Mussels of the Sydenham

Ontario is home to 41 of the 55 freshwater mussel species native to Canada. Unfortunately, the populations of many of these species are in decline and being lost from areas they were once found. The Sydenham watershed is a refuge for mussels, it supports 34 mussel species and is the most mussel diverse watershed in Canada.

The mussels native to the Sydenham all belong to the family of river mussels Unionidae (often collectively referred to as unionids). They come in an astonishing array of shapes, sizes, colours and textures. They can be round and inflated or narrow and elongate, as small as your thumbnail or as large as your hand. Their colours range from a rich, chestnut brown to bright yellow with lime green rays. Their shells can have bumps that look like warts or they can have smooth, satiny ridges.

Collage of eight different mussel species from the Sydenham River that are different shapes, sizes, and colours

Livers of the Rivers

Freshwater mussels live in the sediment of lakes and rivers. They are filter-feeders meaning they use their siphons to take in water and strain out oxygen and food, including algae, bacteria, and zooplankton. These living filters offer a valuable ecosystem service by removing suspended particles and pollutants, which helps to clean water. A single mussel can filter up to 40L of water a day and live to be up to 100 years old – that adds up to a lot of water!

Mussels also stabilize the riverbed and are a vital link in the food chain as they are a source of food for many animals including fish, turtles, birds, and mammals.

Video: 1-hour time lapse of four freshwater mussels (left tank) filtering 9.5L of cloudy river water compared to a tank without mussels (right tank), video by Iowa PBS.

A Complex Life Cycle

Unionids have a complex life cycle, this is because they require a host in order to reproduce. Small mussel larvae, known as glochidia, need to attach onto a host fish to absorb nutrients and develop for a few days or weeks, before falling off to the river bottom as a juvenile mussel. This parasitic life stage does not harm the host fish and because mussels are relatively immobile animals, hitching a ride on a fish is critical for the dispersal of mussels to new areas of habitat. Some mussel species are host generalists while others require a specific host, so if that species is absent, the mussel cannot reproduce.

Hand-drawn diagram of the Rainbow Mussel Lifecycle by Erin Carroll, labels read: 1. Adult male releases sperm into water column; 2. Fertilizes eggs of adult female, which develop into glochidia (larvae); 3. Female uses lure to attract fish; 4. Host fish bites at lure; 5. Female mussel releases glochidia; 6. Close up of microscopic glochidia; 7. Glochidia attach to gills of host fish and absorb nutrients; 8. Glochidia metamorphose into juvenile mussels, drop off host fish to the streambed; 9. Juvenile mussels mature into adults

Depending on the species, unionids use different strategies to deliver their glochidia to the gills of their host. Some mussels have evolved modified mantle tissue that they use as a lure to attract their host. Others produce and release conglutinates that look like host prey items such as worms, insects or small fish but are actually a packet of larvae. Others do not use displays and simply broadcast their larvae into the water column. There are even a few unique species that practice catch and release fishing – these mussels have evolved methods that involve briefly capturing their host.

What are glochidia?
Glochidia are the larvae of freshwater mussels. They attach to the gills and fins of a host fish to absorb nutrients and metamorphose into a juvenile mussel, at which point they drop off the fish to begin their life on the lake or river bottom. Hitching a ride on a host also helps them to disperse to new areas of habitat. It is important to note that the glochidia have little impact on their host fish and do not harm them.

Close up picture showing glochidia attached to the gill filaments of a largemouth bass

How are Mussels Monitored?

The Sydenham River is carefully surveyed for mussels by researchers wading in the river and collecting mussels by hand. In turbid (cloudier), deeper portions of the Sydenham, biologists feel along the river bottom for the mussels – a tactile survey technique often referred to as “raccooning.” If the water is shallow enough, glass bottom viewing boxes can be used to search for the mussels along the river bottom by sight. The mussels are identified, measured and counted, then returned back to the river. Surveys are completed by trained biologists that have permits to complete this work.

Picture of biologists wading in the river surveying mussels by hand

Picture of biologists wading in the river surveying mussels by hand or with orange glass-bottom viewers that are a cone biologists can use to look along the river bottom in shallow water

Why Mussels are Monitored: The Canary in the Coal Mine

Most people are familiar with the expression a “canary in the coal mine.” In the past, miners would bring a canary in a small cage with them into the mines to serve as an early warning system for a build up of deadly gases like carbon monoxide. Trouble with the bird meant there would be trouble for the humans as well, unless they took quick action. Concerningly, two-thirds of Canada’s native mussel species are imperilled, making this group of animals the most threatened in the country. Like the canary in the coal mine, mussel population declines are a signal that cannot be ignored.

Mussels are “biological indicators” of environmental health. Because they are long-lived, sedentary, filter feeders with complex life cycles, they provide valuable information on the health of waterways. Due to their complex life cycle, unionid mussels are not only vulnerable to direct threats like poor water quality, they are also indirectly threatened by factors that impact the survival of their host fish species. This makes them even more sensitive to changes in their ecosystem. Through surveys, biologists are able to learn about the presence/absence of the species, their relative abundance, and distribution patterns. Monitoring helps to detect changes to the mussel communities over time and any threats that may be causing these changes, this helps scientists, land managers, and policy makers to implement actions with the help of land owners and community members.

Picture of a female Northern Riffleshell mussel in a hand, a small, yellowish-orange mussel with narrow green rays, the shells of females extend creating a more oval shape.

The Sydenham: A Key Biodiversity Area
In 2017, the East Sydenham River was designated one of 13 freshwater Key Biodiversity Areas (KBAs) in Canada by the International Union for Conservation of Nature (IUCN) because it supports a great diversity of freshwater species including five globally threatened mussels: the Northern Riffleshell, Snuffbox, Round Hickorynut, Salamander Mussel, and Rayed Bean. Key Biodiversity Areas are areas contributing significantly to the global persistence of biodiversity.

Dreissenids: A Major Threat to Native Mussels

Zebra and Quagga mussels are native to Eurasia but are highly invasive species in the Great Lakes region that were originally introduced through ship ballast water. They are members of the family Dreissenidae (often collectively referred to as dreissenids). Zebra and Quagga mussels are able to reproduce quickly and, unlike the native freshwater mussels that belong to the family Unionidae, they do not rely on a host for their larvae in order to complete their life cycle. The female and male dreissenids release their eggs and sperm into the water column where they are fertilized and develop into free-swimming larvae (known as veligers) that disperse through currents then firmly anchor themselves onto surfaces, forming dense colonies.

Picture of dreissenid mussels covering a weathered unionid mussel valve.

Dreissenids will attach to the shells of native mussels, which become covered, unable to move or filter feed. Dreissenids are a major threat to native mussels and have contributed to the loss of unionids from much of their previous ranges. These invasive mussels have caused extensive damage to the Great Lakes ecosystem as well as human-made structures like boats and intakes for drinking water treatment plants and power stations.

Zebra and Quagga mussels invade tributaries in a downstream direction. They are present in the lower portion of the Sydenham River and there is concern that they could spread if introduced to upstream reservoirs in the watershed. This would be devastating to the native mussels of the Sydenham and the local aquatic ecology. Visit the Ontario Invading Species Awareness Program to learn more about how to prevent the spread of invasive species.

MUSSEL GLOSSARY

Mollusc: refers to the large phylum of soft-bodied invertebrates that includes a wide range of animals. Mussels, snails, octopuses, and squid all fall under different classes in the phylum “Mollusca.”

Bivalve: refers to the class of aquatic molluscs, “Bivalvia,” that have shells formed by two valves that open and close with a hinge. The shells are hard and calcareous meaning they are largely made of calcium carbonate. Bivalves are found in fresh and salt water, they include mussels, clams, and oysters.

Unionid: refers to the family of freshwater river mussels, “Unionidae.” All mussel species native to the Sydenham River are unionids.

Dreissenid: refers to the family of freshwater mussels, “Dreissenidae,” that includes Zebra and Quagga mussels. They firmly attach to surfaces using a byssus. Dreissenids are invasive in the Great Lakes where they are having large, detrimental impacts on the ecosystem and threaten the survival of native mussels.

Glochidia: (singular glochidium) the larvae of unionids. After a mussel’s eggs are fertilized by sperm, they develop into glochidia. The glochidia are parasitic and require a fish host. They attach to their host’s gills, fins or skin, absorbing nutrients for a period of days to weeks before metamorphosing and dropping off to the river bottom as a juvenile mussel.

Valve: a half shell of a bivalve.

Periostracum: the thin, skin-like outermost layer of the mussel shell.

Nacre: the hard, iridescent inner layer of the shell, also known as mother of pearl.

Mantle: also known as the pallium, a layer of tissue that covers the internal organs of the mussel and produces its shell. The mantle can also form other structures like the siphons or lures.

Siphon: mussels have inhalant and exhalent siphons they use to filter water. Siphons allow them to breathe oxygen; consume algae, bacteria and other food; and for females to take in sperm to fertilize their eggs.

Marsupium: (plural marsupia) modified gill pouch of female unionid mussels where eggs are fertilized and develop into glochidia.

Conglutinate: packet of glochidia encapsulated by mucous, look like host fish prey items.

Foot: muscular part of the body, mussels use their foot to move, burrow, and anchor to the sediment; juveniles also use their foot for pedal feeding.

Byssal thread: a strong, fine thread made from protein that some mussels use to attach to gravel, rocks and other surfaces.