Zebra mussel

## Description

The zebra mussel (Dreissena polymorpha ) is a small freshwater mussel. The species was originally native to the lakes of southern Russia and Ukraine, but has been accidentally introduced to numerous other areas and has become an invasive species in many countries worldwide. Since the 1980s, the species has invaded the Great Lakes, Hudson River, and Lake Travis.

The species was first described in 1769 by German zoologist Peter Simon Pallas in the Ural, Volga, and Dnieper Rivers. Zebra mussels get their name from a striped pattern commonly seen on their shells, though it is not universally present. They are usually about the size of a fingernail, but can grow to a maximum length around 2 in (5.1 cm). Shells are D-shaped, and attached to the substrate with strong byssal fibers, which come out of their umbo on the dorsal (hinged) side.

## Appearance

Adult zebra mussels have a pair of roughly triangular shells connected by an elastic hinge, the outside of the shells are usually brown with stripes that give the species its common name in English, but patterns and the darkness of color varies. They grow to about 5cm maximum length. The ventral side of the mussel is flattened, so much so that the mussel shell will stand on a flat surface. In life they attach to substrate with a glue they secrete that forms fibers called byssal threads.

Zebra mussels are ectothermic and heterothermic. Their body temperature as the temperature of their environment changes.

Zebra mussels have a free-swimming larval stage. This stage in molluscs is called a veliger. Veliger larvae are small enough that they can easily drift in the water, only about 100 micrometers long (0.1 mm) in diameter. Zebra mussel veligers have a tiny shell, and a curved sheet of skin called a velum, that is covered with tiny hairs that beat in the water. This helps them swim, and also draws food particles in for them to eat.

## Distribution

Zebra mussels were originally found in the drainage basins of the Black and Caspian Seas, including the Danube, Dniester, Volga, and Ural Rivers. Shipping and canal construction in the 19th century allowed them to spread west into most European rivers and lakes. In the late 20th century they were accidentally brought to North America, probably in ballast water of large ships. They now occur in the Great Lakes basin, most of the Mississippi River drainage, the Hudson River and many other eastern North American rivers.

Zebra mussels live in still or slow-moving freshwater, and attach themselves to any hard surface under water, natural or man-made, including rocks, submerged wood, boat hulls, buoys, docks, and water intake pipes. They need at least moderate concentrations of calcium to grow their shells (approximately 25 mg calcium2+/liter) but can survive for some time in lower concentrations. They do not thrive in pH lower than 6.8, and grow fastest in pH 7.4-8.4.

This species can survive exposure to temperatures as low as -10°C for a few minutes, and warmer sub-freezing temperatures for hours or days. Consequently most mussels live below ice levels, and the most dense populations are usually found between 2 and 12 meters in depth. They can live deeper however, and have been collected at least as deep as 60 m.

They stop growing at about 3°C, and increase their growth and feeding rates as they warm to 20-25°C. Above that they slow down and again, and start to die at 30°C.

## Behavior

Zebra mussel larvae drift in the water, feeding and growing and then settle down on to a hard surface. Once they attach, they do not move again. When they are abundant, they often completely cover any exposed rock or other surface. Even if there are many close together, they do not seem to have any interaction with each other, except for reproduction. Zebra mussels have no heads, and no eyes. However, they are sensitive to chemicals in the water, and can detect gravity, touch and temperature. If disturbed they will close their shells.

## Diet

Zebra mussels filter their food from the water. They eat mainly single-cell organisms, such as bacteria, blue-green algae, small green algae, and protozoans. They also consume very fine detritus particles.

Zebra mussel larvae have no special defense against predators, but they are so small that only small predators and filter-feeders eat them. The larvae are part of the zooplankton in the water, and pretty much any predator that eats zooplankton eats them. This includes many small fish (including the young of large fish), other zooplankton such as copepods, freshwater Cnidaria like hydras, even freshwater sponges.

Most fish can't eat zebra mussels because they can't crush the shells. A few fish species have specialized teeth and jaws that are strong enough to break the shells of mollusks, and some of them do eat zebra mussels. In Europe the roach, is a major predator of zebra mussels, along with bream, and silver bream. Round gobies and common carp, native to Eurasia, have been introduced to North America, and eat zebra mussels where they occur. The black carp is an east Asian species that has been introduced to Europe, and eats zebra mussels there. The pumpkinseed sunfish has been introduced to Europe from North America, and eats zebra mussels on both continents. Besides pumpkinseeds, the several other North American fish eat zebra mussels, including freshwater drums, redhorse suckers, river carpsuckers and smallmouth buffalos.

Some species of waterbirds are important predators of zebra mussels too. These are mostly diving ducks. Species known to feed significantly on zebra mussels include greater scaups, lesser scaups, pochards, tufted ducks, buffleheads, goldeneyes, common coots oldsquaws, herring gulls, and white-winged scoters.

Blue crabs (Callinectes sapidus) consumed many zebra mussels during a study in the Hudson River. Crayfish, including the northern clearwater crayfish, Orconectes propinquus, may prey on small zebra mussels. (Molloy, et al., 1997; Nalepa and Schloesser, 1993; U.S. Geological Survey, 2008)

## Reproduction

Adult zebra mussels start to reproduce in the spring, when water temperatures rise to about 12°C. In habitats where they water stays warm year round, they may reproduce continuously. Females release eggs into the water, and males release sperm, and fertilization occurs after they are released. Females grow and release eggs in batches of up to 40,000, up to four times during the breeding season, which lasts as long as the water stays warm enough. Each can release as many as 1 million eggs each year.

Zebra mussels are mature and able to reproduce when they are 8-9 mm long, usually after about one year of growth.

Zebra mussels life span varies. Most live 3-9 years.

## References

https://animalia.bio/zebra-mussel

https://animaldiversity.org/accounts/Dreissena\_polymorpha/