GASTROPODS: CHARACTERISTICS, REPRODUCTION AND FEEDING - BIOLOGY - 2025

The gastropod, gastropods or univalve are soft-bodied animals with defined head, mostly protected by a spiral shell limestone. This group is included in the phylum Molusca.

Snails that have a shell and slugs that lack a shell are distinguished. They have a muscular foot like a sliding sole that allows them to move, although very slowly.

IMAGE: Snail (Gastropod). pixnio.com

They are both terrestrial and aquatic animals, both marine and freshwater. Terrestrial species prefer humid environments. When the weather is dry they take refuge in shady and humid places, and leave their shelters with the entry of rains.

Some species are of interest to humans as food. Others represent a problem, as they are part of the life cycle of parasites that cause serious diseases such as schistosomiasis or bilharziasis. In some cases they are pests on crops, such as the African snail (Achatina fulica).

In the past, some species of snails were used as coins, such is the case of the cowrie (Moneta moneta).

characteristics

Gastropods or snails, regardless of the shell, are animals of bilateral symmetry. Their body remains constantly moist due to the mucus or snail slime that secretes their skin and prevents desiccation. That slime leaves a shiny trail as the snail moves around.

Snails have been a source of food for humans since prehistoric times. In France they are considered a gastronomic delicacy. Its shells are used to make musical instruments and to make various ornaments.

Among the most common predators of gastropods are birds, fish, Coleoptera larvae, Hemiptera nymphs, and Odonata.

Some gastropods are intermediaries in the cycle of pathogens that cause diseases in humans, such as schistosomiasis, or livestock, such as liver fasciolasis.

In bilharziosis or schistosomiasis, the causative agents of the disease are flatworms of the genus Schistosoma. These flatworms fulfill part of their life cycle in snails of the Biomphalaria and Oncomelania genera.

Exotic species

In the case of species introduced by humans in other environments, the damage can be multiple. For example, Achatina fulica is native to East Africa and has been introduced to other regions, either as food or for the production of snail slime.

Today it is a pest of crops in much of Africa, Asia, Australia and America. On the other hand, this snail is the host of the nematodes Angiostrongylus costaricensis and Angiostrongylus cantonensis, which cause the disease known as abdominal angiostrongylosis.

Additionally, Achatina fulica, as a voracious and rapidly developing exotic species, competes advantageously with local species. In the case of tropical and subtropical America, it threatens the existence of species of the genus Megalobulinos (American endemic).

Taxonomy and subclasses

Gastropods make up a class of the phylum Mollusca and include about 40,000 species. They are traditionally subdivided into three subclasses: Prosobranchia, Opisthobranchia, and Pulmonata. For its part, Prosobranchia is divided into three orders: Archaeogastropoda, Mesogastropoda and Neogastropoda.

For some authors, the Opisthobranchia and Pulmonata subclasses are the same group and are called Euthyneura or Heterobranchia. Likewise, in the case of the Mesogastropoda and Neogastropoda orders of the Prosobranchia subclass, today they are grouped into Caenogastropoda.

In other classifications, gastropods are divided into only two subclasses: Orthogastropoda or "true snails" and Patellogastropoda or "true limpets."

Structure

-The shell

In gastropods or snails the shell consists of a single structure, unlike bivalves. It has an opening, which may or may not be closed by a kind of lid called an operculum.

The shell has a spiral structure around a central column or columella. The winding plane of said spiral generates two possible basic shapes: discoidal or planispiral and helical or trochoid.

The discoidal shape is the product of the spiral being built around the axis, but in the same plane. In the helical form, the spiral reaches different planes in each turn.

The size, diameter versus length relationship, number of spirals and design of the shell surface is highly variable between families and genera.

The apex of the spiral is formed by what was the larval shell, called proto-shell. The rest of the set of turns of the spiral is called teleoconcha.

In the snails of the Opistobranchios subclass the shell may be reduced or even absent. These are the so-called slugs.

-The soft body

Head

Gastropods have a differentiated head. In this structure are the ocular tentacles or commonly known as the antennae or horns of the snail. Additionally, it shows two more tentacles located above the mouth.

In aquatic lung snails, the eyes are located at the base or near the base of the eye tentacles. In land lung snails, the eyes are located at the distal ends.

Gastropods have a mouth with labial palps. They have a horseshoe-shaped jaw and a structure called a radula.

The radula is a scraping organ made up of a central tooth and a large series of small surrounding teeth. These little teeth are renewed as they wear out.

Foot

They have a foot or locomotor organ, formed by a ventral muscle mass. The head and foot form the cephalo-pedal region, which is located in the antero-inferior part of the animal. This region can be outside or inside the shell at will.

The foot may or may not have an operculum. It is a protein cap that, when the animal retracts into the shell, covers the opening. In some species, the operculum is calcified, which makes it harder.

This flat and rough muscle mass in its lower part, allows the snail to move with slow sliding movements.

Visceral mass

Inside the shell and partially coiled in the columella is the visceral mass. The viscera are covered by an epithelium called the mantle, internally attached to the shell.

This mantle joins the cephalo-pedal region at the level of the opening of the shell, by means of a muscular structure called the collar of the mantle.

Organs

The heart, digestive system, reproductive organs and gills or pseudobranchs are located in the mantle cavity or paleal cavity.

In lung snails, instead of gills there is a lung. There is an opening of the respiratory organ to the outside called a pneumostoma.

Nervous system

They have an elemental nervous system, formed by a series of interconnected ganglia. Two of these nodes, called cerebroids, are connected to two vesicles called statocysts.

Small calcareous granites (statoliths) are located within the statocysts. This organ enables the snail to perceive its position and maintain balance.

Columellar muscle

The cephalo-pedal region and visceral mass are attached to the shell by the columellar muscle. As the name implies, this muscle inserts along the columella.

Reproduction

The sexuality

Gastropods can be hermaphroditic or unisexual. Fertilization can be external or internal. From the embryo, a veliger larva forms, provided with a covering and ciliated fins for swimming.

In some species, a trocófera larva can be generated, a ciliated larva of bilateral symmetry.

Hermaphroditic snails possess an organ called the ovotestis, which includes the testis and ovary. Despite being hermaphrodites, in many cases they require the participation of another individual and carry out cross-fertilization. Each individual acts simultaneously as a female and a male.

In those species with unisexual individuals, cross-fertilization or cases of parthenogenesis may occur. In parthenogenesis, the generation of the egg occurs without the need for the participation of a male.

Laterally and behind the head there is a genital or sexual orifice. Through this hole the sexual organs communicate with the outside.

Oviposition

Most gastropods are oviparous, although there is viviparism and ovoviviparism. Shortly after fertilization they lay a large number of small, soft, round eggs.

The oviposition can be in openings dug in the earth for this purpose, such as land lung snails. In most aquatic snails, the eggs have gelatinous shells or capsules that adhere to the roots of submerged plants or rocks.

The eggs can be white or with conspicuous colors (reddish) as in the species of the Ampullariidae family. There are species that retain the juveniles in a hatching sac located at the back of the head, as in the Thiaridae family.

Feeding

Gastropods play an important role in ecosystems due to their condition as detritivores and decomposers. They generally feed on plants, debris or organic remains and the periphyton or plant cover attached to hard substrates in rivers, lakes and lagoons.

The food is scraped and crushed by the friction of the radula against the jaw. Two salivary glands contribute to the predigestion of food.

The food bolus travels to the stomach and then to the intestine, where the secretion of a digestive gland called the hepatopancreas acts, which generates a fermentation process.

Finally the waste is excreted through the kidney through the excretory duct that empties near the anus.

Habitat

Gastropods are aquatic, terrestrial or amphibian animals. Aquatic ones can be marine or freshwater.

Its presence in the different habitats is conditioned by the availability of water or humidity. Other factors are high levels of dissolved oxygen in the water (in aquatic species) and calcium as a raw material for its shell. They tolerate temperatures from 0 ° C to 46 ° C.

Some species are able to survive in places where there is a marked seasonality with dry periods during which they hibernate. To do this, they retract their bodies into the shell and cover the entrance with the operculum or by secreting an epiphragm over the opening.

References

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4. Faber MJ. (2007). Studies on West Indian marine molluscs 58. Marine gastropods from the ABC islands and other localities 14. The family Terebridae with the description of a new species from Aruba (Gastropoda: Terebridae). Miscellanea Malacologica 2 (3): 49-55, 28.III.
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