The Thiaspia is a class of jawless fish with a complex nasal labyrinth. All species are found in or near marine environments that have large amounts of dissolved sulfides, usually either at hydrothermal vents, such as the "black smokers" of Terran fame, or in marine hotsprings, such as those that dominate the coasts of the Malthalassa. The nasal labyrinth, the diagnostic anatomical trait of thiaspids, serves to filter out potentially harmful sulfides before the delicate gill tissue can be damaged. In some taxa, namely the order Amphoranasuta, the nasal labyrinth serves also to extract dissolved nutrients from inhaled water.
Thiaspia is divided into two subclasses, Thiogyrinida and Thiaspida. The Thiogyrinids are found in abyssal hydrothermal vents throughout the world, while the Thiaspids are endemic to the Malthalassa. While current studies suggest that the Thiogyrinids may be just as diverse, species-wide, as the Thiaspids, the Thiaspids are the better-studied group, as thiogyrinids live in the largely inaccessible deep sea.
As the translation, "sulfur tadpoles," suggest, the thiogyrinids bear a superficial, tadpole-like appearance. They are found grazing on biofilms at and around abyssal hydrothermal vents.