Gastropods are members of the class Gastropoda, a highly broad group of mollusks that includes snails and slugs. They have a visceral hump, mantle, muscular foot, eyes, tentacles, and a specialized feeding organ called the radula, composed of many tiny teeth. Snails generally have a coiled, one-piece shell, while slugs are shell-less.
They evolved during the early Cambrian Period, but their number has grown since the Palaeogene. Currently, they are second only to insects in species diversity, with 65,000 to 80,000 extant species found in various habitats, including freshwater, marine, and terrestrial environments, from deserts and mountains to hydrothermal vents and abyssal oceanic depths.
Initially, all gastropods were grouped under four subclasses: Opisthobranchia (gills to the right and behind the heart), Gymnomorpha (lack shell), Prosobranchia (gills in front of the heart), and Pulmonata (with a lung instead of gills). However, with time, this classification scheme has been discarded, and new schemes are constantly being proposed.
Being a diverse group, gastropods vary greatly in size. The largest land snail, Achatina achatina, has a shell length of almost 20 cm (8 in), while the largest freshwater snails of the genus Pomacea measure around 10 cm in diameter. The largest marine snail, Syrinx aruanus, can exceed 2 ft. The largest land slug, Limax cinereoniger, measures over 20 cm, whereas one of the smallest slugs, Angustopila dominikae, is only 0.86 mm.
Gastropods are typically characterized by a head, muscular foot, mantle, and visceral hump. Most species are covered by a characteristic one-piece shell coiled dextrally (right-handed) or conically. However, in slugs, the shell is absent.
The head is usually bilaterally symmetrical, bearing one or two pairs of tentacles with accessory palps. The radula, a feeding organ, and the mouth are located at the head’s ventral margin.
Within the shell lies the visceral hump (or visceral mass) that houses a large inner cavity called the mantle, or pallial cavity, along with all the organ systems. In marine gastropods, the mantle cavity is ciliated and helps lash water towards the respiratory apparatus.
In snails, the visceral hump rotates 180° to one side during development in a process called torsion. Through rotation, the right side of a gastropod’s body is greatly reduced, and most of the body’s contents, including gill combs, the olfactory organs, the foot slime-gland, nephridia, and the auricle of the heart, shift to the left. As a result, the anus lies to one side of the median plane, almost above the head. Moreover, the single genital orifice also lies on the same side of the body as the anus.
This rotation of the body, however, is independent of the coiling of the shell.
It is a highly vascular, fleshy lining that secretes the shell and encloses an inner cavity, the mantle cavity. In terrestrial forms with reduced shells, the mantle extends anteriorly over the neck and the head in the form of lobes and laps.
Some carnivorous marine gastropods possess a mantle collar that extends forward and rolls into a muscular siphon. This structure is chemoreceptive and also helps locate food underwater.
Their foot is broad and muscular, often divided along the anterior-posterior axis into a propodium and a metapodium. In some groups, like limpets and abalones, the foot is largely expanded and serves as an adhesive disk, whereas, in heteropods and pteropods, it helps in feeding. In other groups, like prosobranchs and opisthobranchs, it extends outwards into lateral projections called parapodia, which help in locomotion.
Most snails have a calcareous, spirally coiled shell with an outer protein layer called periostracum and inner layers of calcium carbonate. Some snails have an operculum, a structure that closes the shell’s opening. Some snails, like the Lister’s River snail, also possess an operculum, a keratinous flap that encloses the shell, although it may also be made of calcium carbonate.
Gastropods possess an open circulatory system characterized by a network of sinuses through which body fluid (hemolymph) flows into the tissues. The pigment found in hemolymph is usually copper-based hemocyanin; however, members of the family Planorbidae have iron-based hemoglobin.
Although most marine and freshwater gastropods breathe through gills, most of those living on land respire using pallial lungs. Some land slugs and tropical snails with reduced shells also exchange gasses using their skin or through epidermal projections of the mantle collar.
In a particular group of sea slugs, the nudibranchs, the gills are arranged on the back in the form of a rosette of feathery plumes. Some of these, however, may also respire through their skin.
The feeding organ or radula leads the way to the mouth, where the ciliated inner lining brushes the food particles into the digestive tract. While salivary glands secrete enzymes into the buccal cavity, some digestive glands in the stomach also serve the same purpose. A digestive gland stores the digested food for use in long periods of inactivity.
These invertebrates expel nitrogenous waste from their bodies in the form of ammonia or uric acid through nephridia. While primitive gastropods, such as archaeogastropods, had two nephridia, in most modern forms, one kidney is reduced or completely lost. These nephridia are also critical for maintaining osmotic balance in both freshwater and terrestrial gastropods.
Some species, like those in the genus Haliotis, possess special pericardial glands or Keber’s organs in the body cavity that aid in excretion.
The nervous system in gastropods includes central and peripheral systems, which are characterized by a series of interconnected paired ganglia. These ganglia are the cerebral, pedal, osphradial, pleural, parietal, visceral, and sometimes the buccal ganglia.
Georges Cuvier coined the term Gastropoda in 1795, taking reference from the Greek words gaster and pous, meaning stomach and foot. This taxonomy refers to the position of the foot below the animal’s gut.
In 2004, Brian Simison and David R. Lindberg proposed possible diphyletic origins (derived from two separate ancestral lines) of gastropods based on analyses of mitochondrial gene order and amino acid sequence. A year later, in 2005, Philippe Bouchet and Jean-Pierre Rocroi proposed the Bouchet & Rocroi taxonomy, which combined older classifications with new cladistic data. These two scientists, along with others, further modified their classification scheme in 2017, using unranked clades to denote taxa above the rank of superfamily and following the Linnaean hierarchy for those below it.
Although scientists have not unanimously decided on the relationships at the base of the gastropod phylogenetic tree, they have listed the major groups as follows:
As of 2017, 721 gastropod families are known, of which 476 are extant, and 245 are extinct.
Gastropods are found almost everywhere, from around the Arctic and Antarctic regions to the tropics. They inhabit almost every habitat, ranging from ocean basins and supralittoral zones to deserts and rainforests. While most are terrestrial, including land snails and slugs, some, such as abalone and conches, are marine. A few species, like pond snails, live in freshwater environments.
Some species, like Sphincterochila boissieri and Xerocrassa seetzeni, are found in deserts, whereas scaly-footed gastropods (Chrysomallon squamiferum) have been spotted in hydrothermal vents and oceanic trenches (6 miles under the water surface).
These mollusks are often transferred from one habitat to another by external agents, like birds and other animals.
Being a diverse group, the diet of gastropods varies significantly.
Most gastropods typically move by repeatedly contracting the muscular foot from the posterior to the anterior end. Species with narrow feet, such as those in the genera Strombus and Aporrhais, move in a stop-and-start manner, using their foot and operculum to dig into the substrate. Some smaller species, like mud snails, secrete a layer of pedal mucus from the anterior part of the foot, which they glide over using ciliary action. Slugs employ a similar mechanism for locomotion.
Gastropods often engage in courtship behavior before mating. Some pulmonates make a sharp, calcareous, or chitinous dart called the love dart and shoot it at their potential mates. Except in most primitive gastropods, like abalones, fertilization is internal, and the male releases the sperm inside the female’s body.
The fertilized eggs hatch into a free-swimming trochophore larva, which, by expanding its ciliary girdle, further develops into the veliger larva. The veliger has heavily ciliated lobes and a small shell within which the lobes retract. It undergoes torsion and slowly develops into an adult.
Welcome to AnimalFact - your go-to source for fascinating insights into the amazing world of animals!
