The life history and biology of freshwater mussels are quite complex and makes this group particularly vulnerable to environmental disturbances. For example, unionids are filter feeders. To take up food and oxygen, mussels siphon water from their surroundings. Along with dissolved and suspended organic matter, mussels also ingest potentially lethal contaminants if the water is polluted. Additionally, the complex life cycle of unionids involves a parasitic stage during which larval mussels (glochidia) attach to species-specific fish hosts. As a result, the survival of freshwater mussel populations is heavily dependent on the availability and health of suitable fish hosts. Although vulnerable, mussels perform functions that are critically important for the integrity of their ecosystems. They help to maintain water quality by filtering contaminants and, as primary consumers, they occupy an important intermediate position in the food web, shunting energy from one trophic level to another. For example, unionids are an important food source for consumers such as wading birds, raccoons and muskrats. The decline of mussel populations or the outright disappearance of unionids from a water body can be an early indication of environmental stress or of deterioration of an aquatic ecosystem.