Mobility
Mussels less than 15 mm in length are very mobile, capable of crawling, drifting, and floating for some time in the water column. Movement is believed to be in response to environmental conditions.

Risk Assessment: Severe
Mussels will crawl into any small crack or crevice, into filter floss, water intake systems, and even up out of the water. The narrowness of their shells enables mussels to pass through small openings. For example, 5-mm-long mussels have been known to crawl through 0.5-mm mesh netting. Extra precautions are needed to prevent contamination of all equipment that is in contact with zebra mussels or water in which zebra mussels are known to be present. Do not assume that netting or coarse filters can prevent escape of small mussels.

Reproduction
Zebra mussel fertilization is external, and spawning can continue over a period of several weeks. Mussel reproduction starts when water temperatures are above 12°C. In most temperate regions, water temperature limits the spawning season to May through September. However, reports from Russia and laboratory studies conducted in this country indicate that spawning continues year-round in areas where water temperatures remain above 12 °C. About 10-15% of zebra mussels will reach sexual maturity at a ventral shell length of 2-3 mm. Most become sexually mature at a ventral shell length of 6 mm.

Risk Assessment: Severe
Laboratory colonies held at water temperatures above 12°C can and will spawn continually, increasing the risk of veligers being present in all wastewater.

Food Supply
Mussels are filter feeders and were initially reported by the Europeans to feed and survive only on live algae. However, research done in this country indicate that zebra mussels consume all types of food, including detritus and zooplankton, as well as their own young, and can therefore grow during periods of time when live algae are unavailable. Also note that mussels can survive for up to 11 months without food under laboratory conditions at 4° C.

Risk Assessment: Moderate
Mussels can colonize areas where live algae is limited or areas where the food supply is intermittent (such as drainage pipes).

Growth
Juvenile mussels are capable of rapidly growing to sexual maturity. Juvenile mussels average only 0.4 mm in ventral shell length just after undergoing metamorphosis, and under optimal conditions can reach 13 mm in less than 3 months. Growth begins when water temperatures are over 3°C.

Risk Assessment: Low
Small mussels will grow to sexual maturity under laboratory conditions even if held at less than 10° C, although spawning has not been reported at such temperatures.

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Handling small mussels
Juvenile mussels (less than 1 mm long) are difficult to detect visually without using a microscope. The easiest way to determine if these mussels are present under field conditions is to feel them--they feel like sand grains. They also "stick" to everything, lodging under fingernails, in net handles, on clothing, etc., increasing the risk of accidental release. Extra precautions should be taken to insure proper "decontamination" of all gear, etc. that may have been exposed to juveniles less than 1 mm in shell length.

How to determine if mussels are dead
When mussels die, the shells remain open with body parts exposed. A dull probe can be used to touch mussel tissue to determine if animal is alive or dead. Mussels that float when they are placed in water are not necessarily dead. Live quaggas frequently retain air in the shell valves during handling and will float for hours.

Risk Assessment: Severe
Assume mussels are alive, unless body tissue has sloughed off from the shell.

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Calcium needs. European research indicates that mussels require 30 ppm dissolved calcium for shell growth and 50 ppm for reproduction. However, laboratory studies done in this country indicate that some growth can occur at 20 ppm and reproduction at 35 ppm. Quagga calcium needs have not been tested, but their shells are noticeably thinner than zebra mussels.

Dissolved oxygen. Oxygen needs of zebra mussels have not been documented. However, mussels have been reported from lakes in Europe where summer oxygen levels are less than 2.0 ppm.

pH. In Europe, zebra mussels usually occur in areas where the pH is over 7.5. The degree of acidity in the water that will be tolerated by zebra mussels will in part be related to calcium levels, and is at this time unknown.

Salinity. European studies indicate that zebra mussels will not live in sea water, but can tolerate estuarine conditions. However, Russian literature indicates that some of the other Dreissenidae are more salt-water tolerant than zebra mussels. At this time, salinity tolerance of the quagga mussel is unknown.

Water temperatures. Mussels can survive in temperatures ranging from below 0° to 35°C, if they are submerged. Mussels exposed to the air have a much narrower temperature range (about 6-28°C). To date, spawning has only been seen when water temperatures are over 12°C.

Water velocity. Mussels are positively attracted to water current and will colonize areas with water velocities up to 2 meters per second.

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Development
When water temperatures rise above 12° C, adult mussels release eggs and sperm into the water column. After fertilization, developing embryos remain in the water column, and can drift for some distance from the parent colony. The time required to develop from egg to juvenile mussel varies according to water temperature, but averages about 2 weeks under laboratory conditions at 22°C. Studies in Europe have documented the presence of veliger in the water column for up to one year. Initial size at shell formation is approximately 100 microns (some quaggas are smaller at D-shell, under 70 microns), and 300 to 450 microns at metamorphosis.

Risk Assessment: Severe
Since larvae are microscopic, their presence or absence on sampling gear or in samples cannot be determined unless examined under a microscope. Assume that veligers are present if water temperatures are over 12° C.

Mobiliy
Young larvae have a ciliated organ called a velum that is used for swimming. Older larvae, just before metamorphosis, also have a foot that can be used for crawling. Since the larvae are so small, they are readily picked up by water currents, and can be transported some distance.

Risk Assessment: Severe
Assume that veligers are present if water temperatures are over 12° C. Although veligers are described as planktonic, any object collected in a zebra mussel area during spawning season will have veligers of various ages crawling on it.

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Food. Veligers begin to feed just after shell formation. They are filter feeders, consuming algae, bacteria, and detritus. Initially, veligers feed off of particles less than 4 microns in size.

Settling substrate. Proper substrate must be present during the time veligers under metamorphosis, or the larvae will die. Veligers settle on filamentous material first, undergo metamorphosis, and then move to a hard substrate.

Water temperature. Veligers tolerate the same temperature regime as do the adults. Development rate is directly correlated to water temperature. Live larvae have been held at 4° C for up to one week without food.

Water velocity. Water velocities over 2 meters per second discourage the settling of veligers.