Invasive Mussel Biology

The family Dreissenidae consists of three genera of mussels: Congarea, Mytilopsis, and Dreissena. Collectively, these are known as dreissenids.

This toolkit is focused on two members of the genus Dreissena; the zebra mussel (Dreissena polymorpha) and quagga mussel (Dreissena rostriformis bugensis), and information has been added about the golden mussel because of its first detection in North America in the State of California in 2024.

Quagga and Zebra Mussels (Dreissena spp.)

Although there are differences in the biology of dreissenids, they share many similar life history traits and cause similar adverse environmental and economic impacts. Both species have European origins and were introduced to the United States in the 1980s as the result of ballast water discharge. Both zebra and quagga mussels attach to a broad range of surfaces, including pilings, pipes, rock, cement, steel, rope, crayfish, other bivalves, aquatic plants, and each other, forming dense colonies.

Zebra and quagga mussels seem to have divergent spatial distributions; zebra mussels are primarily warm, eutrophic, shallow water inhabitants whereas quagga mussels prefer deep, oligotrophic, cold-water (MacIsaac 1994). Although this toolkit includes some references specific to zebra mussels (reflecting a larger national focus on the spread of this species), its objectives and tactics also apply to quagga mussels and other dreissenids.

Zebra mussel.

Quagga mussel.

Conrad's false mussel.

Golden mussel. Copyright MP Zhou, some rights reserved (CC-BY-NC)

Density and Food Availability

Zebra mussel densities can vary widely depending on water chemistry, food availability, and breeding population. After their initial introduction, zebra mussel populations can rapidly increase by orders of magnitude, and then similarly decrease. Eurasian zebra mussel population densities range up to 40,000 mussels per square meter (Neumann et al. 1993). Under ideal conditions in the Laurentian Great Lakes, zebra mussel densities reach 700,000–800,000 per square meter (Kovalak et al. 1993). In the lower Mississippi River, densities of 400,000 per square meter have been reported (Kraft 1995). The Mississippi River is an ideal environment for zebra mussels, in part because food resources are abundant (Kraft 1995). The Columbia River’s lower plankton densities in comparison to the Mississippi or Great Lakes, may limit zebra mussel population densities, though this has yet to be quantified.

Water Temperatures

Dreissenids can tolerate a wide range of water temperatures from roughly 32° to 86°F (0 °F to 30° C) (Ohio Sea Grant 1997). North American zebra mussel spawning (release of gametes into the water column) will not generally occur at temperatures below about 12 °C (Claudi and Mackie 1994). There is evidence, however, that quagga mussels in deep waters of the Great Lakes are capable of spawning at temperatures near 5 °C (Roe and MacIsaac 1997) and 9 °C (Claxton and Mackie 1998).

Based on these parameters, a water temperature profile created from data recorded at the smolt monitoring facilities at Bonneville and John Day Dams shows the potential for quagga mussel egg release for approximately 7 months of the year (late March to late-November). However, peak spawning temperatures of 68 °F (20° C) and above occur for 2 months during mid-July to mid-September.

Calcium Requirements

North American zebra mussel populations require 10 mg Ca2+/l to initiate shell growth and 25 mg Ca2+/l to maintain shell growth. Larval development is inhibited at pH of 7. 4. Higher rates of adult survival occur at a pH of 7. 0-7. 5, but populations have been found in the hypolimnetic zone of lakes with a pH of 6. 6-8. 0, and in the epilimnetic zone with a pH of 7. 7-8. 5. Optimal larval survival occurs at a pH of 8. 4, and optimal adult growth occurs at pH 7. 4-8. 0. (Benson and Raikow 2007).

Calcium concentrations can be a limiting factor for dreissenids. Hincks and Mackie (1997) estimated that large populations of zebra mussels are not expected where calcium levels are less than 25 mg/l, however Counihan and Bollens (2017) defined risk as very low (< 12 mg/L), low (>12 and < 15mg/L), medium (>15 and < 25 mg/L), and high (>25 mg/L) . Cohen and Weinstein (2001) found little evidence that zebra mussels can become established at ambient calcium concentrations below about 20 mg/l. Calcium thresholds in the Columbia River West of the Cascades and in particular the Willamette River may be suboptimal for establishment of dreissenid populations (Whittier et al. 2008). There are cases in which dreissenid populations have become established in calcium-limited water bodies at locations that have input from other water sources with higher calcium levels (Cohen and Weinstein 2001). In lentic water bodies, fluctuations in maximum pH can create conditions of higher calcium variability in sites with high pH compared to other lentic water bodies with pH ranges between 7.3-9.4, considered high risk for dreissenid establishment (Prisciandaro 2022).

Conrad's False Mussel

Conrad’s false mussel (Mytilopsis leucophaeta), also known as the false dark mussel, is another invasive member of the family Dreissenidae, and occasionally is found on boats entering the Pacific Northwest. This mussel represents a threat to brackish waters. Conrad’s false mussel has a planktotrophic life-stage and may be difficult to differentiate from Dreissena species using visual identification techniques.

Golden Mussel (the following was excerpted, with some paraphrasing and edits, from the State of California Fish and Game Commission Finding of Emergency and Statement of Proposed Emergency Regulatory Action)

Golden mussel (Limnoperna fortunei), an invasive, non-native freshwater bivalve, was discovered for the first time on October 17, 2024, in the Port of Stockton by California Department of Water Resources (DWR) staff while conducting routine operations. This is the first known occurrence of this highly invasive species in North America.

Shortly after, golden mussels were detected at additional sites in the Sacramento-San Joaquin Delta (Delta), including Turner Cut downstream of the Port of Stockton (October 23), Middle River (October 31), Old River (November 7), and in the San Joaquin River upstream of the Port of Stockton (November 5). Golden mussels were also detected in O’Neill Forebay on October 25 and at the outlet of O’Neill Forebay to the California Aqueduct on October 31. O’Neill Forebay is a forebay of San Luis Reservoir, which is a joint use facility of the California State Water Project and federal Central Valley Project located in Merced County.

Golden mussels in the Delta pose a significant immediate threat to the ecological health of the Delta and all waters of the state, the operations of water conveyance systems, agricultural interests, hydroelectric power generation, infrastructure, water quality, and the economy. Their presence in California is of statewide, national, and international concern. Without actions to prevent further spread, golden mussel have the potential to be moved overland on trailered watercraft and equipment out of the Delta and to nearby and distant fresh and brackish waters, including rivers, lakes, and reservoirs within California and the rest of North America.

Golden mussels are native to rivers and creeks of China and Southeast Asia. They are known to be established outside of their native range in Hong Kong, Japan, Taiwan, Brazil, Uruguay, Paraguay, and Argentina. The initial introductions to these countries were likely the result of ships with biofouling on the hulls and/or ballast water release. In most cases, the invaded range has expanded upstream from the point of introduction, and inland from ports through local, human- mediated pathways. Within the invaded range, significant impacts resulting from the dense colonization of golden mussels on hard surfaces are widely documented.

Golden mussels have similar appearance, biology, and impact as quagga and zebra (dreissenid) mussels. Golden mussels are small, typically under 1.5 inches in length with shell color that is light golden to darker yellowish-brown to brown color. They firmly attach to hard to semi-hard surfaces. Shortly after fertilization, the larvae become mobile, capable of coordinated swimming, and disperse in the water column. Larva are microscopic and themselves cannot swim upstream, but can be carried by human-mediated pathways such as water within watercraft. Once a suitable substrate is found, juvenile mussels settle and attach themselves to the substrate by strong, silky fibers called byssal threads and develop into adults. Golden mussels can grow in dense colonies of hundreds of thousands of mussels per square meter.

Golden mussels can tolerate a wider range of environmental conditions than dreissenid mussels, meaning they are able to establish in environments where dreissenid mussels are unable to invade. Golden mussels require less calcium to survive and reproduce than dreissenid mussels. Golden mussels tolerate higher salinity than dreissenid mussels, making the brackish parts of estuaries, such as Suisun Bay, suitable for golden mussel establishment. They also tolerate warmer water temperatures compared to dreissenid mussels.

Like dreissenid mussels, golden mussels pose an environmental threat because they are ecosystem engineers and can profoundly change the food web of an invaded ecosystem. They can impact native species and sports fish by competing for food sources. They can also increase water clarity due to intense filter feeding, resulting in degraded water quality, algal blooms, and increased aquatic vegetation growth that requires control to maintain navigation.

Like dreissenid mussels, golden mussels pose an economic threat to infrastructure and recreation industries. Heavy encrustations of golden mussels form dense reef-like structures that block municipal and industrial water supplies, agricultural irrigation, and power plant operations, necessitating ongoing biofouling removal.

Golden mussels can also impact recreation by limiting recreational opportunities, encrusting docks and beaches, and colonizing recreational equipment including watercraft hulls, engines, and steering components. Dreissenid mussel infestations resulted in the temporary and permanent closure of waterbodies to the public and have negatively impacted aquatic ecosystems.

Without containment, golden mussels are likely to spread overland on trailered vessels and equipment to other fresh and brackish waterbodies throughout California, and to other ports and inland waters of North America, and potentially abroad.

Click here for risk assessments and literature on the golden mussel.