The Bladder Snail, Physella acuta (Dreparnaud, 1805) is familiar to freshwater aquarium enthusiasts, as they have the uncanny ability to appear “out of the blue” in their tanks. This is exactly what happened at home last week, when my significant other, Kim Nealon, found several, 1/4-inch long, sinistral (left-handed) snails thriving in our home fish tank. The Bladder Snail, also known as Tadpole Snail or Acute Bladder Snail, is a globally invasive species: the snails have the ability, following arrival in a new body of water, to quickly get established and expand the invasive range. As an invasive species, it also has a knack for outcompeting and eliminating populations of native snails. In addition, the species was recently shown to be a vector for trematode parasites known as flukes. Current genetic studies indicate that several other species of the family Physidae from the Americas and Europe are actually one and the same, Physella acuta. The invasion history of the species is confusing, mostly because human activities such as the aquarium trade and boating keep driving invasions in many different directions. A hypothesis based on genetic evidence, however, suggests that the first invasive populations took place in the 1700s, when snails were carried from North America to Europe, most likely via the cotton trade from Mississippi to France. Aquarium enthusiasts regard this aggressively invasive species with mixed feelings. When the population in an aquarium is limited, the snails are useful, being capable of cleaning the glass by removing the algal film that sometimes coats the aquarium glass internally. But population growth may quickly become excessive, with snails taking over an aquarium if there is too much food available. The photo shows a young Bladder Snail measuring about 3 mm (0.12 inch). The head is on the lower right-hand corner. The mouth with the radula (rasping apparatus with teeth) is visible as a golden-colored structure in the center of the head. The little golden-green specks are diatom algae, most likely a species in the genus Tabellaria. (The “snail invasion” in our home tank was most likely caused by hatching of eggs attached to new plants for the aquarium.)