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The Global Decline of Mollusks

Christine E. Parent


Mollusk species, such as snails and clams, are disappearing, but you may not know it since they do not typically stir up strong feelings. However, certain species

  • have commercial value as food and medical research
  • provide important nourishment and shelter (discarded shells) for other animal life
  • keep the water clean by recycling nutrients through their systems

April 2008

If an animal isn’t “cute,” do we care?

Carelia kalalauensis, Nancibella quintalia, and Pupilla obliquicosta are among the numerous species that reportedly went extinct over the past few years. They are not cute little furry mammals, or singing birds that we will never hear again, or brightly colored amphibians that are often pictured on tropical adventure pamphlets—these three species are mollusks1,2 (also spelled molluscs).

Sinistral species of snail (shell opening to the left as opposed to dextral species with shells opening to the right). Photo by cj.samson, Wikimedia Commons.

Invertebrates make up nearly 99% of animal diversity.

The decline and loss of mammals, birds, and other vertebrate species is well documented and often brought to public attention as a consequence of recent human impact on environment. It is indeed alarming to realize that we have lost 135 bird species, 70 mammal species3, and that so many more are under threat because of human activities. However, as guilty we should feel for their loss, vertebrates represent less than half of the documented extinctions. Comparatively, invertebrate species (without a backbone) receive much less media attention, even though they comprise nearly 99% of all animal diversity4 and occupy a central role in the survival or maintenance of most ecosystems.

What is the conservation status of mollusks?

Not all mollusks have shells.

Mollusks are a group of soft-bodied animals that includes snails, scallops, clams, and sea slugs. Most mollusks have shells: univalves, like snails, have one shell; bivalves, like clams, have two. Some mollusks, such as the squid and octopus, have lost their shells during their evolution. The phylum Mollusca (including snails and slugs (Gastropoda), mussels and clams (Bivalvia), squids and octopuses (Cephalopoda), and a few other less well-known species distributed in five other classes) is the second most diverse taxonomic group of animals in terms of numbers of described species, after the very species-rich group of arthropods (mainly comprised of insects, arachnids, and crustaceans).

Their extinction risk has not been studied in detail.

In spite of their large numbers, mollusk populations are declining locally and globally, as suggested by an increasing number of studies:

  • With 41% of the 736 documented extinctions of all animal species, the number of mollusks exceeds that of any other major taxonomic group3.
  • Most documented endangered mollusk species are non marine (they live in freshwater or terrestrial environments)5.
  • The level of threat is poorly documented and almost certainly underestimated: a very small fraction (less than 2%) of known molluscan species has had its conservation status properly assessed3.

Why should we care?

They are slimy, most of them are slow moving, and hardly anybody would have a second thought for them unless they are found munching on the lettuce in their garden. So why should we care if we are losing mollusk diversity?

Mollusks are vital to sustaining many ecosystems.

Mollusks are a diverse group of animals, and play crucial roles in various ecosystems. For example, in the western United States, the Center for Native Ecosytems has identified the following contributions: “Mollusks, which include land snails, freshwater snails, mussels, clams, and slugs, are vital to healthy ecosystems. Land snails, for example, not only provide food for a host of small mammals and birds, but they play vital roles in recycling forest nutrients. Freshwater snails also provide food for fish, including native trout and salmon, and are also important recyclers of plant and animal waste, essentially keeping water clean and healthy. Mollusks are also considered excellent ecological indictors, their status providing a window into the health of entire ecosystems.”6

There may be up to 150,000 mollusk species.

With estimates varying between 80,000 and 150,000 described species, mollusks are the 2nd most diverse animal group (after arthropods), thus representing a large part of evolutionary history that happened on our planet. Despite the fact that mollusks are very susceptible to changes in their environment because of their permeable skin and soft body (often with an exoskeleton in the form of a shell), they have adapted to all of the main environments of our planet—marine, freshwater, and terrestrial. All classes of living mollusks exist in the sea, and all habitats have mollusks7. Mollusks are very abundant in some ecosystems. In many marine communities the dominant organisms of the second trophic level (primary consumers of plant productivity) are mollusks7.

They are good barometers of environmental health.

In addition to their essential role in maintaining the integrity of various ecosystems, mollusks have great importance in our lives and our world:

  • Mollusks produce a wide range of biotoxins and metabolites that are used in medical research8; for example, the lethal toxins produced by cone snails are used to develop a drug called ziconotide for patients with cancer and AIDS who are suffering from pain that cannot be relieved by opiates9.

  • Mollusks provide a sensitive tool for monitoring environmental health. They are found in almost all habitats, but individual species often have small-scale distributions. They are sensitive to changes in their environments, and therefore could provide an early warning of habitat deterioration10.

  • Mollusks may be useful as indicators of conservation needs. For example, in a study conducted in Australia, insects and mollusks were found to be strong predictors of conservation priorities for vertebrates, but not vice versa11.

  • Many species of mollusks are commercially exploited for human consumption (for example mussels, clams, oysters, squids). Compared to the meat of other animals, the food prepared from mollusks has high nutritious value, as it contains high protein content and many amino acids, and they are relatively in low fat content.

Why are populations of mollusks declining?

Most extinct mollusks are non-marine species.

Ninety-nine percent of the documented extinctions in mollusks are of non-marine (terrestrial and freshwater) species7. Although much more research has to be carried out to document population declines and identify with certainty their definitive causes, there is increasing evidence suggesting that human activities are directly related to the declines (for example see reference 12). Scientists are pointing to two main, but not exclusive, potential culprits:

  1. direct habitat destruction by human activities, such as forest clearing, dam construction, and pollution
  2. introduction of non-native or exotic species, intentional or not

Habitat destruction

Mollusk species have their own habitat preferences5. Some are restricted to certain types of woodland and forests; others live in grasslands, wetlands, certain types of rivers and lakes, lower tidal water levels, intertidal and shallow estuarine water, coral habitats, abyssal plains of the ocean floor or open ocean waters.

Altering their habitat can affect survival.

Direct destruction of some of these habitats—because of agricultural and urban development and habitat transformation resulting from dam construction and water pollution—are important causes of mollusk population declines. Most freshwater mollusks species are highly sensitive to water quality partly because of their permeable skins and because they need a good oxygen supply. There are reported cases of species disappearing in association with the acidification of water13.

Freshwater mussels are very vulnerable.

Among the species most vulnerable to pollution are the freshwater mussels (unionids), because their parasitic larval stage is dependent on fish hosts. This group of species reaches its peak of diversity in North America14. At present, only about a quarter of the host fish for the mussels in the USA have been properly identified. Therefore it is difficult to predict the impact that pollution and habitat transformation due to damming and pollution might have on these freshwater bivalve populations.

Introduction of alien species

The exotic rosy wolfsnail devastated native snails.

Probably the best-known example of the demise of a native snail fauna was caused by the deliberate introduction of the predatory snail Euglandina rosea on some of the Pacific islands. The rosy wolfsnail15 (common name of E. rosea) was introduced with the hope that it would control another introduced snail, the giant African snail (Achatina fulica), which is considered to be a crop pest and public nuisance. The plan was not carefully enough thought out, however; not only did the rosy wolfsnail fail to reduce populations of the giant African snail, but it also devastated the native snail populations, especially the slow-growing and slow-reproducing Partulidae and Achatinellinae16.

Introduction of alien species can lead to:

  • habitat alteration
  • asymmetric competition from fast-growing and dispersing alien species
  • predation on naïve native species
  • and, diseases

What can scientists do?

Continuing loss of mollusk diversity is detrimental, not only to ecosystems around the world, but in the long run, to the welfare of humankind itself. Indeed, mollusks are crucial to the integrity of ecosystems, the evaluation of environmental health, and human well-being.

The MSG aims to manage mollusk conservation.

Recognizing the importance of mollusks in the life of humankind and the significant roles they play in ecosystems, the Mollusk Specialist Group (MSG) was formed. In an effort to counter the increasing loss of diversity suffered by mollusks worldwide, five major lines of action have been proposed by the MSG for the conservation of molluskan diversity17:

  1. The acquisition and management of threatened habitats on islands, in aquatic ecosystems, on continents, and on coral reefs;

  2. The development of a database necessary for a better knowledge of molluscan diversity;

  3. The prevention of the introduction of alien species that negatively impact native mollusk species, and control and eradication of those exotic species where such introductions have already occurred;

  4. The establishment of self-sustaining captive populations of endangered mollusk species and support for their eventual reintroduction into native habitats;

  5. The promotion of public awareness and concern for molluscan conservation needs and programs.

We need more data about mollusk declines.

Scientists are focusing on gathering more data on the distribution and abundance of mollusk species across the world to reach a more accurate assessment of the conservation status of species, as well as potential causes of population declines18,19. This information is crucial to the integrated conservation efforts to protect the molluskan diversity of our planet. Such conservation efforts are multiple:

  • habitat protection and management plans for endangered species
  • recovery plans for species that have suffered population declines
  • captive breeding of endangered species
  • farming programmes for commercially exploited species

How can everyone help?

Reduce transport of alien species.

Introduction of alien species is one of the major threats to mollusk diversity. The constant increase in travel by human populations is associated with the increasing risk of alien species being introduced. Quarantine programs were established on islands such as Galapagos20 and Hawaii21 to prevent the introduction of additional species in these very fragile habitats. It is everyone’s responsibility to follow these regulations where they are established, but also to refrain from transporting exotic species when travelling in order to avoid another unfortunate event as the rosy wolfsnail’s appearance on the Pacific islands.

Take an active role in mollusk conservation.

Most mollusks are small, slow moving, and slimy, but they are crucial to our lives and our world; and yet their populations are declining. Much work needs to be done to

  • better understand the current conservation status of most species;
  • monitor the population health of different species,
  • pinpoint threatened areas; and
  • establish the causes of the threats.

While this much-required research is to be done by scientists, the public can help by volunteering as citizen scientists and promoting awareness of the importance of mollusks in our lives through education and public events.

Christine E. Parent is a Ph.D. student in biology at Simon Fraser University, Vancouver, Canada. Christine’s research focuses on the evolutionary ecology of an adaptive radiation of endemic land snails on the Galápagos Islands. Although she is primarily engaged in the study of snails, she pursues a general interest in the formation and maintenance of biodiversity at any spatial and time scale. *Since the original publication of this article, Christine Parent has relocated to the University of Texas at Austin where she is a post-doctoral researcher in the section of integrative biology. More information on her current and previous research can be found at

The Global Decline of Mollusks

BioScience Article

“The Global Decline of Nonmarine Mollusks.”
In this article, Lydeard et al. (BioScience, April 2004) provide an overview of global nonmarine molluscan biodiversity and conservation status—including several case studies documenting the diversity and global decline of nonmarine mollusks. They conclude with a discussion of the roles that mollusks and malacologists should play in conservation, including research, conservation management strategies, and education and outreach. Free to read.

Mollusks at the Smithsonian Institution

Sources of information on mollusks (including lists of books for young and adult beginner malacologists).

Mollusks on the tree of life

Get an introduction to the taxonomy of Mollusks, including Gastropoda (snails, slugs, limpets, etc.) , Bivalvia (mussels, clams, oysters, etc.) , Cephalopoda (octopods, squids, nautiluses, etc.), Polyplacophora (chitons).

Man and Mollusk

Yet another site of resources for students, educators, and anyone wanting to learn more about the fascinating world of mollusks.

The Mollusc Specialist Group of the World Conservation Union (IUCN)

Read articles in the TENTACLE, the newsletter of the group.

Conservation of freshwater mollusks

The Freshwater Mollusk Conservation Society (FMCS) is devoted to the advocacy for, public education about, and conservation science of freshwater mollusks, North America’s most imperiled fauna.

The Samoan snail project

The Samoan Snail Project has as its goal assessing the diversity and historical decline of the native Samoan nonmarine snail fauna, as a first step in its conservation.

Environmental Impacts of Marine Exotics

Read James T. Carlton’s article on for more details about how non-native species can affect their new environments.

The IUCN Red List

The Red List of the IUCN (World Conservation Union) Species Survival Commission: learn about the Red List programme, search the database, make a donation to help save species.

The American Malacological Society

An international society dedicated to the study and conservation of mollusks. Anyone interested in mollusks (professionals, students, serious amateurs) are encouraged to become a member.

Collecting shells

Website of Conchologists of America: learn more about collecting shells, access information about clubs, events, conservation and collection of shells.

Por los amantes de los moluscos!

A website for mollusk lovers, all in Spanish.

Caring for pet snails

Provides potential snail owners with information on keeping snails as pets, and is a point of reference for existing snail owners.

Invertebrate Dribbling

Students practice proper dribbling skills and to work on identifying invertebrates into the four major types. Lower Middle School.

Man and Mollusc

A directory of lesson plans about a variety of mollusks for different grade levels.

Zebra Mussels

This exercise focuses on the characteristics of the zebra mussel, why it is so invasive, and what the economic impact is. Numerous Internet sites are available for research on this non-indigenous invading species. High School.

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  2. For an excellent, detailed description of the mollusc (mollusca), including models and classifications see: (accessed 20 April 2008).
  3. IUCN 2006. 2006 IUCN Red List of Threatened Species. (accessed on 24 May 2007).
  4. Ponder, W. F., and D. Lunney (eds.) 1999. The Other 99%: The Conservation and Biodiversity of Invertebrates. Mosman, Australia: Royal Zoological Society of New South Wales.
  5. Lydeard C., R. H. Cowie, W. F. Ponder, A. E. Bogan, P. Bouchet, S. A. Clark, K. S. Cummings, T. J. Frest, O. Gargominy, D. G. Herbert, R. Hershler, K. E. Perez, B. Roth, M. B. Seddon, E. E. Strong, and F. G. Thompson. 2004. The global decline of nonmarine mollusks. BioScience 54(4): 321–330.
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  7. Russell-Hunter, W. D. (ed.). 1983. The Mollusca. Volume 6 Ecology. Academic Press Inc: London.
  8. Cimino, G., and M. Gavagnin (eds.) 2006. Molluscs: Chemo-ecological study to biotechnological application. Marine Molecular Biotechnology, volume 43. Springer: New York.
  9. Staats, P. S., T. Yearwood, S. G. Charapata, R. W. Presley, M. Byas-Smith, R. Fisher, D. A. Bryce, E. A. Mangie, R. R. Luther, M. Mayo, D. McGuire, and D. Ellis. 2004. Intrathecal ziconotide in the treatment of refractory pain in patients with cancer or AIDS. Journal of American Medical Association 291(1): 63–70.
  10. Wells, S. M., and J. E. Chatfield. 1992. Threatened Non-marine Molluscs of Europe. Nature and Environment, No. 64. Council of Europe Press: Strasbourg.
  11. Moritz, C., K. S. Richardson, S. Ferrier, G. B. Monteith, J. Stanisic, S. E. Williams, and T. Whiffin. 2001. Biogeographical concordance and efficiency of taxon indicators for establishing conservation priority in a tropical rainforest biota. Proceedings of the Royal Society of London, Series B 268: 1875–1881. Okland, J. 1990. Lakes and Snails. Backhuys, Oegstgeet, Netherlands: Universal Book Services.
  12. Edgar G. J., and C. R. Samson. 2004. Catastrophic decline in mollusc diversity in Eastern Tasmania and its concurrence with shellfish fisheries. Conservation Biology 18(6): 1579–1588.Lydeard, C., and R. L. Mayden. 1995. A diverse and endangered aquatic ecosystem of the southeast United States. Conservation Biology 9: 800–805.
  13. Okland, J. 1990. Lakes and Snails. Backhuys, Oegstgeet, Netherlands: Universal Book Services.
  14. Lydeard, C. and R.L. Mayden. 1995. A diverse and endangered aquatic ecosystem of the southeast United States. Conservation Biology 9: 800-805
  15. __. About the Rosy Wolfsnail problem in Hawaii, with photos. (accessed 20 April 2008)
  16. Coote, T., and É. Loève. 2003. From 61 to five: endemic tree snails of the Society Islands fall prey to an ill-judged biological control programme. Oryx 37: 91–96.
  17. Kay, A. (ed.). 1995. The Conservation of Molluscs: Proceedings of a Symposium held at the 9th International Malacological Congress, Edinburgh, Scotland, 1986. Occasional Paper no. 9. IUCN Publications: United Kingdom.
  18. Killeen, I. J., M. B. Seddon, and A. M. Holmes (eds.). 1998. Molluscan conservation: a strategy for the 21st century. Journal of Conchology Special Publication No. 2.
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  20. Galapagos Conservancy. Winning the battle against invasive species. (accessed on 30 May 2007).
  21. Hawaii Department of Agriculture. Animal Quarantine Information. (accessed on 30 May 2007).


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