gastropod

The gastropods , gastropods , univalves ( Gastropoda , from the Greek γαστήρ gaster, “stomach” and ποδη poda, “foot”) or single-shell creatures, are the best known, numerous and varied in terms of:
– The shape of their shells : convex, globose, flat, turriculate, conical, spirally wound, etc.
– Color patterns and sculptures. Currently, at least 30,000 species of univalves are known, including marine, freshwater and terrestrial species, which makes the Gastropods the most important Class within the phylum of Molluscs .

Environmental changes and genetic mutations that have occurred over millions of years have generated a variety of forms, a very efficient foot with which they move, dig their lairs or defend themselves from a predator; a pair of eyes, radula and a respiratory system that allows those species that live on land or in the lake environment to breathe air. The univalves that remain in the water still maintain the gills.

Summary

[ disguise ]

• 1 Parts of a Gastropod
  • 1 Shell
    • 1.1 External parts of a shell
      • 1.1.1 Periostracum
      • 1.1.2 Middle prismatic layer
      • 1.1.3 Inner mother-of-pearl layer
      • 1.1.4 Sculpture
      • 1.1.5 Axial sculpture
      • 1.1.6 Spiral sculpture
      • 1.1.7 Coloring
    • 2 Mantle (or canopy)
    • 3 Head
    • 4 Visceral Mass and Paleal Cavity
    • 5 Foot
  • 2 Vital Functions and Systems
    • 1 Digestive System
      • 1.1 Extracellular
      • 1.2 Intracellular
    • 3 Respiration, Circulation and Excretion
      • 1 Breathing
        • 1.1 Aquatic respiration
      • 2 Circulatory system
      • 3 Excretion
    • 4 Nervous System and Sense Organs
      • 1 Nervous system
      • 2 Sense organ
    • 5 Reproduction and Development
    • 6 Nutrition
    • 7 Longevity
    • 8 Habitat and Distribution
    • 9 Importance
    • 10 Fonts

Parts of a Gastropod

Gastropods are made up of: shell, mantle, head, visceral mass and foot.

Shell

The shell is a resistant limestone covering that most mollusks have to protect the animal from unfavorable environmental conditions or possible predators.

The variety of shapes and colors they present is a differentiating character in systematics (Conchiology). In the case of slugs, the cover is a small calcareous layer called limazel that is inside the body, giving certain rigidity and protection to certain organs.

The protoconcha is the first thing that forms during the larval stage. It is a very small spiral (or turn), also called apex or nuclear turn. The protoconcha differs in texture, color and sculpture from the postnuclear coils, which form as the mollusk develops, and which together form the whorl.

Columella or axis of a Northia pristis. (UCACM)

Columella

The last turn is the largest and is called the body turn. In its external part, between the suture and the base, there is an area called the periphery, which is where it reaches its greatest amplitude. The suture is where the loops join and can be fine or very ribbed.

At the back of the body is the opening (or stoma) which is where the animal comes out. The shape can be round, oval or slit.

The limit of the turn of the body borders the opening and is known as the lip (peristome) and is composed of an external lip and an internal lip or parietal wall; both can be smooth or serrated.

The external one may also have tubercles or spines. The inner lip is extended into a columella or axis, from which the turns develop. Many marine gastropods extend the columella forward to form the siphonal canal through which the respiratory tubes exit (Murex). Some genera also have a posterior or anal canal (Bursa) that forms with the upper part of the opening.

The columella can be solid or furrowed by a central canal. When the turns do not touch, a navel forms at the base of the shell that can be quite wide and deep.

File:Classification according to the type of spiral.png

Classification according to spiral type

The anterior part of the shell is where the opening is and the posterior part is where the apex is. The distance between both ends is known as length, although it is sometimes called height.

The shells of mollusks are mostly right-handed because the spiral is coiled to the right; If we hold a shell in front of us with the apex facing up, the opening is on the right side of the shell. A few species are left-handed (roll to the left).

To the far left a specimen of Glyphyalinia sp. and to the right one of Neocyclotus dysoni, both exhibiting a very deep navel. In the center, Beckianum beckianum (right-handed) and Beckianum sinistro (left-handed) show the position of the mouth, as do Succinea recisa and Physa nicaraguana on the far right.

The shell is composed mainly of calcium carbonate (taken from food or the surrounding environment), a minimal amount of calcium phosphate and conchiolin on which the calcareous material is deposited.

External parts of a shell

Calcium carbonate (CaCo ₃ ) usually crystallizes in two forms: aragonite or calcite, with shells formed by aragonite being more fragile. The crystals can be arranged in the form of prisms or sheets providing different properties to the structure of the shell. Basically the shells are the result of three layers:

Periostracum

The periostracum (from the Greek peri = around and ostrakon = helmet) a very resistant and thin horny integument, formed by conchiolin. Sometimes it may be missing, very thin and transparent, or only a light tint (as in some scrolls and moon shells). Its function is to protect the animal from harmful environmental influences ( acidic waters ), prevent damage to sponges, prevent the attachment of other organisms to its surface and provide mimicry to the shell. It generally appears worn due to use, sometimes with only a few tatters remaining.

Middle prismatic layer

Calcite prisms attached to each other and arranged perpendicular to the surface of the shell.

Inner mother-of-pearl layer

P: periostracum; CA: aragonite layer; CN: nacre layer (Photo by Callil & Mansur, 2005 )

Consisting of conchioline and sheets of aragonite oriented parallel to the surface of the shell, giving the internal surface a porcelain appearance. Or they can be very fine and wavy, mother-of-pearl, with typical iridescence as light breaks down into various colors.

The periostracum of Melongena patula forms vertical threads that cover the entire shell. The last two layers are collectively called ostracum.

 

Sculpture

The exterior sculpture of the shells is of calcareous formation and generates ribs, knots, varicose veins, tubercles, spines or bristles, edges, cords, threads, indented lines, holes, etc.; They are grouped into two types:

axial sculpture

Marks, edges or lines that go from suture to suture. It is sometimes called longitudinal or radial sculpture, Epitonium). Varicose veins and growth lines of the outer lip are axial forms.

spiral sculpture

It follows the direction of the suture, so we can talk about spiral colored bands. Natica unifasciata) The axial ribs are formed when the mantle retracts at the end of each growth phase, then the material is deposited at the same points, creating an accumulation. Spiral lines and radial ribs appear when the edge of the mantle is wavy.

The combination of both factors gives rise to knots and varicose veins, but differences in the rate of secretion cause fingerings and spines.

When the axial sculpture and the spiral intersect, it gives rise to the canceled sculpture. The reticulated sculpture is similar but the lines intersect at right angles. There can be many combinations and the wide variety of sculptures that we can find depends on it.

Shell growth is produced discontinuously throughout the life of the animal and is controlled by sex hormones, diet, water acidity , temperature , state of health and stress. Therefore, it could be concluded that under favorable conditions growth will be rapid, the lines will be relatively separated from each other; If the conditions are not good it will be slow and the growth lines will appear very close together; and if it becomes very unfavorable, it stops completely. With this data it is possible to know the environmental conditions of the area where the mollusk lives , as long as there are a certain number of specimens with similar variations.

Coloration

Most of the colors in the shells and soft parts of the body are produced by pigments obtained from food and distributed throughout the body by the blood system. Glandular cells concentrate these pigments and then mix them with the fluid calcite just before the outer layer of the shell hardens.

The cells that produce colors are generally located along the edge of the mantle. When they are distributed continuously on the edge of the mantle, the color of the shell is uniform, without patterns. But if they move and spread from place to place, lines or rays form as the shell increases in size. If the cells continue moving they leave a trail that will give rise to the formation of circles, triangles and any type of figures. The spots are outlined when the production of pigments is not constant.

The various colors of the shells are obtained by different combinations of yellow , black , green , blue or red pigments . Although some, such as iridescent blue-green, are the effect of light refraction .

The usefulness of colors and drawings seems to have a defensive purpose, because it provides camouflage that helps them go unnoticed, or they function as a warning of the animal’s danger. Nudibranchs, which lack a shell, are an example of the wonderful coloration that the soft parts can have.

Mantle (or canopy)

It is a viscous and soft membrane that surrounds the animal and offers an infinite variety of shapes, colorations and arrangements. It is responsible for producing the shell and also the ribs, spines, nodules, etc., that decorate it. Its edge contributes to the growth in diameter, while the internal area is responsible for the growth in thickness. A thicker shell will always be older than a thinner shell, regardless of its size.

Different parts of a Prosobranch.

Parts of a Prosobranch

Due to the torsion it is asymmetrical. In some genera it forms a tube, the siphon, through which the water current passes towards the pallial cavity. When the mantle tissue is undamaged, it is capable of correcting damage to the shell, otherwise malformations occur.

Head

The radula of Euglandina cumingi with hundreds of aligned teeth

The figure shows how the radular system is composed

It contains eyes, tentacles, mouth, proboscis and sensory organs. They may have two pairs of tentacles and the eyes may be at the base or at the end of a pair of tentacles. The second pair may have touch and chemoreceptor cells. In nudibranchs the second pair are called rhinophores and have platelets that increase the surface area for chemoreception.

In the pharynx the radula is on a structure called the odontophore that moves it back and forth like a saw, to scrape and tear food particles. This movement causes the teeth to continually wear down, but between 4 and 6 rows of teeth can be replaced per day. The number of teeth varies from 16 to more than a thousand, their position and shape is very varied and it is a character that helps differentiate one species from another.

Visceral Mass and Paleal Cavity

The visceral mass is the set of internal organs that are totally or partially covered by the mantle and contained in the greater whorl of the shell. The mantle together with the body wall delimits a space called the pallial cavity in which the visceral mass, the excretory and reproductive orifices, the anus and the gills or ctenidia are found (two in the most primitive and one in the others). At the edge of the gills is a sensory organ called the osphradium that acts as a chemoreceptor to determine the quality of the water and the amount of sediment in the inhalant stream.

In terrestrial gastropods, the pallial cavity is highly irrigated to facilitate gas exchange, so that it acts like a lung.

Foot

It is the locomotive organ. To move, they do so by crawling, a movement they achieve when waves run along the sole of the foot and make the animal move forward. On rough surfaces, a pedal gland secretes a mucus that prevents friction and facilitates movement.

The foot is inserted into the columella by means of a retractor or columellar muscle that has the function of removing the head and foot of the mollusk inside the shell. This locomotor organ is modified for different species depending on whether they swim or burrow. In those that remain fixed on a surface, it can take the shape of a suction cup to adhere strongly, as in the case of limpets.

Some gastropods, such as Melampus, extend the front part of the foot, and then drag the rest of the body behind it. The Litorinas have the foot divided into two halves, and they move by advancing first one side of the foot and then the other.

Different types of opercula

In the Harps the animal is capable of amputating its foot, which is left waving violently to distract the predator. In Strombus, it has a claw-shaped operculum with which it moves in jumps. The operculum is generally like a disc attached to the top of the foot which is formed by horny (organic) or calcareous material and fits perfectly into the opening to close it once the animal retracts into the shell to hide from its enemies. to protect yourself from harmful liquids or the dehydrating effects of the sun and air. It can be spiral or concentric and is sometimes very useful as an identifying character.

Different types of opercula

When the lungs lack an operculum, they have another mechanism to defend themselves against adverse conditions, the epiphragm. This is nothing more than a mucus that the animal spreads over the opening and once it dries it leaves it sealed and stuck to the substrate.

Different types of opercula:
A) Fasciolaria;
B) Melongena;
C) Cones;
D) Nerite;
E) Muricidal;
F) Melongena.

Vital Functions and Systems

Digestive system

The mouth is flanked by a small, horny jaw; The pharynx is followed by one or two pairs of salivary glands that lubricate the radula and agglutinate ingested food particles. They then pass into a tubular esophagus where (a pair of glands will secrete) more mucus is produced to envelop the food particles that will go to the stomach . In some genera (the glands of the esophagus are capable of producing) a toxic secretion is produced that paralyzes the prey for subsequent digestion.

Mollusks carry out digestion in two ways:

Extracellular

This takes place in the stomach. There the food is crushed in a gastric shield and subsequently sent to a classification area where the large particles are discarded in the intestine and the small ones are directed to the digestive gland duct (liver).

intracellular

It is carried out in the digestive gland duct by reabsorption and storage of the food slurry and substances (carbohydrates, fats and lime). The waste generated here goes to the intestine.

The main function of the intestine is the formation of feces so compact that they reduce the risk of contaminating the mantle cavity and gills as the excrement is expelled through the anal opening. Some gastropods have a crystalline stylet like bivalves. This is inside a sac rotating and rubbing against the gastric shield.

This rotating action causes the end of the stylet to wear and release enzymes that break down and absorb carbohydrates, as well as helping to mix the stomach contents.

The presence of the crystalline stylet in gastropods is related to the type of diet, which is basically phytoplankton or organic detritus.

Respiration, Circulation and Excretion

Breathing

Respiration can be aerial or aquatic.

Aquatic breathing

It is carried out by gills that are housed in the pallial cavity and their number varies depending on the species. The Pulmonates carry it out through the strongly vascularized pallial cavity, which acts as a lung.

Circulatory system

The heart is dorsal and formed by two atria in primitive forms. In more evolved forms, due to the coiling of the shell, the right gill and the right atrium disappear. The remaining gill uses the atrium and then the ventricle to supply the head, foot, and visceral mass.

Excretion

In primitive gastropods, excretion is carried out through nephridia. These at one end empty into the pericardial cavity from which it receives waste that is then reduced to urine; The other end connects to the pallial cavity where it sends urine to be expelled.

Nervous System and Sense Organs

Nervous system

It is formed by a nervous ring that surrounds the esophagus and from which two pairs of nerve cords emerge: a ventral pair that innervates the muscles of the foot and a visceral pair that innervates the mantle, the columella muscle and the viscera.

There are a pair of cerebral ganglia in the posterior part of the esophagus that are responsible for the eyes, tentacles, statocysts, and a pair of buccal ganglia that innervate the radula and accessory structures.

sense organ

Tactile cells are scattered on the surface of the body, but concentrated mainly on the head, edge of the foot and mantle. In the head we find tentacles with touch cells and chemoreceptors. Also the eyes, which in most mollusks only detect changes in light intensity . Some marine pulmonates have eyes (ocelli) at the edge of the mantle and can form images.

In the foot they have a pair of statocysts sensitive to gravity, although their function is not yet very clear. In nudibranchs the statocysts are modified and are called rhinophores. The most characteristic sense organ of gastropods is the osphradium. It is located in the gills and evaluates the quality of the water that enters the pallial cavity.

Reproduction and Development

Most gastropods have a gonad (ovary or testicle) that pours the product into the pericardial cavity from where they are transported to the outside by the nephridia.

Fertilization can be external or internal. Most aquatic species expel unfertilized sperm and eggs into the water, where (external) fertilization takes place. In terrestrial gastropods, copulation occurs by means of a penis and with very specific mating positions (coupling).

Reproduction

Individuals can be unisexual (Prosobranchs) or hermaphrodites (Opisthobranchs and Pulmonates) with cross-fertilization, which is the most general.

Hermaphrodites first function as males and then as females (like some crepidulas and limpets). The sex change is related to the number of females there are at the time of fertilization, and it is believed that pheromones are responsible for these transformations. In other rarer cases, such as nudibranchs, the animal is simultaneously male and female but the crossing occurs with another individual.

Almost all mollusks lay eggs and can be very fecund. This is the case of some nudibranchs whose ovaries can have up to 60,000 eggs; Land snails are less prolific, with 30 to 100 eggs, and they lay their eggs in holes they dig in the ground.

Nutrition

Because they have adapted to different environments, their nutrition is very variable, so they can be carnivores, herbivores, scavengers, ingest suspended particles, and also parasites.

Generally, Gastropods feed on algae or sessile animals that they tear off surfaces with their radula. Many mollusks live fixed on the substrate, moving several centimeters around their place of attachment during low tide to search for their food. This is the case of limpets.

Carnivorous species are usually larger. They feed on echinoderms, bivalves, other gastropods, polychaetes, fish and crustaceans.

There are detritivorous and scavenger gastropods. The Nasarios and the Olivas detect the presence of dead fish through the suspended particles that enter their palatal cavity with the inhalant current, once they identify it they go to the dead organism to devour it.

On the other hand, slugs and many land snails are herbivores, although there are also carnivorous families such as Spiraxidae.

Longevity

The lifespan of snails varies between species. Land snails and slugs from temperate climates live two years; others can live up to 20 years in captivity.

In their natural habitat, Acatinas live between 5 and 7 years, those of the Helix genus approximately 2 to 3, and apple snails (Pomacea) only 1 year. Limpets 5-16 years old. Nudibranchs are the ones that live the least because they don’t even live a year.

 

Habitat and distribution

Sawmills

The rocky coast is the place where many species of mollusks prefer to live despite being an inhospitable place, since it is precisely there where the waves break over the rocks.

In summary, some gastropods are distributed from the intertidal zone to abyssal depths, but there are also swimming and floating species. On the other hand, others have abandoned the salt waters of the sea to adapt to the fresh waters of rivers, lakes, etc.; and those who have conquered the mainland .

Many of the coastal species are able to stay out of the water for hours and in some cases weeks; This helps them cope with desiccation when the tide goes out.

In summary, some gastropods are distributed from the intertidal zone to abyssal depths, but there are also swimming and floating species. On the other hand, others have abandoned the salt waters of the sea to adapt to the fresh waters of rivers, lakes, etc.; and those who have conquered the mainland.

Importance

Throughout the history of the world, men and snails have been closely associated. Since primitive times, man collected oysters, shells and other mollusks for food or to use as ornaments, currency or household implements. Currently, both men and mollusks continue to be linked through their use in jewelry, ornaments, culinary uses, prestigious collections, etc.

Other mollusks are important in the field of medicine since they can be intermediate hosts for many trematodes that are parasites of man and domestic animals. Marine snails also serve as hosts for trematodes parasitic on fish and coastal birds.

In the natural environment, their importance lies in the fact that they are part of the food chain of other animals. Among its main enemies are fish, water birds and small mammals.