Hydrobiidae

Hydrobiidae, commonly known as mud snails, is a large cosmopolitan family of very small freshwater and brackish water snails with an operculum; they are in the order Littorinimorpha. Hydrobiidae are found in much of the world, inhabiting all continents except Antarctica. In Australia alone there are over 260 species in the family. These are very small or minute snails, with a shell height of less than 8 mm. The dextrally-coiled shells are smooth (except for growth lines conforming to the shape of the outer lip) and are usually rather nondescript. The shell offers very few robust characteristics to the systematist who is attempting to classify the species within this family. This difficulty is compounded by a high degree of intraspecific variation. Descriptions often have to be based on the characteristics of the operculum, radula and penis. The shell of species within this family varies from planispiral to needle-shaped. The shell may have an open umbilicus or a plugged umbilicus. The thickness of the shell can vary from thin to fairly solid. The shell may be transparent and horn-colored, or colorless. The number of whorls in the shell varies between two and eight. The shell can sometimes even assume a corkscrew or hornlike shape by loosening of the attachment of body whorl. The periostracum (outer layer of the shell) is usually thin, and is often colored. It can sometimes show hair-like projections. The sinuous aperture is entire, not interrupted by a siphonal canal or other extensions. It is sometimes thickened. The protoconch is usually shaped like a dome, and usually shows a few spirals. The corneous operculum is usually well-formed and shows only a few spirals. The head, foot, mantle and visceral coil are colored pale gray to dark purple-black with melanin pigments. Subterranean species are often unpigmented.