In previous posts on this site (see here and here), I've introduced you to members of the Hydrobiidae, a diverse family of mostly freshwater gastropods. Hydrobiids have long been recognised as a tricky group to work with, because of their small size and general shortage of distinctive shell features. In recent years, an understanding has developed that the 'hydrobiids' may include a number of lineages that became independently adapted to fresh water, and a number of previously recognised subfamilies of the Hydrobiidae have come to be recognised as their own distinct families. One of these ascended subgroups is the Lithoglyphidae.
The Lithoglyphidae are a family of about 100 known species, mostly found in the Holarctic region (Strong et al. 2008), though they have also been recorded from South America. Most lithoglyphids have distinctively squat, relatively thick shells, and for a long time this was treated as one of the main defining features of the group. However, Thompson (1984) pointed out that the sturdy lithoglyphid shell was probably an adaptation to living in fast-flowing streams and rivers, and could also be found in other 'hydrobiid' groups. As well as reducing the shell profile, the lithoglyphid shell possesses a broad aperture that allows for a proportionately large foot, increasing the snail's clinging power. Thompson (1984) identified a number of other features characteristic of lithoglyphids, including a spirally sculptured protoconch and a simple, blade-like penis that lacks accessory lobes or glandular structures. As the soft anatomy of many 'hydrobiids' has not yet been described, it is still possible that some taxa currently identified as lithoglyphids are in fact impostors. Conversely, confirmed lithoglyphids now include some taxa more divergent in shell shape, such as the limpet-like Lepyrium showalteri from Alabama. This species is distinctive enough that when first described it was identified as a neritid, a member of a group of gastropods not even closely related to lithoglyphids (imagine a new species of rodent being identified as a ratfish). Sadly, Lepyrium is also now endangered, being extinct in one of the two river catchments it was historically known from (see here). Thompson (1984) notes that another North American lithoglyphid genus, Clappia, may be entirely extinct. For at least one species, the cause of extinction was pollution from coal mining; no cause was specified for the other species, but according to Wikipedia its native habitat in the Coosa River has been modified by the construction of hydroelectric dams.
Also closely related to the lithoglyphids are the Benedictiinae, a group of 'hydrobiid' gastropods endemic to Lake Baikal in Russia. A single species of benedictiine has been described from Lake Hövsgöl in Mongolia, but has not been collected there since; it seems likely that its original location was an error (Sitnikova et al. 2006). Baikal is a remarkable place: one of the world's largest freshwater lakes (and easily the largest in terms of the volume of water it contains), it is basically a freshwater sea. While other large lakes such as the Rift Lakes of Africa are poorly oxygenated at deeper levels, effectively restricting most animal life to the surface layer, Baikal has oxygen-rich deeper waters allowing a rich deep-water animal community (this may also be related to the numerous hydrothermal vents in the depths of Baikal). Some of you may have heard of the endemic Baikal seal Phoca sibirica, but Baikal is also home to a wide diversity of endemic fish (including a dramatic radiation of sculpins), a remarkable array of endemic amphipods, and even its own endemic family of sponges. The Benedictiinae are currently classified as a separate subfamily of Lithoglyphidae, with the remainder of species in the Lithoglyphinae (Bouchet et al. 2005), but as the relationship between the two subfamilies has not yet been examined in detail it is possible that the lithoglyphines are paraphyletic to the benedictiines. The benedictiines generally have thinner shells the lithoglyphines, possibly related to the differences in their usual habitats.
Bouchet, P., J.-P. Rocroi, J. Frýda, B. Hausdorf, W. Ponder, Á. Valdés & A. Warén. 2005. Classification and nomenclator of gastropod families. Malacologia 47 (1-2): 1-397.
Sitnikova, T., C. Goulden & D. Robinson. 2006. On gastropod mollusks from Lake Hövsgöl. In: Goulden, C. E., T. Sitnikova, J. Gelhaus & B. Boldgiv (eds) The Geology, Biodiversity and Ecology of Lake Hövsgöl (Mongolia), pp. 233-252. Backhuys Publishers: Leiden.
Strong, E. E., O. Gargominy, W. F. Ponder & P. Bouchet. 2008. Global diversity of gastropods (Gastropoda; Mollusca) in freshwater. Hydrobiologia 595: 149-166.
Thompson, F. G. 1984. North American freshwater snail genera of the hydrobiid subfamily Lithoglyphinae. Malacologia 25 (1): 109-141.