**2. Origin and distribution of snails**

feeding, respiration and sometimes locomotion [1]. It constitutes the second largest, and most variable, invertebrate phylum. The living species of the mollusks are divided into seven classes, that is aplacophora, polyplacophora, monoplacophora, gastropoda, cephalopoda, pelecypoda and scaphopoda [2]. Gastropods are the largest group of mollusks, comprising about 80% of the living mollusks with ca. 62,000 living species. The first gastropods originated during the late Cambrian period and over 500 million years ago. Since then, gastropods have radiated into marine, freshwater and terrestrial environments, changing their food preferences from herbivorous to carnivorous, endo‐parasitism or symbiont‐mediated chemoautotrophy [3].

The class gastropoda is the most speciose among animals to inhabit a variety of habitats such as oceans, rivers, etc. and are the ones that have inhabited the land among mollusks [4]. The aquatic forms have adapted to benthic forms while others remained pelagic. The life span ranges from months to decades [5, 6] and in some cases life is marked by varying periods of dormancy [7]. All gastropods are commonly called head‐foot or cephalopodium which is a typical character of all gastropods because the head and foot arise from the same region making it very difficult to differentiate where the head ends or the foot begins [8]. The head of gastropods typically has two or four sensory tentacles with eyes and a ventral large foot, which gives them their name (in Greek, *gaster* is stomach and *poda* is feet). The anterior divi‐ sion of the foot, that is, propodium, is used for crawling. The shell in the larval stage is called protoconch. Most gastropods have a shell that typically opens on the right‐hand side. Several

Most species of gastropoda include slugs and snails where the snails possess coiled shells on their body. The term snail is often used to describe marine and freshwater snails, along with terrestrial ones. More generally, the term is applied to land snails than to those from the sea or freshwater [9]. Snails generally thrive in habitats rich in calcium, limestone, marl and places with concrete and cement. They are hermaphrodite but reciprocal copulation is required to produce viable eggs. Eggs are laid 8 days after copulation producing about 400 to 1000 eggs per year [10]. Cool and moist soil is necessary for the egg hatching producing juvenile snails that eat their egg shells and remain burrowed for 2 weeks. The juveniles feed on tender shoots of plants while the adult can also digest detritus. Under unfavorable conditions, snails can

The terrestrial snails like *Achatina fulica*, *Achatina achatina* and *Archachatina marginata* are large‐sized terrestrial mollusks that can grow up to 20 cm in length and 10 cm in diameter. In these snails, the brownish shell having dark stripes generally covers half of the body [12]. Among these, the shell of *A. fulica* is smaller that can grow up to 3–4 inches, while *A. achatina*

Snails are both ecologically as well as economically very important animals. In the modern era of technology, the utility of snails is largely neglected, particularly in developed countries. Since snails dwell in a variety of niches, they could harbor a militia of micro‐biota which could be exploited for various biotechnological purposes. This work provides insights into the microbiome of various snails. Furthermore, for the first time, we assessed the probable applications of snails in general and their gut micro‐biota in particular for various biotechnology‐based industries.

species have operculum that is used to close the shell opening.

bury themselves under soil and remain inactive from months to years [11].

has a larger shell size of 10–11 inches [9].

190 Organismal and Molecular Malacology

The families Lymnaeae and Planorbidae originated from the common ancestor approximately 250 million years ago during the Permian period. Some fossils belonging to family Bulininae and Planorbinae of the upper cretaceous have been obtained from Africa and India [13, 14]. The first fossil record for the family Achatinidae was obtained from the Pleistocene in Africa [15, 16] but the family clearly evolved much earlier. In the 1950s, Mead described the earliest achatinids that originated in Cameroon and Gabon, northward of the river Zambezi in Africa, which later spread to both arid and the sub‐arid areas of the southern continent and other moist parts east of the great watershed [17, 18]. This indicated that temperate species were directly evolved from tropical ancestors. Nonetheless, little is known about the evolutionary history of the achatinids.

The habitats of terrestrial snails range from dense tropical forests in Africa to the fringing riparian forests of Savannah [19, 20]. The members of the family Achatinidae comprise more than 200 species in 13 genera that are native to Africa. Several species have attained pest nature within their native African range when the habitat was modified by human activities and cropping. Furthermore, due to the increased mobility of humans and globalization of trade and travel, several alien species have been accidentally or purposefully transported to areas outside of the African continent. In these new areas, Achatinidae have caused signifi‐ cant economic and ecological impacts [21]. Due to its invasive capacity, *Achatina fulica* has spread from East Africa to many regions around the globe including rainforests, tropics, subtropics, etc. Apart from anthropogenic activities [22], the higher adaptability of this snail to variety of habitats is often contributed by its gut micro‐biota that it selectively chooses from the favorable environments for successful dispersion [23]. However, terrestrial spe‐ cies have a great capacity of adaptation, survival and may contain an intriguing micro‐biota serving in the efficient degradation [24] of ingested lignocellulosic plant biomass into many useful products. Due to its fast distribution and voracious feeding, this species is now con‐ sidered as the most destructive terrestrial gastropod [12]. The *A. fulica* has been blamed as an intermediate vector of many worms and microorganisms, causing a variety of ailments [25]. The species was introduced to the USA in 1939, to India in 1947 near Kolkata and to Brazil in 1980s.

The widespread distribution of *A. fulica* is caused by a number of factors [26]. Sometimes, it has been deliberately introduced by humans as pet and in some cases as a source of food or for ornamental and medicinal purposes (**Figure 1**). It is also transported unintentionally with agricultural, horticultural and other commercial products or in containers in which they are shipped. They were also transported accidentally with military equipment in many countries [27]. The land snail, *A. fulica,* spreads extensively along rivers and streams, either on floating mats of vegetation or by surviving long enough in the water to float downstream.

The pulmonate snails are native to Africa but are currently found in Asia, the Pacific, Madagascar, Indonesia, Australia, the Caribbean Basin, the United States and South America (Colombia, Venezuela, Ecuador, Brazil and Argentina) [28, 29].

**Figure 1.** Different methods involved in the isolation and identification of microbes of the snail gut.

## **3. Uses of snails**

From prehistoric times, it is quite evident that mollusks have a precarious relationship with humans. Many snails are known to damage the wooden ships and poison scuba divers. Researchers have also found that snails actually harbor a secret that could help humans to stay healthy and pain free. Even some authors quoted that guts of mollusks contain a unique set of microorganisms that might save human lives. During evolution, snails have also coevolved with ancient bacteria that reside in their guts. In return, the bacteria also express some drug‐ like molecules that help the snail's proper functioning and ward off diseases [30]. For example the *Leuconostoc mesenteroides* strain isolated from the gut of *Cornu aspersum* produced some bac‐ teriocins‐like substance that inhibited the growth of the pathogen *Propionibacterium acnes* [31].

#### **3.1. Snails as food**

The gastropods, particularly snails, have been used both as food as well as treatment for a variety of human diseases. The fossil remains of prehistoric shellfish found in caves indicate that snails have served as a delicacy for humans for thousands of years. The snails are easy to culture and majorly composed of muscles. Snails are a rich source of proteins containing high amounts of essential amino acids [32]. From the twentieth century, the food qualities of snails were so appreciated that it was a highly sought‐after food. They are preferred as a food source in certain parts of Africa, Asia and South America. In recent decades, the snail's consumption has increased throughout European countries, which consequently lead to their gradual dis‐ appearance from freely dwelling areas. This decrease in population contributed to predation of the species and introduction of pathogens that harmed the productivity of snails [33, 34]. The inedible parts of snails are also used in animal‐feed preparations as shown in **Figure 1**.

#### **3.2. Medicinal uses**

Hippocrates reportedly said that crushed snails can be used to relieve inflamed skin and pain. Two decades ago, slime of the Chilean snail was reported to quickly heal the skin lesions with no scars. This innovation later led to the production of "*Elicina*" which is snail slime‐ based fairness products. Recently in 2010, Missha, a USA‐based cosmetic company launched a branded fairness cream, "Aqua Cell Renew Snail Cream", containing 70% slime. The com‐ pany also claimed that this cream reduces pigments, acne, scars and combats wrinkles [35]. Though snail slime contains unusual crystals of calcite, it may find some use in orthopedics also. This is because scientists at the Herriot‐Watt University stated that calcite may be used for the development of bone cement by using inorganic crystals in organic matrix [36].

#### **3.3. Religious importance of snails**

In southern Miami, snail invasion is very severe because they are linked to religious rituals. In Candomblé religion, coloration of the shell is considered very important for offerings to their gods, Orishas, and symbolizes the personality of an Orisha (e.g., red indicates fire and fury, white indicates tranquility and age while yellow is for prosperity and wealth). However, the color preference can vary between nations of different areas of the religion. For example, sac‐ rificial animals or their parts that are offered to Obatala (white Orisha) should be completely white such as the white blood of *A. fulica* [37].

#### **3.4. Ecological importance**

**3. Uses of snails**

192 Organismal and Molecular Malacology

**3.1. Snails as food**

From prehistoric times, it is quite evident that mollusks have a precarious relationship with humans. Many snails are known to damage the wooden ships and poison scuba divers. Researchers have also found that snails actually harbor a secret that could help humans to stay healthy and pain free. Even some authors quoted that guts of mollusks contain a unique set of microorganisms that might save human lives. During evolution, snails have also coevolved with ancient bacteria that reside in their guts. In return, the bacteria also express some drug‐ like molecules that help the snail's proper functioning and ward off diseases [30]. For example the *Leuconostoc mesenteroides* strain isolated from the gut of *Cornu aspersum* produced some bac‐ teriocins‐like substance that inhibited the growth of the pathogen *Propionibacterium acnes* [31].

**Figure 1.** Different methods involved in the isolation and identification of microbes of the snail gut.

The gastropods, particularly snails, have been used both as food as well as treatment for a variety of human diseases. The fossil remains of prehistoric shellfish found in caves indicate that snails have served as a delicacy for humans for thousands of years. The snails are easy to culture and majorly composed of muscles. Snails are a rich source of proteins containing high amounts of essential amino acids [32]. From the twentieth century, the food qualities of snails Some snails that climb the trees rasping on the surface of leaves can influence biosphere com‐ munity succession and nutrient cycling. Snails also provide some antimicrobial barriers to the plants by secreting the wax layer which contains antimicrobial compounds [38]. Mucus secreted by the gastropods has been shown to have selective antimicrobial properties as well [39, 40]. Moreover, some snails are also used for monitoring the environmental pollution. Such is the species of *Arianta arbustorum*, which can tolerate higher concentrations of the heavy metals, like cadmium, lead and copper, indicating elevated levels of metal pollution in their niches [41].
