Palicypods or  Bivalvia, referred to as the Lamellibranchiata and Pelecypoda, comprise a class of marine and freshwater molluscs are most affected group of organsims and will work as good bioinducators, These have laterally compressed bodies enclosed by a shell consisting of two hinged parts. They have no head, and they also lack a radula. Bivalves include clams, oysters, cockles, mussels, scallops, and numerous other families that live in saltwater, and well as a number of families that live in freshwater. The majority are filter feeders that is by they will take in contaminants with the sea water after accumulation in their body animals will  show a range of symptoms pertaining to pollution related effects. Hence, they will be good indicators of environmental pollution.

Agree with Dr. Upadhyay'a answer.You can refer "International Mussel Watch" for your requirements.

Dear Shankadeep, yes bivalves are one of the best bioindicators but this also differs in the area you are looking forward to sample/ study. Oysters are good for estuaries, clams for sandy beaches, cockles for reef or seagrass etc.. You can choose on specific bivalve families to work on.

best regards, Deepak

The marine pollution studies have been carried out by mussel watch programme. This is widely distributed, so it is suitable for this pollution studies, it can be consider as bioindicators. In general pollutants accumulate in the sedentary animals like mussels and stable even in higher toxic level when comparing with any lower organisms.

I'm not sure but I think that water supply system in Warsaw (Poland) uses Dreisena polymorpha

For aquatic pollution bivalve molluscs (clams, oysters, mussels) are suitable indicators. Bacterias, metals, pesticides, hydrocarbons are accumulated in these organisms.

Hi Shankhadeep, has broadly explained in the other answers bivalves are considered good bioindicators, but some gastropods as well, even if there are only few studies about their usefulness. Currently several research groups are testing their ability of recording isotopic concentration and trace metals.

The best advice that I can give is that if you want to find a good bioindicator you should take into account few points that could help you in selecting the best target species.

A good bioindicator is:

1- An organism with a long life span.

2- A species with a wide distribution (and easy to find), not species limited to secluded areas such as lagoon, brackish water or low salinity environments. This means a species that can be used to monitor a wide geographical area.

3- A species with an hard part usually (i.e. shell), which growth is progressive (daily or annually growth), in order to “record” periodical events.

4- An organism with a growth affected by environmental parameters, such as organism with a seasonal growth or a growth proportional to salinity, pH levels etc. This means that the organism can “record” the variations of this biotic and abiotic environmental parameters.

If a mollusc comply with these points, it could be considered as a potential bioindicator for the marine environment.

I think that bivalves are the most potential bioindicator class within mollusca due to their link with the sediment features (grain size distribution, organic content) and the water column (for filter feeders). A large number of bivalve species are very sensitive to changes in the sediment characteristics. Bivalves have species with fast and slow growth which makes them a good target group for testing effects of environmental and human impacts of different sources. Moreover, filter feeding and deposit feeding bivalves have physiological responses to food quality (and environmental quality) that can be easily measured plus they generally include contaminant particles and toxins in their body which can be used for indirect measurements of environmental quality. Finally their shell is composed of calcium carbonate which allows to test effects of ocean acidification.

Dear Shakadeep, one of the first question that should arise about choosing a good bioindicator is what kind of stress are you looking at ? According to the type of stress, disturbance, pressure or whaterver term you could use to describe what will cause a change in your population or community, you should choose what is the most convenient to get a feedback from a biological point of view. Filtering bivalves are very often used to detect chemical presence or disturbance in the water and sediment. They are mostly integrating the water quality. But several other species groups could also be used in terms of sensitivity to disturbance like organic matter loads or more simply high sensitivity to hypoxia. Then the information is not "registred" in the muscles and organs of the animal but it is simply reflected by abundance or even presence/absence. Look at the following ref for more info.

Borja, A., Muxika, I. and Franco, J., 2003. The application of a Marine Biotic Index to different impact sources affecting soft-bottom benthic communities along European coasts. Marine Pollution Bulletin 46, 835.

Borja, A., Dauer, D.M., Diaz, R., Llanso, R.J., Muxika, I., Rodriguez, J.G. and Schaffner, L., 2008. Assessing estuarine benthic quality conditions in Chesapeake Bay: A comparison of three indices. Ecological Indicators 8, 395-403.

I agree with Dr. Mario Lepage, and I do have something else to add: if you are looking after a bioindicator of chemical pollution, the best species (or class of molluscs) depends on the pollutant in question. Gastropod molluscs are known to be the best bioindicators of TBT contamination (usually arising from now banned antifouling paints), since females usually display imposex when exposed to these compounds, while for most of the other types of chemical pollutants (such as trace metals, etc.) bivalves are more appropriate as said by our colleagues. If you look after "imposex" and "TBT" you will find plenty of literature on this. A good understanding of your study site, its diversity and baseline levels of pollution are desirable when picking up your bioindicator species, as well as how the type of stress you expect to study is expected to impact a candidate organism.

Bivalve mollusks and gastropods are the ideal organisms to study the bio accumulation because of their feeding behavior. Moreover, there is also a report on the use of inter tidal gastropods as the bio indicators to measure heavy metal contamination. So as rightly said by all the authors, the objective of the study (which contaminant that you are looking for) also matters when it comes to the selection of bio-indicator. See the reference

http://www.canberra.edu.au/researchrepository/file/af0308ff-5069-d3e7-10f4-9b090d6aa170/1/introductory_pages.pdf 

Your choice of mollusc as bioindicator depends on what you wish to measure.  For example, grazing herbivorous gastropods or burrowing molluscs of several classes may be good for detecting materials deposited on sediments or other benthic substrates.  For detection of pathogens or other material suspended in the water column or pollutants dissolved in the water itself, the best bet is probably bivalves.  In particular, mussels that attach to hard substrates (e.g., Mytilus and others) have direct feeding access to the water column and filter large volumes of water each day, sequestering both chemical and particulate material in their systems.  We have used Dreissena mussels and Corbicula clams as bioindicators of human and zoonotic pathogens in freshwater environments for many years, and other authors have done similar work with marine bivalves in marine and estuarine environments.  Below, I have noted some of our publications that are available through ResearchGate.

Article Human waterborne parasites in zebra mussels (Dreissena polym...

Chapter Dreissenid Mussels

Conference Paper Sentinel organisms as biomonitors for emerging zoonotic path...

Conference Paper Invasive freshwater molluscs as sentinel biomonitors to pred...

Chapter Asian Freshwater Clams (Corbicula fluminea) and Zebra Mussel...

Article Biomonitoring for zoonotic protists in terrestrial and aquat...

Chapter Navigational buoys (navaids) as monitoring platforms

Shankhapdeep,

This is a very good question  I have wondered about this myself but never did anything about it.  I think it all depends on the context and what you plan to do.  Bruce has shown how bivalves can be very useful but for monitoring for human microparasites and in an effective.  So if it is for general biodiversity for a region then this would be different and different according to the financial resources that you might have.  I would suggest gastropods as being a useful group, should biodiversity be the main area of your interest.  These are like hoverflies (Syrphidae) some having quite specific types of habitats http://pollinators.biodiversityireland.ie/hoverflies/syrph-the-net/ and provide a good indication of the range for a region.  I have been using stranded shells on the high shore as the most cost effective means for evaluating the habitats in a nearby region in order to have an idea of what habitats might be present.  Gastropods range from parasites, commensals, suspension feeders to predators and some have specific requirements.

No references I am afraid as this is based on my geriatric opinion

Glad to see what others think

I have now uploaded some more relevant articles.  Also, some of the articles on our earlier work with the Asian Clam, Corbicula fluminea, are referenced below.  Corbicula continues to remain abundant in many areas, and continues to invade others.  This makes it a good choice for biomonitoring in freshwater systems.

Article In Vitro Interactions of Asian Freshwater Clam (Corbicula fl...

Article In vitro phagocytosis of Giardia duodenalis cysts by hemocyt...

Article Evaluation of the recovery of waterborne Giardia cysts by fr...

Article Recovery of waterborne oocysts of Cyclospora cayetanensis by...

Article Recovery of Waterborne Cryptosporidium parvum Oocysts by Fre...

Article Molecular markers and sentinel organisms for environmental monitoring

Conference Paper Long-term biomonitoring for zoonotic waterborne pathogens: N...

I totally agree with the first sentence of the first answer from David Bruce Conn (and one of the last sentences from Sreekanth gb): the best bioindicator depends on what do you expect to measure, or the organism to indicate. Although bivalve mollucs are usually studied as bioindicators - as filter feeders they actively uptake contaminants - sometimes gastropod mollucs would be more adequate. If you are, for instance, willing to measure the effects of organotins (such as TBT = tributyltins, formerly used as biocides in antifouling paints), gastropods are more proper bioindicators than bivalves, as female gastropods usually develop imposex (***) in response to these compounds, and the degree of imposex is usually well correlated with environmental concentrations of organotins. On the other hand, if you are willing to screen an aquatic environment for pollutants, you should probably use bivalves. The shells of  any mollusc class are also good bioindicators, as they make a semi-permanent record of the conditions in the water column while they were alive. See for example the beautiful work of Yves-Marie Paulet here on ResearchGate. 

https://www.researchgate.net/profile/Yves-Marie_Paulet

Article Use of Antifouling Paints on Ship Hulls over Past Four Decad...

Intertidal clams and gastropods are the best bioindicators of heavy metals, petroleum hydrocarbons and pesticides present in coastal waters

Arthropods like Teredo and Bivalves, especially oysters. Mussels have been used for along time for these type of studies.