Molluscs are highly diverse invertebrates. In terms of global abundance and diversity, molluscs are only surpassed by arthropods. About a quarter of all living marine species are molluscs. All molluscs have a mantle, a cavity for breathing and excretion, and a nervous system. Within this basic design, molluscs have wonderful diversity in body form. Many molluscs are important food sources and culturally important.

Molluscs are subdivided into at least seven classes: bivalves, cephalopods, gastropods, the polyplacophora (marine chitons), scaphopoda (marine tusk shells), aplacophora (marine worm-like animals), and monoplacophorans (limpet-like animals). Of these, the first five classes live in the NWT.

Marine molluscs mostly live below the lowest tides, away from shoreline ice scour. Deep-water species are little known. NWT species of marine molluscs belong to one of five classes: Bivalvia, Cephalopoda, marine Gastropoda, Polyplacophora, Scaphopoda. Only the first two classes are included here. The other classes will be included in future reports.

There are 57 species of marine bivalves, 22 species of freshwater bivalves, five species of cephalopods, 37 species of freshwater gastropods, and 25 species of terrestrial gastropods confirmed present in the NWT. One species of terrestrial snail is alien to the NWT.

Bivalves

Bivalves are so named because they have two “valves” that are similar in shape and face each other, forming a two-part shell. The two valves come together at the “hinge”. Freshwater bivalves include the strictly aquatic unionids (freshwater mussels), and the pea and fingernail clams, some of which can survive extended periods of time being dry.

Freshwater mussels feed by filtering water and eating plankton and other fine particulate organic matter. Mussels use their foot to anchor or half bury themselves at the bottom of water bodies. Mussels often live together in a group. Because they filter large quantities of water and spend a large portion of their life in one area, mussels are excellent indicators of aquatic ecosystem quality. The sudden disappearance or a decline in growth rate of a species of freshwater mussel could indicate declining aquatic ecosystem health. Mussels are also food for muskrats, river otters, and humans, among others. Separate male and female mussels produce sperm and eggs with the male releasing his sperm into the water. The female filters the sperm out of the water and transports it to a specialized area of her gills where her eggs are fertilized and develop into larvae (called “glochidia”).

Except for a couple of species, freshwater mussels produce parasitic glochidia that attach to the gills or fins of fish. The larvae of some mussels are species-specific and can live only if they attach to the appropriate host fish species. All larvae eventually detach from their host and, if they fall in suitable habitat, will develop into adult mussels. The fat mucket has 14 known host fish species, including yellow perch and walleye. It is found in southern NWT where it is considered abundant. New records for 2015 extend its previously known range to the Johnny Hoe River, just south of Great Bear Lake. The giant floater may be found across the NWT along the Mackenzie River watershed, but its host fish is unknown and there is no information on numbers or population health. The best-known and most studied population of giant floater can be found at the aptly named Shell Lake, near Inuvik. These two species can easily be told apart.

The fat mucket has protuberances or teeth on the hinge inside the shell whereas the giant floater’s hinge is toothless. Marine bivalves are mostly filter-feeders, but some of the deep-water groups are predators. The Arctic Ocean, including the Beaufort Sea is rich in marine bivalves. These molluscs are the preferred food of walruses.

Cephalopods

The most advanced molluscs are Cephalopods. They are highly evolved marine molluscs with a prominent head and eight arms and, in squids, two additional tentacles, which are positioned around the mouth. They are excellent swimmers, using water expelled from the body to propel themselves. The arms, provided with suckers, capture prey and bring it to the mouth where a beak, similar in shape to that of a bird, takes bites of the prey. Cephalopods are well known for their remarkably high intelligence among invertebrates, their ability to learn and to communicate using changes in skin pigmentation, skin texture, and bioluminescence. They are also known for the complex structure of their eyes.

Freshwater gastropods

Two major types of freshwater gastropods occur in the NWT: those that can breathe air (pulmonates) and those with gills (prosobranchs) used to extract oxygen from the water, much like fish. All have shells that are either cone-shaped or flat with all except one species group having a spiral shell. The prosobranchs have a trap door (called operculum), which they can close to seal their bodies into their shells. All except one family of freshwater snails in Canada (Physidae) have shells that twist to the right.

Most freshwater prosobranchs have separate sexes (only members of the family Valvatidae are hermaphroditic), whereas all pulmonates are hermaphrodites, having both female and male sex organs. They are micro- and macroherbivores, scavengers, and detritivores, scraping their food from surfaces using a multi-toothed “tongue” called a radula. Some can even filter-feed using their gills as do the mussels. They are found in all types of water bodies, from large lakes and rivers to small ponds, streams, and marshes with or without permanent standing water. Birds, fish, and small mammals eat them. Some birds even eat empty snail shells to gain the calcium that is then used for the development of their own eggs. Shells range in size from a diameter of only 3 mm to a shell length of over 5 cm.

Only one species of freshwater gastropods is currently known to be endemic to northern Canada. The Western Arctic stagnicola (Stagnicola kennicotti) is found nowhere else in the world except in the NWT and Nunavut. Surveys of several hot springs in Nahanni National Park Reserve in 2003 found some of the springs to be inhabited by at least three species of freshwater snails. One species was originally thought to be a common freshwater snail found throughout much of Canada but in 2015, DNA barcoding results from specimens collected in 2003 suggested it may be a different and potentially new species. Two other thermal spring snails were not fully identified and could either be new species or closely related to other species found in thermal springs in Kamchatka, Russia. They have not yet been subject to DNA barcoding or thorough analysis.

Many of the records of freshwater snails in the NWT come from historical museum collections. The first large survey of Canadian freshwater molluscs occurred from 1959 until 1969 with this collection being held in the Canadian Museum of Nature, Ottawa. While most of the collection sites were in southern Canada, there are also collections from the NWT by the Fisheries Research Board.

The entire NWT has not been adequately surveyed for freshwater snails, but progress is being made. For example, the Mollusc Species Specialist Subcommittee of COSEWIC had the opportunity for two days of surveys around Hay River in 2013. This brief survey added one previously unknown species to the list of NWT freshwater snails. Additional survey and work on already collected specimens could very well increase the size of the list and provide data for further status ranking.

Terrestrial gastropods

Terrestrial snails and slugs belong to the class Gastropoda, which is also represented in marine and freshwater environments, and have adapted to life on land. Gastropods are the most diverse class in terms of numbers of species, morphology, and adaptation to varied aquatic and terrestrial environments.

Terrestrial snails have a single, spiral, external calcareous shell that serves as protection against predators and desiccation. Slugs have evolved from snail-like ancestors in separate lineages and are simply snails in which the shell is much reduced in size and usually internal or sometimes altogether absent. Semislugs, such as the western glassnail, have a body form and shell that are intermediate between slugs and snails.

There are just about 200 species, counting both native and introduced ones, of terrestrial snails and slugs in Canada.

Most, if not all the NWT species of snails and slugs have expansive ranges, likely across much of the Arctic, boreal North America, or beyond, into northern Eurasia. At least one species, grey fieldslug, is introduced to the NWT. Knowledge of the presence of this species in NWT was the result of a 2013 Hay River survey. Specimens collected, or vouchered, years ago have been re-identified using very recently published taxonomic revisions. Species unrecorded from the NWT continue to be discovered.

In 2018, the northernmost terrestrial mollusc population in Canada was recorded on Banks Island. The presence of meadow slug, a species native to Canada and the NWT but usually found further south, is important as it is an intermediate host of lungworm parasites that infect caribou and muskox.

Little is known about the natural history of most NWT terrestrial snails and slugs. In general, terrestrial species need moisture to survive and seek shelter within loose accumulations of fallen leaves, under logs, rocks, and other objects. Worldwide, terrestrial snails occur in almost every imaginable habitat, some not generally thought to be ideal for snails. In the NWT they should be expected in boreal, subarctic, and tundra ecosystems, as well as in modified habitats in towns. Habitats include a wide variety of wetlands and forests, tundra, and coastal beach ridges/dunes.

Terrestrial snails and slugs are mostly herbivores and scavengers and feed on dead and living plant material, fungi, and carrion. A few are active predators on invertebrates, including other snails. In general, NWT terrestrial snails and slugs are simultaneous hermaphrodites, like all the air-breathing freshwater snails. That is, both male and female reproductive organs are present at the same time in all individuals. Cross-fertilization between individuals, however, is still usually required, although self-fertilization is known to be common in some species. All NWT species are probably short-lived, under one or two years.

With a few exceptions, all species in the NWT are tiny, less than 6 mm. The small size of most NWT land snails has likely contributed to the lack of knowledge on these animals in the NWT, as has the remoteness of many parts of the territory. Taxonomic difficulties, the absence of accessible reference books, and the presence of just a few experts on terrestrial molluscs in all of Canada have also prevented better knowledge of this group.

Little of the NWT has been surveyed for terrestrial snails and slugs. While most or perhaps all species in the territory are predicted to be secure based on what we know about them elsewhere, we have few data on which to base territorial ranks. New records, in the form of vouchered, expertly identified collections are needed. For example, NWT’s tundra areas and various kinds of wetlands are almost unsampled for terrestrial molluscs, and the Mackenzie Mountains may be of interest. Additional species known from Yukon and elsewhere in Canada probably also occur in NWT but need “discovering”.

However, casual searches are unlikely to find most species, and collections of minute snails may be best made by gathering accumulations of fallen leaves and dead grasses, drying and screening to remove large material, and finally picking though the remaining debris for snails.