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One reason zebra
mussels are so prolific is their reproductive cycle. A mature female can
produce up to 1 million eggs per year, although the average is probably
closer to 300,000. The eggs are fertilized outside the body and spawning
peaks at a water temperature of around 68 degrees Fahrenheit. During
spawning, the water may be so thick with microscopic larvae that a coffee cup
scoop of water contains more than 100 larvae.
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Each of the
microscopic larvae, called a veliger, is about the same diameter as a human
hair. (You can see the veligers at the
left side of the top photo.) Veligers live in the open water and move about
with the water currents.
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Two to four
weeks after hatching, the free-swimming veligers need to settle. These
juveniles have latched on to a beer can, but any solid object will do,
including other zebra mussels. Zebra mussels can colonize soft, muddy bottoms
when hard objects deposited in or on the mud such as pieces of native
mussel shells or grains of sand serve as a substrate.
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Finally, you
normally find zebra mussel adults growing in colonies in water about 1 to 30
meters deep. Their cousins, the invasive species known as the quagga mussel,
are very similar but are more often found in deeper and colder conditions.
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