*3.2.8 Superfamily: Truncatelloidea*

Families within this superfamily were earlier included in the Rissooidea which was one of the largest and most diverse molluscan superfamilies, with about 23 recognized recent families, including marine, freshwater, and terrestrial members. The freshwater, brackish water, and semiterrestrial families and genera were moved to Truncatelloidea [47]. Most families contain small-sized species (**Figure 8**) and several species have medical and/or veterinary importance. The following families belong to this superfamily: Amnicolidae, Assimineidae, Bithyniidae, Cochliopidae, Helicostoidae, Hydrobiidae, Lithoglyphidae, Moitessieriidae, Stenothyridae, and Tateidae. Detailed reviews of these families are found in Refs. [16, 72–79]. Here, we present a brief overview of selected families.

#### *3.2.9 Assimineidae*

The species are mostly amphibious, spending most of the time outside the water on wet mudflats under stones, on decaying wood or in the stumps of palms [29]. Some species, however, are fully aquatic [29]. They are found in drainage creeks, in the estuaries of rivers, and in trenches and ponds in freshwater within the tidal zone [29]. The animals are oviparous with free-swimming larvae. *Assiminea lutea* was reported to harbor microcercous cercariae of *Paragonimus*, probably a species infecting non-human final hosts [29]. Brandt [29] checked several thousand specimens of *A. brevicula* and *A. obtuse* from Thailand for cercariae but no infected snails were found.

#### *3.2.10 Bithyniidae*

The family (**Figure 8**) is very important in Asia because some species are intermediate hosts of liver and intestinal trematodes. Species identification based on only morphological characters may be difficult. Species are commonly found in shallow reservoirs and wetlands including rice fields and may often be exposed to desiccation. Although some snails die during desiccation, some survive through aestivation to recolonize the habitat when water returns. Species within this family may feed both by grazing and by filter feeding. Bithynid snails are often found in aquaculture ponds in the Red River and Mekong deltas and occasionally at high density but they are more commonly found in small canals and rice fields. During the spring planting of rice fields, density of *Bithynia* spp. can be extremely high in newly planted fields.

#### *3.2.11 Pomatiopsidae*

With approximately 170 species, the Pomatiopsidae is among the most speciesrich freshwater gastropod families. The highest diversity can be found in Southeast Asia and the Japanese archipelago (>140 species), followed by sub-Saharan Africa with approximately 10–11 species, southern Australia with ca. 9 species, the northwestern Palearctic with 1–8 species, North America with 5–6 species, and South America with ca. 2 species [80]. The Pomatiopsidae comprise two subfamilies, the

Pomatiopsinae Stimpson, 1865 and the Jullieniinae. The Asian intermediate hosts for *Schistosoma* species belong to this family.

The Triculinae in Asia is very diverse with an endemic fauna that includes over 90 species occurring along a 300 km stretch of the lower Mekong River in Thailand and Laos [29, 80–82]. Relatively few species are reported from Vietnam [83], but this is likely because relatively little work has been done on the Vietnamese part of the Mekong River. Within the Triculinae, several species have been described from Vietnam [83], i.e., *Tricula ovata*, *T. similunaris* and 11 species of a new genus *Vietricula*. Liu et al. [82], however, believed these snails to be *Pachydrobia*. Some of the pomatopsid species are intermediate host for *Paragonimus westermani* [18, 84]. Doanh et al. [85] found two *Vietricula* spp. (originally identified as *Oncomelania*) infected with *Paragonimus heterotremus* and Doanh [86] experimentally infected the two species with *P. heterotremus*.

### *3.2.12 Hydrobiidae*

Hydrobiidae, commonly known as mud snails, is a large cosmopolitan taxonomic family of very small freshwater snails and brackish water snails. These are small snails, with a shell height of less than 8 mm. The dextrally coiled shells are smooth and renders few robust characteristics to the systematist. Furthermore, there is considerable intraspecific variation in shell characteistics. Description is mostly based on the characteristics of the operculum, radula, and penis.
