Mussels found during a boat inspection at Blue Mesa Reservoir
NPS Photo
Zebra and Quagga Mussels
Zebra mussels (Dreissena polymorpha) and Quagga mussels (Dreissena bugensis) are small freshwater bivalve mollusks (animals with two shells). They are relatives of clams and oysters.
Zebra and quagga mussels were introduced into the Great Lakes from discharged ballast water or attached adults on anchors of transoceanic vessels. After the introduction into North America, they “hitched a ride” to other waters on watercraft or traveled down tributaries.
Description
The shell of both mussels alternates between a yellowish and darker brown, often forming stripes. They range in size from microscopic up to two inches (five centimeters) long. Unlike native North American freshwater mussels, which burrow in soft sediment, adult zebra and quagga mussels can attach to most hard and semi-soft surfaces via tiny threads called byssal threads.
Both zebra and quagga mussels can survive cold waters but cannot tolerate freezing temperatures. They can endure temperatures between 33°F–86°F (1°C–30°C). Zebra mussels need waters above 54°F (12°C) to reproduce, while quagga mussels can reproduce in waters as cold as 48°F (9°C). Adult mussels are light sensitive and prefer to live in water around 200 to 300 feet (60 to 90 m) deep. They can thrive in a wide range of conditions including oxygen-depleted water.
Life Cycle
Zebra and quagga mussels have three life stages: veliger, settler, and adult.
The embryos are microscopic larvae, called veligers. They are free-floating plankton that float in the water column or are carried in the current for about four to eight weeks. The veligers undergo metamorphosis and develop shells while settling out of the water column onto solid surfaces.
The juvenile form of the mussel is known as the settler stage. At this point in their life cycle, they settle into the deep benthic zone of the water column.
Upon maturation, adult mussels become sessile, meaning fixed in one place or immobile. They are attached semi-permanently with their byssal threads. Adult mussels typically form dense clusters in which they pile up on top of each other essentially smothering the generation beneath them. Their typical lifespan is four to five years.
Invasive Characteristics
Prolific Reproduction
Zebra and quagga mussels reproduce exponentially. They can spawn year-round when conditions are favorable. A single female mussel can produce up to one million eggs a year.
Byssal Threads
Zebra and quagga mussels can attach, via byssal threads, to almost any surface that sits or floats in the water, such as boats hulls, anchors, ropes, buoys, rocks, aquatic plants, artificial surfaces (cement, steel, plastic, etc.), crayfish, native clams, and other mollusks, to name a few. They attach and can form dense clusters that impair facilities and impede the flow of water. Zebra and quagga mussels also degrade water quality and can alter the taste and smell of drinking water.
Filter Feeding
Invasive species can change aquatic ecosystems by altering native plant and animal communities. As filter feeders, they remove large amounts of microscopic plants and animals that form the base of the food chain. Zebra and quagga mussels are one of the few species that can crash the entire food web by removing the base of the food chain, plankton, and smother benthic organisms that are a source of food for larger fish. Each mussel can filter over a liter of water per day. Invasive mussels would directly compete with Kokanee Salmon present in Blue Mesa Reservoir.
Questions and Answers
Zebra mussels are native to Eastern Europe, specifically the Black, Caspian, and Azov seas. They were discovered in the Great Lakes in Lake St. Clair in 1988 and have since spread across North America.
Quagga mussels are native to the Dnieper River drainage in the Ukraine. They were discovered first in the Great Lakes in the Erie Canal and Lake Ontario in 1989 and have since spread throughout the country.
Zebra and quagga mussels were introduced into the Great Lakes from discharged ballast water or attached adults on anchors of transoceanic vessels. After the introduction into North America, they “hitched a ride” to other waters on watercraft or traveled down tributaries.
Zebra mussels were found in Colorado at Highline Lake State Park in September 2022. More museels were discovered in October 2023 on buoys. Highline Lake is considered infested status by Colorado Parks & Wildlife.
In 2024, the Colorado River upstream of Highline Lake tested positive for zebra mussels. This section of river feeds into Highline for irrigation purposes. The source of this infestation has not been determined.
Positive detections in Colorado:
Pueblo Reservoir tested positive for zebra and quagga mussel veligers in 2007 and for quagga mussel veligers in 2007, 2008, 2009 and 2011.
Grand Lake tested positive for zebra and quagga veliger in 2008.
Granby Reservoir, Shadow Mountain Reservoir, Willow Creek Reservoir, Tarryall Reservoir, and Jumbo Reservoir tested positive for quagga mussel veligers in 2008.
Blue Mesa Reservoir tested positive for quagga mussel eDNA in 2009, 2011 and 2012.
Green Mountain Reservoir tested positive for quagga mussel veligers in 2017.
De-listing Positive Waters (Mussel Free):
In January 2014, Granby Reservoir, Grand Lake, Shadow Mountain Reservoir, Willow Creek Reservoir, Tarryall Reservoir, Jumbo Reservoir, and Blue Mesa Reservoir were de-listed after multiple years of negative testing.
Pueblo Reservoir was de-listed for quagga mussels in January 2017 after five years of negative testing.
Green Mountain was de-listed for quagga mussels in January 2021 after three years of negative testing.
Invasive Mussel Impacts
Zebra and quagga mussels can have tremendous recreational and economic impacts to waters and surrounding infrastructure. Zebra and quagga mussels can clog watercraft engine cooling systems potentially resulting in expensive repairs due to overheating. As watercraft remain in mussel infested waters, additional hull and motor fouling can occur and increase the removal time and operational maintenance. They also impact fish populations and reduce sport-fishing opportunities. Their sharp shells can cut the feet of unsuspecting swimmers and beach goers.
Infrastructure located in infested waters needs expensive maintenance when the mussels clog water-control structures. This increased maintenance cost is passed down to the public through expensive utility bills for power generation, water-control structure maintenance, and water treatment.
Blue Mesa Reservoir is part of the Wayne N. Aspinall Storage Unit, along with Morrow Point and Crystal reservoirs. A mussel infestation would directly impact the hydroelectric generation that services the local communities. Any mussel introduction in the headwaters of Curecanti National Recreation Area would allow veligers to float downstream and compromise ecosystems and infrastructure. Native fisheries destroyed by mussels cause economic loss with reduced tourism and recreation.
Asian Clam shell, Zebra mussel shell, and Quagga mussel shell (left to right)
Photo courtesy of Colorado Parks & Wildlife (CPW)
Other AIS in Colorado
For more detailed information and locations where these AIS occur in Colorado, visit the Colorado Parks & Wildlife Aquatic Nuisance Species website.
Asian Clams
Asian clams (Corbicula fluminea) are small non‐native bivalves that are commonly mistaken for zebra and quagga mussels. Asian clams were first detected in the South Platte River in Colorado in 1993 and have since expanded their range to include the Arkansas River, Gunnison River, San Juan River, and Colorado River basins.
Their shells have striations (or ridges) which give it the appearance of having stripes, but they do not possess byssal threads.
They prefer fast moving water because currents provide food for these suspension feeders. However, they are commonly found on the shorelines of lakes and reservoirs.
They create for power plants and water canals because large numbers of clams block water intake valves.
They also compete with native bivalves for food.
New Zealand Mudsnail
New Zealand mudsnails (Potamopyrgus antipodarum) are small aquatic snails. They were first discovered in North America during the late 1980s in the Snake River (Idaho) and Madison River (Montana).
New Zealand mudsnails (NZMS) range in size from a grain of sand to an eighth of an inch in length.
They have an operculum (cover) which can close off their shell and allow them to survive out of the water for extended periods of time.
NZMS can tolerate a wide range of temperatures, ranging from near freezing to 82°F.
NZMS spread into new river systems primarily by humans, although they can be carried on the feet of dogs and wildlife. Anglers, boaters, researchers, and others can carry NZMS to uninfested locations on their boots and gear.
They can survive up to 50 days on a damp surface, giving them ample time to be transferred from one body of water to another on fishing gear.
NZMS compete with native invertebrates, including native mollusks, for space and food resources. NZMS may reduce the availability of native invertebrate prey for fish—particularly mayflies, caddis flies, and chironomids. They are not a viable food sources themselves because their hard shell allows them to pass through a fish gut unharmed.
Rusty Crayfish
Rusty crayfish (Faxonius rusticus or Orconectes rusticus) are native to the Ohio River Basin. They were first discovered outside of their native range in the 1960s. Rusty crayfish were introduced by anglers who used the crayfish as bait and threw unused bait back into the water. Although they are often introduced as bait, they do not make good bait due to their aggressive nature.
They have brown bodies and large grayish‐green to reddish‐brown claws with dark black bands on the tips.
They live in freshwater lakes, rivers, and streams and prefer deep pools and fast currents with cover from predators.
Rusty crayfish eat small fish, insects, and fish eggs. They also eat aquatic vegetation, damaging underwater habitat that is important for fish spawning, cover, and food.
They are aggressive and displace native crayfish.
Eurasian Watermilfoil
Eurasian watermilfoil (Myriophyllum spicatum) is native to Europe, Asia, and northern Africa. It was once commonly sold as an aquarium plant and was introduced to the eastern U.S. as early as the late 1800s to 1940s. Eurasian watermilfoil is known to many reservoirs, in and outside Colorado. It is detected in new locations almost every year within the state of Colorado.
Eurasian watermilfoil is a submerged, rooted perennial with long, branching stems and soft feathery leaves attached in whorls of four.
The plant tolerates a wide range of water conditions and depths, but prefers nutrient‐rich substrate. It reproduces by seeds, fragmentation, and winter buds. Any plant fragment can start a new infestation, this is the most common way of introduction.
It forms dense mats that restrict swimming, fishing, and boating, clog water intakes, and choke out other native aquatic plants. The decaying native plants decrease oxygen levels in the water, foul lakeside beaches, and disrupt the habitat and food needed by fish and birds.
Eurasian watermilfoil slows the flow of water in irrigation ditches and canals and creates standing water that is ideal mosquito habitat.
