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.
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?
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).
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 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
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.
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?
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:
- direct habitat destruction by human activities, such as forest clearing, dam construction, and pollution
- introduction of non-native or exotic species, intentional or not
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.
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.
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
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.
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:
The acquisition and management of threatened habitats on islands, in aquatic ecosystems, on continents, and on coral reefs;
The development of a database necessary for a better knowledge of molluscan diversity;
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;
The establishment of self-sustaining captive populations of endangered mollusk species and support for their eventual reintroduction into native habitats;
The promotion of public awareness and concern for molluscan conservation needs and programs.
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?
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.
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.
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