Mollusks: What are they and how many types are there?

The molluscs They constitute one of the most fascinating and varied groups in the animal kingdom. With approximately 100.000 living species, these invertebrates are found in a wide range of environments, from the ocean depths to terrestrial ecosystems.

What are mollusks?

The molluscs They are a diverse and extensive group of Invertebrate animals belonging to the edge MolluscaThey are characterized by having a soft body and, in many cases, a hard shell, which may be present or absent.

Mollusks inhabit a variety of environments, from oceans to terrestrial regions, and exhibit diverse feeding, reproductive, and body structures. They are known for their ecological, economic, and evolutionary biological importance.

Characteristics and anatomy of mollusks

The anatomy of molluscs is divided into three main parts: the head, the muscular foot and the visceral mass. The foot, a key structure in the mobility of these animals, shows a remarkable diversity, adapting to various functions such as anchoring, digging or even transforming into tentacles in the case of cephalopod mollusks.

Among the characteristics Of these living beings, the following stand out:

Soft body: Mollusks have a soft, muscular body that is usually protected by an external or internal shell, although some lack one.
Shell: In many mollusks, the presence of a shell is a distinctive feature. It may be an external shell, like that of a snail, or a Ostra, or an internal shell, as in the case of squid.
bilateral symmetry: Most mollusks display bilateral symmetry, meaning their body can be divided into two approximately equal halves along a central axis.
Muscular foot: They have a muscular foot that they use for locomotion, allowing them to crawl, swim, or burrow, depending on the species.
Mantle: Mollusks have a mantle, a membrane that covers their body and, in some cases, secretes the shell.
Food scraper: Many mollusks have an organ called a radula, which they use to scrape and collect food from surfaces.
Diversity: This phylum includes a wide variety of animals, from land snails to squid and marine octopuses. They are found in a variety of habitats, both on land and in water.

Types of Mollusks

It is estimated that there are around 85,000 species of molluscs around the world, making them one of the largest phyla of the animal kingdomThese species are grouped into various classes and orders.

Next, we will show the most common types of Mollusks:

Class Cephalopoda (Cephalopods)

The cephalopods They are a group of very ancient mollusks with around 800 living species and approximately 7,500 known fossil species. These organisms are typically pelagic, meaning they they live in open waters, although many have adapted a benthic lifestyle, that is, associated with the seabed.

Within the class Cephalopoda, found various species, including:

Squid: These cephalopods have an elongated body and a mouth surrounded by tentacles. Some squid species have small, transparent internal shells.
Octopus: Octopuses also have elongated bodies and tentacles, but lack a shell. They are known for their intelligence and ability to change color.
Cuttlefish: Cuttlefish are similar to squid in appearance, with elongated bodies and tentacles, but their inner shell is larger and more visible. They can also easily change color to adapt to their surroundings.
Nautiluses: Nautiluses are the most primitive cephalopods still in existence. They have a spiral-shaped external shell.

cephalopods They use the movement of water in its mantle cavity both to move around to carry out gas exchange. Some species have evolved to reduce their gills or even eliminate them completely, depending on breathing through the surface of your body.

In addition, cephalopods have a more complex circulatory system than other mollusks, with a closed system of vessels lined with endothelium. Their blood contains hemocyanin, which is responsible for transporting oxygen. These mollusks They also have a highly developed nervous system, with a brain that coordinates movement and advanced sensory organs, such as highly evolved eyes.

Class Bivalvia

The bivalve They are a group of mollusks characterized by having two shells that are joined by a “hinge”This class includes a wide variety of species that typically live in aquatic environments, such as oceans, rivers, and lakes. Some examples of bivalves include:

Clam: These bivalves generally have thinner, oval shells. They live buried in sand or marine mud and are known for their edible meat.
Mussel: Mussels are bivalves with teardrop-shaped shells that often form colonies on rocky surfaces. They are also prized as food in many cultures.
Oysters: Oysters are bivalves with rough, bumpy shells. They are famous for their delicate flavor and are farmed on oyster farms.
Scallops and scallops: Scallops are bivalves with fan-shaped shells. They have a large, meaty muscle that is used in gourmet cooking.
CocklesThey are characterized by their heart-shaped shells. Inside its protective shell, the animal has a soft body that includes vital organs such as gills, which it uses for both breathing and feeding.

Bivalves are known for their filter-feeding lifestyle, where they pump water through its gills to absorb particles of foods suspended. This process is also essential for obtaining oxygen.

Class Gastropoda, gastropods or univalves

The gastropods are largest class of mollusks and come in a wide variety of shapes and sizes. Snails, slugs, and snail shells are common examples of this class. Some key characteristics of gastropods include:

Spiral shells: Many gastropods have spiral shells that grow in a coiled or coiled manner.
Muscular body: They have a muscular body that crawls on the substrate, allowing them to move slowly.
Well developed head: Gastropods have a head with eyes and sensory tentacles.
Radular organ: They have an organ called a radula, which allows them to scrape food from surfaces.

This class of mollusks inhabits a wide variety of habitats, from terrestrial to marine environments. Some species are herbivorous, while others are carnivorous or detritivorous.

Class Polyplacophora

Polyplacophorans, or "parasols", are mollusks characterized by having shells formed by eight superimposed hard platesThey often attach themselves to rocks in intertidal habitats. Some key aspects of polyplacophorans include:

Benthic life form: They are found on the seabed, attached to rocky substrates.
Algae scrapers: They feed by scraping algae and other materials from rock surfaces with their radula.
Slow movement: Although they are able to move, they do so slowly and are not active swimmers.

examples of this class of mollusc are: Lepidochitona cinerea, Acanthopleura granulata and Mopalia ciliata

Class Scaphopoda

Scaphopods are tube-shaped mollusks that live buried in marine sand. Some characteristics of this class include:

Tubular shells: They have elongated, conical, tube-shaped shells, resembling small fangs.
Habitat in the sand: They live buried in sandy or muddy substrates in shallow marine waters.
Filter feeding: They capture food particles suspended in the water by extending a capturing tentacle.

examples of this class mollusk are: Dentalium octangulatum, Antalis vulgaris, Entalina tetragona

Class Monoplacophora

Monoplacophorans are little-known molluscs that were believed to be extinct until they were discovered in deep waters in the 20th century. Some characteristics of this class include:

Disc-shaped shells: They have flat, disc-shaped shells.
Deep water habitat: They are found in deep waters and usually live on cold, dark seabeds.
Little studied: Due to their rarity and inaccessible habitat, monoplacophorans are the subject of limited study. They are considered living fossils.

examples of this class Molluscs are: Neopilina galatheae, Laevipilina antarctica and Laevipilina hyalina

Reproduction and Conservation

The reproduction of mollusks varies significantly between species. Some, such as bivalve mollusks, are unisexual, while others, like many snails, are hermaphroditic and can even self-fertilize. Unfortunately, one-fifth of terrestrial mollusk species are threatened with extinction, highlighting the importance of sustainable practices in fishing and seafood trade. You can see how we do it. here

List of the 15 most common mollusks

Squid: Cephalopod with long tentacles, used in various culinary preparations, such as the popular Roman-style squid.
Octopus: Cephalopod prized in Mediterranean and Asian cuisine, known for its unique texture.
Cuttlefish: Cephalopod similar to squid, used in Mediterranean dishes, such as “grilled cuttlefish.”
Mussel: Bivalve widely consumed in gastronomy, especially in seafood dishes.
Ostra: Bivalve consumed raw or cooked, considered a delicacy in seafood gastronomy.
Clam: Bivalve commonly used in soups, stews and seafood dishes.
Viera: Bivalve with a fan-shaped shell, prized in haute cuisine.
Zamburiñas: Very similar to Scallops, so much so that it is important to know the Differences between Zamburiña and Scallops
Sea snail: Marine gastropod mollusk that is consumed in some regions, such as Spain.
Limpet: Marine mollusk consumed in some coastal regions, especially in Portugal.
Nautilus: Cephalopod with a spiral external shell that lives in deep waters.
Aloe: Terrestrial gastropod often found in gardens and feeding on plants.
Hush: Marine mollusc belonging to the class Polyplacophora, with shells formed by eight superimposed plates.
Sea snail: A shelled marine gastropod sometimes found in aquariums.
Garden snail: Land snails that are often found in gardens and are prized as pets in some places.