**3. The first intermediate hosts (overview of major clades of the Gastropoda)**

Trematodes require one or two intermediate hosts to complete their life cycle. The first intermediate host is specific species of freshwater water (and for some trematode species brackish or marine) gastropods. Due to the necessity of passing through the gastropods, control of these snails could, at least for some of zoonotic trematodes, be an important way to reduce their transmission (see later).

The class includes the snails, which are superficially asymmetrical and possess a spirally coiled shell; the limpets, which possess a low, conical un-spiraled shell; and the slugs, which possess a concealed shell or no shell at all. A recent paper [47] estimates the number of named and valid recent species as about 63,000 in 476 families. There is a great diversity among the freshwater gastropods. Gastropod taxonomy has undergone considerable revision and still undergoes revision as new DNA data become available. Here we use the classification as described in Bouchet et al. [47].

The class, Gastropoda, contains the following subclasses: Patellogastropoda, Neomphaliones, Vetigastropoda, Neritimorpha, Caenogastropoda, and Heterobranchia of which the last three are represented in freshwater. Many of the existing identification keys to freshwater gastropods follow the classification of Thiele [48] where Gastropoda was divided into three sub-classes Prosobranchia (Streptoneura, i.e. crossed nerve system), Pulmonata and Opisthobranchia (Euthyneura). Using the existing keys for species identification of freshwater snails, however, does not pose a real problem. Thus, Prosobranchia (often called prosobranchs) equates Caenogastropoda plus Neritidae and Pulmonata (often referred to as pulmonates) equates Hygrophila within the Panpulmonata. We shall restrict our discussion to primarily the freshwater gastropods.

#### **3.1 Subclass: Neritomorpha**

#### *3.1.1 Neritidae and Neritilidae*

The Neritidae are one of the most abundant groups of freshwater snails in the coastal streams of tropical and subtropical regions worldwide, as well as in the inland waters of the European continent [49]. The Neritiliidae, previously a subfamily in the Neritidae, include 23 described species in seven genera from low latitude areas of the World. Species of *Neritilia* occur in freshwater streams to brackish estuaries [50]. These are species of medium-sized snails with a characteristic shell, radula and operculum. Neritids require hard substrata for their grazing and locomotion, and some species of *Dostia* and *Septaria* can exclusively be found on branches and drift logs in estuaries and mangrove swamps [49]. These markedly euryhaline neritids have the potential to survive in fully marine conditions for some extended period of time and to disperse as benthic adults and eggs on drift logs [49]. The families are not known as intermediate hosts for medical or veterinary important trematodes, but some species may harbor other trematode or nematode infections. Examples of species are shown in **Figure 12**.

#### **3.2 Subclass: Caenogastropoda**

#### *3.2.1 Viviparidae*

The family (**Figure 12**) has a global distribution and moderate diversity [51] in the extant fauna (125–150 valid, described species). Viviparids are distributed primarily in

#### **Figure 12.**

*Selected species of Neritidae (a), Viviparidae (b) and Ampullariidae (c). Neritidae: Dostia violacea (1, 2), Neritina afra (3, 5), N. natalensis (4, 6, 7), N. oweniana (8). Viviparidae: Cipangopaludina chinensis (1), C. lecythoides (2), Mekongia lithophaga (3), Angulyagra polyzonata (4), Filopaludina sumatrensis (5), Sinotaia aeruginosa (6), various African Bellamya spp. (7–15). Ampullariidae: Saulea vitrea (1), Lanistes spp. (2–4, 7, 8), Marisa cornuarietis (6), Pila ovata (5), Afropomus balanoidea (9), Pila africana (10), Pomacea canaliculata (11), Pila sp. (12), P. conica (13), P. ampullacea (14), P. polita (15).*

lakes, rivers, and streams in temperate to tropical regions. Although they can be found in freshwater of all kinds, many species prefer, or are restricted, to one habitat type only. Their greatest diversity occurs in tropical and subtropical regions of Asia, where some 60–85 species occur. These species are medium to large snails usually with a conical shell. Tentacles are short and pointed and the right tentacle of males is transformed into a copulatory organ. The females are ovoviviparous with a uterine brood-pouch. Size and number of mature embryos may be of help to taxonomists [29]. The family is quite diverse in Asia where representatives are commonly consumed by humans. Metacercariae of the Echinostomatidae and possibly other trematodes are commonly found in viviparid snails and since many species are eaten by local people they could serve as intermediate hosts for human trematode infections if consumed insufficiently cooked. Species within the family are also reported as first intermediate hosts of some species of echinostome [51]. Some if not all species within the family are suspension feeders giving them a competitive advantage over species that only graze.
