Aquariums and Zoos play an important role in conservation, by breeding, rearing large numbers of offspring, and releasing ("propagating" in conservation jargon) the animals back into their natural environment. Such in-house conservation efforts usually go hand-in-hand with work that aims to mitigate the cause or causes for the endangerment of the species in the first place.
Freshwater mollusks are one of the most imperiled groups of animals in the United States. One of the organizations that raise endangered mollusks with conservation goals is the Phoenix Zoo in Arizona. Their staff currently works with two small species of freshwater snails, the San Bernardino Springsnail, Pyrgulopsis bernardina, listed as Endangered in the Red List prepared by the International Union for Conservation of Nature, and the Huachuca Springsnail, Pyrgulopsis thompsoni, treated as Near Threatened in the Red List. The two species are found in freshwater springs in a small area in Arizona and Sonora, in Mexico. Among the factors that cause the decline of those species are depletion of groundwater and the effects of wildfire on stream quality and flooding. Despite their small size (between 2 and 9 mm, or 0.08 and 0.36 inch), springsnails play an important role in their environment, by controlling growth of algae and recycling nutrients from dead plant material. You can follow the progress of the Phoenix Zoo’s efforts toward the conservation of springsnails here.