Freshwater Molluscs

WILDLIFE IN BRITISH ACOLUMBIAT RISK Freshwater Molluscs Many freshwater Ministry of Environment, Lands and Parks mollusc species are at risk due to loss and degradation of aquatic habitats. mussels directly or may reduce the avail- different than that of oceans. In the ability of host fish that they depend ocean, tides result in complex water upon to disperse their young. The intro- flow patterns, whereas inland waters duction of non-native mollusc species are always ultimately flowing in one What are freshwater molluscs? can result in unnatural competition for direction – toward the ocean. In ithin the large group of animals food and habitat, as has occurred in east- order to live in fresh water, molluscs known as molluscs, three subgroups ern North America with the introduc- had to evolve reproductive adapta- – snails, mussels and clams – have tion of the Zebra Mussel. So little is tions that would ensure their eggs Wrepresentatives that live in fresh known about the B.C. freshwater mol- and sperm could meet and that lar- water. Because all freshwater molluscs lusc fauna that it is difficult to assess vae and juveniles could disperse have hard shells that are often washed up which species are currently at risk. without being drawn into that one- on shores, they are some of the most con- way flow to the sea. spicuous of the freshwater invertebrates. Where do they live? About 180 species of freshwater mol- reshwater molluscs live in all types of Freshwater snails luscs are known to occur in Canada. Of wet habitats from large lakes and nails have shells that come in a vari- these, at least 85 species, including 54 rivers to roadside ditches and back- ety of shapes, from flattened, cone- snails, 5 mussels and 26 clams, are found Fyard ponds. They are most common- shaped and uncoiled to elongated, in British Columbia. Although a few ly found along the shallow edges of water Smulti-coiled and high-spired. Those freshwater molluscs are believed to have bodies where warmer temperatures and with coiled shells may spiral to the left survived the last period of glacia- additional light or to the right, depending on family. tion in B.C., most species have Each shell, provide them Some freshwater snails evolved repopulated the province within the each crawling with more food directly from marine species and still past 10 000 years, moving in from than is available have gills like their relatives in salt adjacent unglaciated areas. insect holds a in deeper water. water. However, most of British Freshwater molluscs are an inte- rank … Some species of Columbia’s freshwater snails evolved gral part of the complex web of life which lost freshwater mol- from snails that had first moved from that supports biodiversity. Like luscs can be the ocean onto land and had devel- other invertebrates, they play an Would break found in tempo- oped a lung to breathe air. When these important role in nutrient cycling, the chain rary wet areas, snails took up life in fresh water, many functioning as decomposers and as where they sur- maintained the need to return to the critical links in the food chain. and leave vive by burrow- surface to breathe air. behind a gap ing into the sub- Snails eat by grazing on organic Why are freshwater Which strate to wait out material that accumulates on plants, molluscs at risk? dry periods. Bur- rocks or soft bottom sediments. They lthough freshwater molluscs in Nature’s self rowing is also a scrape these surfaces using a unique, B.C. are not being lost to direct would rue. strategy used by file-like mouthpart called a radula. exploitation, much of their some molluscs to Snail reproduction involves inter- Ahabitat is being lost or degrad- stillingfleet, survive winter nal fertilization. After mating they lay ed. The hazards they face vary. For circa 1882 freezing. Others their eggs in jelly masses or egg cap- the many snails that must return to are able to avoid sules attached to a solid surface. the water surface to breathe, pollutants the ice by retreating to unfrozen areas. Unlike their marine relatives that gen- such as oils and soaps that accumulate All freshwater molluscs evolved erally have free-swimming larvae, a around the edges of water bodies are from marine ancestors. The shift freshwater snail hatches from its egg as potentially disastrous. Rare, hot water between radically different environ- a miniature adult and crawls away. adapted snails suffer habitat loss when ments required the development of This ensures that the juvenile snail is hotsprings are diverted for human use. specialized adaptations. For example, not swept downstream, as a swim- A decline in water quality may affect the movement of fresh water is very ming larva might be. Freshwater snails at risk Due to its very limited distribution a study of freshwater molluscs in Hotwater Physa and the precise (though not yet fully northern B.C. found Rocky Mountain Physella wrighti (Te and Clarke) known) habitat requirements that Capshells at six additional locations. This small, five-millimetre-long snail is restrict it to such a small area, the This is an example of how the lack of the first mollusc to be recognized both Hotwater Physa is at risk. The popula- scientific knowledge about British tion is so localized that a single Columbia’s freshwater molluscs can calamity could result in the loss of this affect assessments as to which species species forever. are at risk. Rocky Mountain Capshell Threeridged Valvata Acroloxus coloradensis (Henderson) Valvata tricarinata (Say) Within British Columbia, this small This snail is quite rare in western North limpet-shaped snail is found only in the America. The only record of it occurring federally and provincially as an endan- in B.C. is from 1969, when it was collect- gered species. It has been placed on ed at Kootenay Lake. This lake has since British Columbia’s Red List, and has been altered by chemical contaminants been designated as Endangered by the from industrial activity and by the Committee on the Status of Endangered introduction of a foreign species of Wildlife in Canada (COSEWIC). The shrimp in an attempt to enhance the only place in the world that the Hotwa- number and size of fish in the lake. ter Physa lives is a 34-metre reach of Survey work is required to confirm the warm stream that forms part of the continued existence of this species in Liard River Hotsprings complex in far British Columbia. northern B.C. During the last ice age, the major ice sheets did not meet at this location. The climate between the ice sheets would have been inhospitable to east-central region. It may have survived most organisms, but the lack of ice cover glaciation in high altitude lakes above and the presence of incoming hot water the ice sheets. For years it was known allowed this species to survive glaciation. from only one B.C. location – Purden It is believed to have lived at this site for Lake, east of Prince George – and was at least 100 000 years. considered very rare. Recently, however, In the ocean, marine mussels mussel releases the glochidia, which reproduce by releasing sperm and immediately hook on to the host. eggs into the water, where they unite However the attachment is brought and develop into swimming larvae about, the fish now grows tissue that are carried off to settle in other around this parasitic glochidium. suitable habitats by the complex Inside this cyst the glochidium devel- water flow patterns. In fresh water, ops into a miniature adult, which Attenuate Fossaria the unidirectional flow has resulted breaks out generally after 10 to 30 days Fossaria truncatula (Müller) in unique reproductive adaptations. and takes up life in a new location – one In Canada, this snail has been found that may be far upstream from its only in the far northwest and far south- parent. It is estimated that it may take east of British Columbia, where it lives the production of 100 million glochidia in water bodies with mud bottoms. to guarantee that one will reach matu- This disjunct distribution pattern can rity. Some mussel glochidia are general- be interpreted in two ways. One is that ists, able to develop on any one of a this species was eliminated by glaciation number of different fish; others are spe- and has re-entered the province from cialists and require a particular fish unglaciated refugia in the far north as - species to act as a host. well as from areas to the south. The sec- . ond possible interpretation is that this Freshwater mussels at risk geographic gap indicates a knowledge Freshwater mussels release sperm ore than half of all North American gap. Such a divergence in the geograph- into the water but retain their eggs species of freshwater mussels are ic zones in which the Attenuate Fossaria within their shells. The eggs are considered imperilled. As aquatic has been found suggests that it may be fertilized by sperm inhaled by the Mhabitats are degraded or become tolerant of a wide range of habitat con- female. The fertilized eggs develop altered for other uses, the habitat for ditions. If so, this snail may be present into specialized larvae called glochidia freshwater mussels is disappearing. Even throughout the province and our min- (singular glochidium) that are often where conditions allow the continued imal information on freshwater mol- equipped with sharp hooks. Some existence of the mussels themselves, if lusc distribution provides only scat- species release the glochidia onto the the habitat can no longer support the tered records of its occurrence.

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