphistomiasis, and paragonimiasis and other trematode parasites. Efforts to control schistosomiasis and other snail-borne diseases depend to a great extent on knowledge of the freshwater snails which serve as intermediate hosts for the parasites.

Freshwater molluscs are the carrier for schistosomiasis (Bilharziasis) in humans. The only report of this disease in the assessment region is in Gimi Village in Ratnagiri District of Maharashtra State (Gaitonde et al. 1981). The freshwater molluscs species Ferrissia tenuis (LC) (Bourguignat 1862) is a target species for control programmes against Schistosoma haematobium. Other species from which schistosome infection has been reported are in L. exustus and L. luteola (Table 11.2). The question of the transmission of urinary schistosomiasis elsewhere in India still remains a mystery. Future research should focus on this (Table 11.3).

Table 11.2 Disease associated with freshwater mollusca in India
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Table 11.3 Medical importance of freshwater mollusca
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Distribution

In India distribution of the freshwater molluscs shows some peculiarity that is scattered and discontinuous distribution of major species. The freshwater gastropod fauna belongs to the clades Neritimorpha, Caenogastropoda and Heterobranchia (including the Pulmonata) (Strong et al. 2008). They are distributed in a wide range of habitats including rivers, lakes, streams, swamps, springs, temporary ponds, drainage ditches and other ephemera land seasonal waters. Highest diversity occurs in the tropics with decreasing species richness and endemicity at higher latitudes (Strong et al. 2008) and altitudes. The Neritidae, Assimineidae and Iravadiidae are brackish water marine families. In the assessment region, seven species from Assimineidae and two species from Iravadiidae are represented, and they are exclusively brackish water species.

Thirteen gastropod families are in 33 genera comprising 112 species (Table 11.1) from the Caenogastropoda and Pulmonata. The Thiaridae is the most dominant family representing 19 % of species within the region followed by Planorbidae (17 %) and Lymnaeidae (9 %). In terms of genera representation, the Planorbidae has the highest number of genera with 19 %, followed by Neritidae (13 %) and Bithyniidae (13 %).

Freshwater bivalves of the India belong to five families (16 genera and 74 species). Family Unionidae is the dominant group, containing 66 % of species found in the region, followed by Sphaeriidae (21 %) and Corbiculidae (10 %) (Table 11.4). Family Solecurtidae is represented by only one species, Novaculina gangetica, endemic to coastal areas of West Bengal.

Table 11.4 Up-to-date list of species, identifying characters, their habitat and distribution
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Many type localities need to be resurveyed to confirm if described range-restricted freshwater molluscs are still present or have already become extinct and to confirm the taxonomic status of previously described species. Except for a few commonly occurring species, information on ecology, population structure, and dynamics, distribution, and habitat preference is not known. A greater degree of taxonomic research and training is also required to ensure that widely accepted taxonomic concepts are adopted. Taxonomic research is central to ecological studies and conservation, but it is one of the most neglected disciplines (Stuart et al. 2010), especially in counties rich in biodiversity but poor in resources. Training in taxonomic expertise and enhanced communication and outreach are basic requirements of biodiversity conservation. Taxonomic knowledge of freshwater molluscs of India is severely lacking. Preston’s Fauna of British India written in 1915 is still the fundamental book comprising the taxonomic account of freshwater gastropod and bivalves of the Indian subcontinent, though Subba Rao (1989) updated the taxonomic knowledge of the region. Since then the taxonomic knowledge has not been developed among regional taxonomists. There are many under- and unexplored areas that can be expected to contain both undescribed species and new populations of currently known species, but there are hardly any new descriptions of molluscs in the region in the decades that have passed since Subba Rao (1989) with few exceptions. It is possible that many of the data deficient species, and especially those that have not been re-collected since description, may be synonyms of common or widespread species when fresh material is collected for study. There is an urgent need to undertake a thorough taxonomic review of the molluscs, combined with the collection of fresh study material and research into species distributions.

Many species are widely distributed in India and its neighbouring countries. However, some species are highly restricted in distribution particularly in streams of the Western Ghats. For example, Cremnochonchus syhadrensis, C. conicus, and C. carinatus belong to the family Littorinidae (periwinkles) and are the only freshwater genus in an otherwise entirely marine family; they are adapted to the spray zone of perennial waterfalls from a few localities in the Karnataka and Maharashtra region of the Western Ghats. Another restricted range species is Pseudomulleria dalyi (Etheridae), an endemic cemented bivalve confined to couple of rivers in the central Western Ghats that is also a rare Gondwanaland relict (Madhyastha 2001). The family Etheridae shows unique discontinuous distribution, with recognized genera, viz., Acostea (South America), Pseudomulleria (India), and Etheria (Africa) (Smith 1898; Bogan and Hoeh 2000). The hill stream genus Turbinicola (Pilidae), which is an inhabitant of streams around Khandala, in Maharashtra resembles the South American hill stream genus Asolene, suggesting convergent evolution (Prashad 1928). Some species such as Sulcospira huegeli show a disjunct distribution, being found in the central and southern Western Ghats and in northeastern Indian states (Subba Rao 1989).

Recently, two new species of freshwater molluscs belonging to the genus Paracostoma have been described from the Western Ghats (Köhler and Glaubrecht 2007). The genus Paracostoma is monophyletic and is restricted to few streams in the central Western Ghats and nested within a clade of Southeast Asian taxa composed of Brotia and Adamietta. These authors argue that the “origins of the Indian biota are more complex and diversethan assumed under the standard Mesozoic vicariance model.” Hence, zoogeographically, the Western Ghats freshwater mollusks offer a great opportunity for biologists.

A few sporadic studies were carried out in northern Western Ghats, mainly from Pune by Tonapi (1971), and Tonapi and Mulherker (1963). Recently, Patil and Talmale (2005) reviewed land and freshwater molluscs of Maharashtra State and listed 72 species and varieties. Most of these studies were concentrated on distributional aspects and no indepth study on ecology. In India very scant attention has been paid to the biology and ecology of molluscs and in particular of bivalves (Subba Rao 1989), and therefore the ecological needs of a great majority of the Indian freshwater molluscs are not known. Apart from Volume IV of Fauna of British India by Preston (1915), there are only two other books that deal with Indian freshwater molluscs; these are Subba Rao (1989) and Ramakrishna and Dey (2007). The book on Indian Freshwater Molluscs gave updated information on the distributions with maps (Ramakrishna and Dey 2007).

Freshwater molluscs of the Western Ghats region are better known than in other parts of India or other species-rich areas within South and Southeast Asia. This assessment is based on the scattered published work, mostly coming from the northern Western Ghats and our own field studies. Still, a large amount of work needs to be done, examining the ecosystem services these species provide, the impact of aquatic-invasive plants, the distribution patterns, population status and dynamics of molluscs, and their species-specific threats. Most of the data deficient species identified here have not been collected since their description (often in nineteenth or early twentieth century) or have very meagre collection details. In many cases, the description of the species is based on either single specimen or very few specimens, and no natural history or ecology is detailed. It is important to revisit the type localities of these species to get adequate information on ecology and threats, to see if they are still present or have already become extinct, and in many cases to confirm their taxonomic status (Budha et al. 2011). Here we consolidate our study on the basis of available literature and personal findings from different surveys in Tables 11.4 and 11.5.

Table 11.5 Statewise distribution of freshwater Mollusca of India
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Endemism

A detailed study on endemism of freshwater molluscs available reveals that 47 species are endemic from India out of the 202 listed of which 33 are gastropods and 14 are bivalves (Fig. 11.1a, b). Two species are endemic from Andhra Pradesh, one from Punjab, seven species from Assam, three from Manipur, one from Meghalaya, two from Mizoram, 13 from West Bengal, two from Tamil Nadu, six from Maharashtra, three from Andaman and Nicobar Islands, five from Kerala Backwater and three from Jammu and Kashmir are endemic species.

Fig. 11.1
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(a, b) Distribution of endemic freshwater mollusks of India

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Endemism from North Himalayan Region

Mollusca are originated from various invasion of freshwater fauna into the Himalayas since the Pliocene period (low-middle mountain fauna). The transitional zone of Nepal and the Himalayas has mixed freshwater faunas, from Oriental and Palaearctic regions with numerous hitherto neglected endemics, such as Tricula species and Pisidium spp. They are originated from several so-called inner valleys of the Himalayas which were seasonal lakes and wetlands during Pleistocene glaciations (Nesemann et al. 2007).

Endemism from Northeastern Himalayan Region

Northeastern Himalayan region shows a high degree of endemism especially within bivalves comparative to the other parts of India. Consistency for the long period of endemism is probably due to the geographical barrier and selective habitat by the species. Among the 64 bivalve species from India, 14 species are endemic from this region i.e. about 21 %, among gastropods it is quite less i.e. near about 6 %, and among gastropods eight species are endemic from this region. Some species mentioned known from type locality also is not reckoned here as endemic because further information on the availability of the particular species is not yet ascertained.

Endemism from Western Ghats

Nearly 212 species of freshwater molluscs have been reported from India (Subba Rao 1989). Of these, only 60 species were recorded from the Western Ghats hot spot by Shivaramakrishnan et al. (1998); however, it is likely that many species were missed. This is substantiated by recent records of species such as Arsidopsis footi and Neritina reticulata, two species of Paracrostoma and five new species of Cremnochonchus from the Western Ghats. This region is also home to some of the most important zoogeographical and Gondwanaland relict species, such as Pseudomulleria dalyii (Etheridae), a cemented freshwater pearl species, and three species of Cremnochonchus (Littorinidae) found in the spray zones of waterfalls at high elevations in the Western Ghats. With very few ecological studies having been carried out on these unique and cryptic freshwater taxa in India, it is critical to work out their habitat requirements and distributions for their suitable conservation strategies to be developed.

In Western Ghats, two species of Neritidae (Neritina pulligera and Neritina violacea), three species from Corbiculidae (Villorita corbiculoides, V. cornucopia and V. cyprinoides) and two species from Iravadiidae (Iravadia funereal and I. ornata) are found exclusively in brackish water. The genus Villorita (Corbiculidae) is confined to the backwaters and estuaries of the west coast. These species cannot withstand high salinity levels and are usually found in the upper regions of the backwaters where the salinity is below 15 %. Here they burrow into the soil to escape unfavourable condition during summer when salinity increases above 15 % (Cherian 1968, Aravind et al. 2010).

A total of 77 species of freshwater molluscs is studied of which 52 gastropods (12 families and 23 genera) and 25 bivalve species (five families and eight genera) of which 28 species are endemic from this region (Aravind et al. 2010).

Conservation

Molluscs play essential roles in almost every known ecosystem. Many molluscs are links in food chains, the pathways between green plants and the animals that are food for humans and other animals. Molluscs show a great specialization of ecological niches in freshwater environments, making them more vulnerable to modifications in their environment (Bouchet 1992; Lydeard et al. 2004). Freshwater environments are some of the most fragile and highly threatened ecosystems in the world. Consequently, molluscs in the freshwater systems have suffered a severe decline in the diversity, distribution and abundance due to human-induced alteration of habitats, pollution, siltation, deforestation, poor agricultural practices, the destruction of riparian zones and invasion by introduced species (Biggins et al. 1995; Pimm et al. 1995). In India, because of urbanization to accommodate more and more people by destructing natural habitat like lakes, pools, ditches, small water bodies, making dams, etc., lead to extinction of species from that area within a very short time span. Speciation is a natural procedure which is controlled by natural selection, but loss of speciation by human being is a great loss of biodiversity with unasserted valuable assets. Agricultural and urban water pollution, over harvesting, dams, urban development and mining are the major threats to freshwater molluscs today.

Conservation Strategies and Implementation of Key Models

  1. 1.

    Conservation strategies for freshwater mollusca should emphasize establish biological monitoring programmes at the local level to aid in the assessment of the current status of regional molluscan diversity. Monitoring programmes are multipurpose management tools which will provide for:

    1. (a)

      Data on the biotic and abiotic characteristic of the environment which identify regions of greatest molluscan diversity

    2. (b)

      Immediate warning of nonacceptable impacts of human activities and their waste products on the environment

    3. (c)

      A long-term database to evaluate and forecast natural changes and impacts of human activities

    4. (d)

      Identification of endangered habitats and threatened species

    5. (e)

      Identification of stocks of molluscs of potential use in fisheries, the shell trade, and biomedical research

  2. 2.

    Establish baseline from distributional information available in systematic works and unpublished information for mapping habitat and at species level.

Conservation Status of Freshwater Mollusca in India

According to a report of IUCN, seven species (12 %) are assessed as threatened which are Cremnochonchus syhadrensis, C. carrinatus, Arcidopsis footei and Pseudomulleria dalyi are assessed as Endangered and Cremnochonchus conicus, Parreysia khadakvasiensis and Scaphula nagarjunai are assed as vulnerable. The majority (88 %) are assed as least concern (Aravind et al. 2010). However, much on the conservation status is not adequately known in the absence of baseline data on their distribution and population with respect to time and space and therefore, more information is required on this.

Role of Govt. Organizations Towards Freshwater Mollusca Conservation and Management

The Zoological Survey of India, since its inception, has in its custody and care collections of the natural history museum, in Calcutta, India, that are over 200 years old, as well as subsequent collections made by scientists and staff of ZSI since 1916. As per Section 39 of the Biological Diversity Act 2002 of India, ZSI is notified as Designated National Repository for Zoological Collections (NZC) of India. The NZC housed at ZSI now contains more than 3,000,000 authentically identified specimens comprising over 90,000 known species of animals. The NZC, the national heritage of the country, was acquired from the museum of the Asiatic Society of Bengal, the zoological section of the Indian Museum, and collections through various surveys till now which started in the early part of nineteenth century. Many distinguished naturalists such as John McClelland, Edward Blyth, W. Blanford, H. Blanford, T. Cantor, Francis Day, H.H. Godwin Austen, T. Hardwicke, B. Hodgson, G. Nevill, H. Nevill, F. Stoliczka, W.M. Sykes, W. Theobald, S.R. Tickell, J. Anderson and H. Wood-Mason significantly contributed in documenting the fauna of Indian subcontinent which are part of the NZC. This work mainly based on the available literature on freshwater Mollusca from India, specimens present in the NZC of ZSI and specimens received in Mollusca section of ZSI from different parts of the country for identification. The authors also have personally collected and examined large number of specimens through field surveys conducted by the Mollusca section of ZSI. ZSI cannot alone able to successfully conserve the national biodiversity with limited strength. Therefore, universities, colleges, schools and nongovernment organizations require capacity building on further studies on molluscan taxonomy, diversity and conservation in India.