Freshwater mussels displaced by zebra mussels
Zebra mussels are native to regions around the Caspian Sea in the west of Russia. Their scientific name is Dreissena polymorpha. They are bivalve molluscs. In appearance, they live up to the species part of their name as "polymorpha" means many forms and the shells of these species do come in various patterns and colours. They reach a maximum size of 3-5 cm
Typical habitats are estuaries, rivers and lakes, particularly where there are firm surfaces suitable for attachment.
Zebra mussels are filter feeders. This means that they draw water into their bodies from their surroundings, sift out and eat phytoplankton and detritus found in the water and then expel the filtered water.
After zebra mussels filter out all the particles from the water, they pick out their favorite bits like fish feces, bits of decaying organic matter and microscopic organisms. What they pick out, they digest as food, and spit out the rest — including cyanobacteria (blue-green algae). 1
Juveniles of the species are free swimming, but adults fix themselves to a solid substrate by means of a byssus (a tuft of tough silky filaments by which mussels and some other bivalves adhere to rocks and other solid objects). Hard surfaces may include rocks, wood, glass, metal, native mussels, and each other (or abandoned shopping carts dumped in a river!).
Zebra mussels are very adaptable to a range of environments. They can tolerate starvation for extended periods, desiccation, extremes of high and low temperatures, and highly variable dissolved oxygen levels. They can adapt to brackish water to some extent.
In their natural environment, zebra mussels, in both larval and adult forms, have a range of predators, pathogens and organisms that can compete with them. This keeps their numbers in check. In the USA and Canada, where D. polymorpha are invasive and considered to be a significant threat, zebra mussels do not have many natural predators. It has been documented that several species of fish and diving ducks have been known to eat them. But many potential predators are unable to crack their hard shells, and given the small amount of edible flesh inside, this makes it not worthwhile.
Like most invasive species, zebra mussels have a very efficient means of reproduction and dispersal. Male Dreissena release a cloud of sperm into the water. Female Dreissena release a cloud of eggs. A female zebra mussel can produce 30,000 to 1,000,000 eggs in one year! The fertilized eggs quickly develop into tiny free-swimming larvae called veligers which feed on microscopic phytoplankton and, as they increase in size, they begin to grow shells. Water currents can cause the immature zebra mussels to travel great distances. At 3 - 4 weeks, their shells weigh enough to cause them to sink. They then seek out any hard surface available to attach to.
The young zebra mussels reach sexual maturity during their first year, at about 10mm in size and are then ready to continue the cycle. The life span of an individual is thought to be approximately between 3 and 9 years.
Zebra mussels are included in the list of the World's 100 Worst Invasive Alien Species as determined by the ISSG, IUCN, Species Survival Commision and Bionet.
Zebra mussels are capable of disrupting entire freshwater ecosystems as will be explained shortly, but one group is particular is threatened by their introduction.
Freshwater mussels (referred to as FW mussels from now on) are bivalve molluscs that live mostly live in flowing water. A few species live in still water. They have no head, eyes, ears, or appendages, just a muscular "foot" that they use for burrowing. Adult mussels tend to bury themselves in sediment at the bottom of the river bed. This makes them very inconspicuous to human observers, and visually, it downplays their vital role in the community. Although similar in appearance to marine mussels, the two groups have been separated for at least 200 million years and have very different biology.
There is a wide diversity in the size of FW mussels from a few millimetres across to almost 20 centimetres across and shells vary in shape and colouring.
Mussels’ names echo their diverse shell shapes and are equal parts poetry and cartoon. Common names for mussels include the threehorned wartyback, sheepnose, fatmucket, heelsplitter, rabbitsfoot, pistolgrip, pigtoe, monkeyface, and snuffbox.
Mussels are slow growing and may not reach adulthood for 5 - 6 years. Some species live to over 100 years under favourable conditions. They are considered to be slow growing and slow to reproduce.
FW mussels are an integral part of the ecosystem in which they occur and provide a range of services for humans -
- Water filtration
- Carbon, Nitrogen and Phosphorus sequestration
- Environmental indicator of ecosystem health
- Food for other organisms in the ecosystem
- Food for humans
- Raw materials - mother of pearl, button manufacture, tools
For more information about FW mussels in ecosystems, see my page Fresh water Mussels
Where is the invasion?
Continents: North America, Europe
Countries: USA, Canada, northwest Russia, central and western Europe, Scandinavia, Britain, Ireland
Primarily freshwater mussels are out-competed for food and literally smothered by zebra mussels. Also a large infestation of these aliens can alter the entire ecosystem of a fresh water environment.
When zebra mussels feed on plankton, they remove incredible amounts of food from the water. Each one can filter about 1 litre of water each day. They leave the water clear, sometimes too clear. The zebra mussels grow on top of the native mussels and smother them. With plankton removed from the water, more sunshine reaches the bottom. Plants living here grow rapidly. They also use zebra mussel droppings as fertilizer. Bottom-feeding fish feast on the waste produced by the zebra mussels. Their numbers increase. Zooplankton and small fish which feed on plankton have less to eat. Their numbers decrease. Larger fish which feed on the small fish decrease in number. The zebra mussels take away space, food, and oxygen, causing the death of native mussels. 2
So the introduction of the zebra mussels disrupts the entire ecosystem.
Remediation and prevention
Getting rid of zebra mussels is difficult due to their initial small physical size and the huge size of the population of the infestation in any given body of water. Manual cleaning can be very labour intensive.
One promising method is a biocontrol agent - a bacterium which apparently does not effect non-target species. Another is the use of a surfactant that does not allow attachment. This would only be applicable in a limited range of situations, and further study would be needed to investigate side effects and toxicity. Other research is ongoing.
Public awareness programs to stop the spread of this pest are being used - these consist of education programs and public signage.
Zebra mussels can also disrupt human systems. They can get into the water intake systems of power plants and water treatment facilities and significantly reduce the flow of water. They also can clog up the cooling systems of boat engines. This clogging costs millions of dollars each year to unclog!