The mollusc phylum is the second largest in the animal world, behind the arthropod phylum. It is estimated that there may be around 100,000 living species of molluscs and 70,000 fossil species have been described.
Molluscs are invertebrate animals with a soft body protected by a calcareous shell. Their respiration is mostly gill, but there are also species of molluscs with lung or cutaneous respiration. They inhabit terrestrial and aquatic areas (marine and freshwater) and some species live in extreme conditions, such as those belonging to the classes of Neomeniomorphs or Chaetodermomorphs, which live in deep waters.
They present a surprising morphological diversity, but despite the variety of their shapes, sizes, life cycles and environments in which they are found, all groups of molluscs present a basic body organization in which the cephalo-visceral region and the foot are distinguished.
The cephalic region contains an organ with a digestive function exclusive to molluscs, the radula. It is a ribbon of curved chitin teeth that acts as a scraping organ during feeding. When removing the radula, the teeth slide over the surface of the food and, when retracting it inward, they dig into the food and carry it towards the mouth.
The visceral mass is protected by the shell, which is secreted by a specific area of the epidermis called the mantle. It is in the mantle where the respiratory and sensory organs are located, as well as the outputs of the excretory, reproductive and digestive systems. The shell usually protects the body of the animal, but it can also be found inside the body or even have disappeared, as in octopuses and sea slugs.
The foot is the muscular organ that can be used for locomotion or be reduced and serve for anchoring to the substrate of the organism, as in the case of bivalves.
The Mollusc phylum is divided into 8 natural groups: Bivalvia, Gastropoda, Neomeniomorpha, Chaetodermomorpha, Polyplacophora, Cephalopoda, Scaphopoda and Monoplacophora:
Bivalves or Pelecypods: There are more than 8,000 described species of bivalves, which make up the second largest group of molluscs. They owe their name to the fact that their shell is made up of 2 articulated valves that protect the animal. They are aquatic and mainly marine. This animals are characterized because their body is laterally compressed and their foot is reduced, since it is not used for locomotion, but is used to dig and hold onto the substrate. Bivalves include both sessile and burrowing species, as well as propulsion swimmers, such as scallops, which propel themselves through the water by jet propulsion: the rapid closing of the valves expels water from the mantle cavity and the animal "swims" in the opposite direction. Oysters, clams, mussels and razor clams belong to this group.
The oldest part of the shell, the umbo, can be recognized as a large hump at the anterior end of the dorsal side of each shell.
Most marine bivalves go through a trochophore stage before developing into a free-swimming veliger larva. This type of larva looks like a miniature bivalve with a row of cilia along the edge of the mantle.
Freshwater species lack these stages. Instead, some go through a larval stage known as the glodychium. Instead of swimming freely, a glodychium attaches to fish or other objects that will not be swept away by the current.
Gastropods: They are the largest group of molluscs. Currently there are 60,000 living species and 15,000 fossil species described, but it is estimated that there may be close to 150,000 living species. They are marine and terrestrial (it is the only type of mollusc that has managed to live outside of water).
These molluscs are characterized by having an elongated body with a large foot at its base that, in general, serves for locomotion by dragging these animals, although in some of them it is modified for swimming or digging. They have a single spiral-shaped shell into which the animal's body can enter. In some species, the shell has been reduced and internalized and others have directly lost this shell throughout evolution, as is the case with slugs. This group includes snails, sea hares, sea and land slugs, and limpets.
Neomeniomorphs and Chaetodermomorphs: Currently there are 390 described species of both groups, but it is possible that many more exist. They are the most unknown groups of molluscs, due to their small size (some measuring only 2 mm) and their usual habitat, as most live in deep ocean waters buried in the sand. These molluscs are characterized by the lack of a shell.
Polyplacophorans: Group of exclusively marine molluscs that has about 1,000 described species. They are characterized by having a shell made up of 8 plates articulated with each other, which gives them a certain capacity for body articulation.
In these animals, the mantle cavity where the gills are located consists of a closed channel that runs longitudinally through the body and communicates with the outside through two anterior orifices (where water enters the cavity) and two posterior ones (where it exits). In this way, the water currents necessary for breathing circulate.
Cephalopods: It is the group of which the most fossil species are known (about 7,500), while only around 600 survive. These animals are characterized because their body has elongated in a dorsoventral direction. The most current species have a reduced shell (such as cuttlefish and squid) or no shell at all (such as octopuses). Only nautiluses have a clearly developed shell. They usually live in areas far from the coast. In cephalopod animals, the mantle is intensely muscular and its contractions allow jets of water that have previously entered the mantle cavity to be expelled, allowing efficient swimming that enables them to be active predators or to escape attacks from other predators. Depending on the species, they have a different number of gills, with some specimens having evolved to completely reduce their gills, so that they breathe through the surface of the body.
Scaphopods: They are exclusively marine and can inhabit sandy bottoms at great depths. Currently there are more than 800 species described and they are characterized by having a shell that looks like a tusk. They are located on the seabed with their heads downwards and with their feet they dig into the sediment, from where they obtain food. They are placed with the widest part of the shell facing the sediment, while the narrower part has a hole that allows water to enter and leave the mantle cavity, along with waste material. Thanks to this current of water, gas exchange occurs through the epidermis, since they lack gills, as well as a heart and vessels for circulation, which is carried out through simple hemolymph sinuses.
Monoplacophorans: This class has the smallest number of living species described, since only 30 have been discovered. These small marine molluscs have existed since the Cambrian and are currently only represented by 2 genera, Vema and Neopilina. The current specimens found have been located in abyssal trenches, between 2,000 and 7,000 meters deep, in the Atlantic, Indian and Pacific oceans. They are symmetrical organisms and have a unique shield-shaped shell below which there is a serial repetition of morphological structures (gills, nephridia, atria, retractor muscles...) along the body.
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