When they have grown enough, the larvae settle onto something and attach themselves - in a sea chest, for example. Because their growth depends on temperature, and the river temperature changes so much as the larvae ride downstream, it is hard to predict exactly where and when they will settle. Some vessels may have the bad luck to be in the "wrong place at the wrong time". When zebra mussels settle, they are smaller than the head of a pin, and they can settle by the hundreds of thousands. After one summer of growth, they will be about 1/4 - 3/4 inch long (6-19mm). They attach quite strongly to a surface with a bundle of threads. The maximum size for zebra mussels after several years growth is about 1.5 inches (38mm).

Most infestation on vessels comes by way of larval attachment (***diagram). It is, however, possible for zebra mussels to detach themselves from a surface and reattach to a new surface. This is the main mode for zebra mussels to infest upstream locations.

1. The towboat operates in an infested area where zebra mussel larvae are actively settling.
2. The vessel then becomes infested.
3. These newly settled mussels are then carried upstream attached to the vessel.
4. At any point on the trip upstream, the mussels can detach themselves and reattach on the river bottom.

If adult mussels are killed inside the sea chest or piping system, the shells will be sucked further into the piping system and may cause further blockage. If zebra mussel control is done on a monthly basis, the shells will be small are relatively fragile. Problems associated with shell blockage will be reduced.

When the mussels die or are removed from a surface at dry dock, the bundle of attachment threads will remain strongly attached to the surface. This rough surface may provide an improved habitat for new settlers.

If zebra mussels die suddenly, the soft tissue may remain associated with the shell. This can lead to significant odor problems if the shells and tissue are not flushed from the system.

Salt Water - Zebra mussels are primarily freshwater organisms. They can tolerate very slightly brackish water but are generally confined to fresh waters. They have not been found on vessels that normally operate or make frequent trips through salt water.

Soft Water - Zebra mussels require a minimum concentration of calcium in the water to form a shell and develop. They are therefore not found in very soft waters. This may be the reason they have not been able to colonize the Tenn-Tom waterway or the Ouachita River.  

Low Oxygen - They also tend not to be found in very stagnant waters that lack oxygen for long periods of time.

Upstream Spawning Source - Because zebra mussels reproduce by sending out free-floating larvae, they can have problems maintaining a population in a river. There must be some means of retaining a self-sustaining local population. It will be difficult for zebra mussels to establish themselves in rivers without pockets of extremely low flow. Zebra mussels have not been able to establish themselves in the Missouri river. The rapid current and the absence of backwater areas and lock and dam structures may be preventing zebra mussels from establishing any self-sustaining populations on the river.

Recently, a single zebra mussel has been found about 15 miles south of Sioux City. This finding indicates that the water quality of the Missouri River may be suitable for zebra mussels. They may not be able to form any stable colonies, however because even during summer low water conditions, the current is rapid enough to sweep all the larvae downstream and out of the river.