The mollusks They are a phylum of animals that are characterized by presenting a soft body protected by a kind of shell. This is the reason for its name, since it comes from the Latin word mollis, what does soft mean.
Mollusks are a group of animals that have managed to remain on the planet for a long time, since, according to the first fossil records collected, they date from the Paleozoic era, specifically the Cambrian period..
They were described and systematized for the first time by Carlos Linnaeus, a renowned Swedish naturalist in 1758. At present they constitute the second most abundant group of animals, with almost 100,000 described species..
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The taxonomic classification of mollusks is as follows:
-Domain: Eukarya
-Animalia Kingdom
-Subkingdom: Eumetazoa
-Phylum: Mollusca
Mollusks are classified as multicellular eukaryotic organisms, since they are made up of cells whose genetic material is arranged within the cell nucleus, conforming to chromosomes..
Likewise, the cells that make them up, during their embryonic development process undergo a differentiation process, through which they specialize in various functions. That is why they are multicellular (many types of cells).
They are also triblastic animals, because they present the three germ layers: ectoderm, mesoderm and endoderm. They are also protostome.
They present an internal cavity known as coelom, thanks to which they are part of coelomed animals and have bilateral symmetry, since they are made up of two equal halves, divided by an imaginary line drawn by the longitudinal axis of the animal..
These are ubiquitous animals, that is, they can be found in practically all ecosystems on the planet, except the most arid ones such as deserts.
From a reproductive point of view, most species are dioecious, that is, they have separate sexes. However, there are some exceptions, such as certain gastropods that are hermaphrodites..
They reproduce only and exclusively in a sexual way, by internal or external fertilization, they are oviparous (reproduction by means of eggs) and most have indirect development, except for cephalopods that have direct development..
The main characteristic of mollusks is a soft body, which is divided into head, foot and visceral mass. In addition, most mollusks are protected by a shell that is secreted by the mantle..
It is generally very well developed. It has the mouth orifice, which, in some species, is surrounded by some extensions called arms and tentacles. The head is also the site of the sensory organs, such as the eyes, which in some groups, such as the cephalopods, are quite developed.
This is the part of the body in which the different organic systems that make up the animal are contained. In addition, it has a kind of cover that goes from the visceral mass until it falls on both sides of the body..
The space between the mantle and the visceral mass is known as the paleal cavity. The mantle has the function of secreting the shell of the animal.
It is a characteristic element of mollusks. It is made up mainly of muscle tissue and its function is related to the locomotion and movement of the animal. In some mollusks, the function of the foot has been modified and is responsible for keeping the animal fixed to the substrate, among other things.
It is a rigid and resistant structure that is secreted by the mantle. Not all mollusks have a shell. This is made up of three layers: the periosteal, which is the outermost; the intermediate layer, known as the prismatic layer, composed of calcium carbonate; and the pearly layer, which is the innermost, which is in permanent contact with the mantle.
The digestive system of mollusks is complete, with an entrance hole (mouth) and an exit hole (anus). Inside the oral cavity there is an organ that is exclusive to mollusks: the radula. This is elongated and presents on its surface a series of small structures with a chitinous texture similar to teeth..
After the oral cavity is the esophagus and immediately after the stomach, which is where most of the digestion takes place. Then there is the intestine, the place where the absorption of nutrients takes place and finally the anal opening.
It is important to note that the digestive system has attached glands, whose ducts lead to the stomach. Their function is the production of substances that contribute to the process of food degradation..
The nervous system of mollusks varies according to the level of complexity of the species. For example, cephalopods (which are the most complex) develop a cluster of ganglia at the level of the head that function as a brain. From this fibers emerge towards the rest of the body.
In the case of the simplest mollusks, the nervous system is represented by nerve fibers that surround the esophagus, from which the different nerves that innervate all the structures of the body are released..
The respiratory system depends on the habitat in which the mollusks develop. In most of them, which are those that inhabit aquatic environments, respiration is of the gill type. The gills are located in the paleal cavity. In the case of terrestrial gastropods, they have managed to develop lungs to be able to breathe.
The excretory system is represented by a pair of metanephridiums, which have two ends, one communicates with the coelom and the other end opens towards the paleal cavity by the nephridiopores.
Most mollusks, with the exception of cephalopods, develop an open circulatory system. They have a heart that is divided into three chambers: two atria and one ventricle. The circulating fluid is the hemolymph.
The phylum Mollusca includes a total of 11 classes, of which 2 are extinct.
This class corresponds to snails. They are generally small in size, but there are also exceptionally large ones. The main characteristic of the members of this class is that the visceral mass undergoes a torsion process in which it rotates on the head and the foot. This occurs during embryonic development..
In addition, most gastropods develop shells of different morphologies, some very showy and colorful..
Gastropods comprise two subclasses: Eogastropoda, which is made up of so-called limpets, and Orthogastropoda, which includes the rest. The latter are the so-called "true snails".
Bivalves are represented by oysters, mussels and clams, among others. Its main characteristic is the presence of two flat shells or shells that are generally held together thanks to some ligaments or also by means of hinges..
These are purely aquatic, found mainly in shallow waters, although there are a few species that live at great depths..
This class is made up of five subclasses:
- Anomalodesmata: with a single order (pholadomyoida)
- Heterodonta: which includes six orders, of which only two are currently present (Myoida and Veneroida)
- Palaeoheterodonta: composed of two orders (trigonoid and unionoida)
- Protobranchia: with one extinct order (precardioid) and two current ones (nuculoida and solemyoida).
- Pteriomorphia: made up of four current orders (arcoida, mytilloida, ostreoida and pterioida).
This class of mollusks corresponds mainly to the so-called chitones. Its distinctive element is a shell formed by the union of eight plates superimposed one on top of the other. Hence its name. They have an oval shape.
Its visceral mass is covered by the shell only on its dorsal surface, while the ventral surface remains exposed. Here they present the muscular foot that allows them to move through the substrate.
Polylacophores are made up of two subclasses:
- Paleoloricata: extinct
- Neoloricata: integrated in turn by two orders (lepidopleurida and chitonida).
They are a little known class of mollusks that, unlike most of these, lack a shell. They also do not have a muscular foot because they do not need to move on the substrate, since they are animals that instead of moving on it, dig into it..
Their body has an elongated shape, similar to a worm and, despite not having a shell, they have a chitin covering that provides protection.
This class is made up of only one order, Chaetodermatida, which is made up of three families: prochaetodermatidae, chaetodermatidae and limifossoridae..
It corresponds to a class of mollusks that do not have a shell. There are species belonging to this class that do not live freely, but are associated with some cnidarians such as anemones. They are quite simple and primitive organisms.
This is a broad and diverse class of mollusks that includes squid, cuttlefish, and octopus. Cephalopods are animals that do not have an external shell, although some have one inside the mantle.
Its body is made up of the visceral mass that in some, like squid, is very long; a head of a smaller dimension, from which some extensions known as arms and tentacles emerge. In most species, these possess suckers.
The organs of sight are very well developed, being here the largest eye in the animal kingdom, in the squid..
Cephalopods are made up of three subclasses: nautiloid (totally extinct), ammonoid (extinct) and coleoid. The latter in turn comprises two cohorts: belemnoidea (extinct) and neocoleoidea that encompasses six current orders (sepiida, teuthida, sepiolida, octopoda, spirulida and vampyromorphida)..
These animals are very similar to the caudofoveados. They do not have a shell and are small in size, barely reaching a few centimeters in length. Its body is slim and has an elongated shape.
They develop some calcareous spicules on their surface and are exclusive to marine habitats. Some species lack the characteristic radula of mollusks.
This order is made up of two superorders: aplotegmentary, with two orders (neomeniamorpha and pholidoskepia); and pachytegmenaria, which includes two orders (sterrofustia and cavibelonia).
Scaphopods are very peculiar animals that are mainly buried in the substrate, with a small portion of their body sticking out. Its appearance is similar to that of an elephant's tusks, since the shell that covers them is whitish, elongated and of small diameter..
At its cephalic end, which is inside the substrate, it has extensions called captacles, with which they perceive possible food particles.
This class is made up of two orders: gadilida and dentallida.
This is a class of mollusks of which only a single order remains today, the Monoplacophorida. They have a shell that is shaped like a saucer or disk, which protects its dorsal surface. They are marine animals that are mostly found at great depths.
It covers a single current order: monoplacophorida.
It was an extinct class of mollusks. The collected fossils have made it possible to determine that their visceral mass experienced a torsion similar to that of gastropods, in addition to being extremely small, reaching just a few millimeters in size..
Through the collected records, four orders in this class have been identified: onichochiliformes, pelagialliformes, khairkhaniiformes and helcionelliformes.
This is a class that is also extinct. Externally, it closely resembled bivalves, due to its shell and according to the records, they were sessile organisms, so they did not experience any displacement through the substrate. They could measure up to 10 cm in length.
Mollusks are animals that are widely distributed throughout the world. In all geographic regions there are.
However, they are more abundant towards the area near the tropics, where temperatures are warmer. In cold areas, near the poles, they are not very abundant, being mostly represented by members of the cephalopoda class..
Although many believe that they are exclusively aquatic, this is not the case, since in the group of gastropods there are species that inhabit terrestrial environments.
However, for mollusks to be found in a specific habitat, it must meet an essential characteristic: having a high level of humidity..
Mollusks require humid environments to stay hydrated and to carry out their vital functions satisfactorily..
There are some, such as cephalopods that are only found in marine habitats. Within these it is possible to find them in coastal areas, as well as at great depths.
Likewise, other mollusks such as scaphopods remain buried in the substrate, also in marine environments. Some are attached to certain substrates such as rocks. Such is the case of polyplacophores.
In the case of bivalves, these are mainly found in coastal regions. Gastropods are practically the only mollusks that can be found in habitats of terrestrial types such as forests or grasslands, although they still require a large amount of humidity to be optimally maintained..
Mollusks are animals that reproduce exclusively sexually. This means that it involves the fusion of male and female sex cells (gametes), through a process of fertilization, which can be internal or external..
Sexual reproduction is very valuable from an evolutionary point of view, since it contemplates genetic variability and this in turn is responsible for the survival of species on the planet, thanks to the fact that they are capable of adapting to the different changes that it experiences. the surrounding environment.
Importantly, most mollusks reproduce through a structure known as a spermatophore. This is secreted by the males and contains the sperm. Sometimes the male introduces it directly into the female or releases it to the environment for her to do so.
Mating rites are a type of behavior widespread in the animal kingdom. It involves a series of actions through which some individual (generally males) tries to attract the attention of their potential partner in order to start the reproduction process..
In this sense, within the mollusks there are several groups that have very particular mating rituals. In the case of cephalopods, there are rituals that involve a great display of skill in swimming, as well as the occasional fight between several males for the attention of the females..
On the other hand, gastropods (snails) have one of the most curious mating rituals ever observed. This is a slow process that can take more than 10 hours..
It begins with a small approach between two snail specimens, which are slowly palpating and caressing, even some specialists have described that there are species that bite their genital pores.
Finally, when they are ready for fertilization to occur, the snails shoot the so-called “love darts”. These are nothing more than dart-like structures that are made up of calcium. Its function is to hold the snails together.
These are just some of the courtship and mating rites that can occur in the phylum of mollusks.
Fertilization is defined as the process by which gametes unite or fuse to give rise to an embryo. In the case of mollusks, the two types of fertilization that exist can be observed: external and internal.
Now, in species that present a type of external fertilization, the gametes are expelled or released to the outside, generally through the gonopores. Once in the water, the eggs and sperm must meet.
Specialists believe that this encounter occurs mediated by a chemotaxis process that involves the secretion and uptake of chemical substances through the signaling and functioning of receptors located on cell membranes. Once they join, the fusion occurs and therefore the fertilization and formation of the embryo.
On the contrary, in the case of species that have internal fertilization, the copulation process must necessarily occur. Some have copulatory organs, such as cephalopods. In these, one of its arms is modified (hectocotyl) to carry out fertilization inside the body of the female..
After fertilization occurs, the embryo is formed, which develops inside an egg. Taking this into account, it is then affirmed that mollusks are oviparous organisms.
Most mollusks develop heterolecyte-type eggs. These have abundant yolk (nutritive substance), which is unevenly distributed throughout the cytoplasm, especially in the vegetative pole. Cephalopods are an exception, since the type of egg they present is telolecito. These contain a lot of yolk and this occupies almost all the internal space of the egg.
The type of segmentation that mollusk embryos undergo is uneven holoblastic. In this, what happens is that the blastomeres do not have the same dimensions, but there are small so-called micromeres.
Later it undergoes the gastrulation process and finally a trochophore-type larva is formed. This is why mollusks have an indirect development, with the exception of cephalopods and land snails..
When the egg hatches, the trochophore larva emerges from them. This is small in size and in some cases has a characteristic belt of cilia. Eventually this larva undergoes another transformation process and becomes another type of larva, the veligera larva..
The larva internally presents the different organs that make up the adult animal systems, as well as a shell. Later it descends to the substrate and acquires the characteristics of the adult individual.
All mollusks are heterotrophic organisms. This means that they do not have the ability to synthesize their own nutrients, so they must feed on other living beings or substances made by others..
Mollusks have different ways of feeding. There are carnivores, herbivores, filter feeders and browsers.
In the case of carnivores, such as cephalopods, they feed on other animals such as some fish, marine arthropods and even other mollusks. Other mollusks that are carnivorous are cones (a type of gastropod).
There are also herbivorous mollusks, which are those that feed on algae and plants. These include slugs and land snails..
On the other hand, filter feeders are, for the most part, those that have little mobility, so they cannot move around to look for food. Because of this, they must filter it directly from the water stream. These include bivalves such as mussels and clams..
The browsers are those that, with the help of the radula, scrape from the surface of some substrates such as rocks, the remains of algae or organic matter that are adhered there. The group of mollusks that present this type of feeding are the polyplacophores, as well as some gastropods.
Once the food is ingested, in the oral cavity it is subjected to the action of the secretion of the salivary glands and becomes a mass of mucous consistency known as the prostate..
Later it passes to the esophagus and from there to the stomach. In this it is subjected to digestive enzymes that degrade it so that later, at the level of the intestine, the absorption of nutrients occurs. The compounds that are not absorbed are released to the outside through the anal orifice.
It is a species of terrestrial gastropod. It has a shell that is generally whitish in color, furrowed by dark brown lines. Because it inhabits terrestrial habitats, its breathing mechanism is based on lungs. It is found only on the European continent.
It belongs to the class Polyplacofora. It is found only on the coast of the Pacific Ocean in Mexico. Its characteristic element is its shell made up of 8 plates superimposed on each other. That shell is dark, brown or black in color.
This is not a proper species. They constitute the genus Architeuthis. It is the largest invertebrate on the planet known so far. They are usually located in cold waters such as those of the Arctic Ocean and at great depth. Due to this they have been very little studied.
Better known as the blue-ringed octopus. Its main feature is a series of bright blue rings that are distributed throughout the body. It can measure up to approximately 10 cm and synthesizes a neurotoxin-type venom that is deadly, even for humans..
It is a bivalve that belongs to the Ostreidae family. Its distinctive element is a dark-colored shell that can measure just over 15 cm. Its habitat is the Atlantic Ocean, being particularly abundant on the coasts of the Gulf of Mexico.
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