Types of Mollusks - Characteristics And Examples

Mollusks are invertebrates with a soft integumentary system similar to annelids. Some have a protective shell, while others do not. Mollusks are the second-largest group of invertebrate animals after arthropods. There are roughly 100,000 species of mollusks, of which 60,000 are gastropods. The estimated number of fossil species is around 30,000.

The majority of mollusks include benthic sea mollusks, meaning they live on seabeds. Many others are terrestrial, like snails. The great diversity within the phylum Mollusca has led them to colonize a multitude of different habitats. Consequently, their feeding habits depend largely on the species and their environment. For instance, some mollusks have developed specialized feeding structures to adapt to unique ecological niches.

For more, we recommend reading our article where we discuss the different types of snails, including marine and terrestrial.

Before we delve into the different types of mollusk characteristics and classification of mollusks, let’s consider the spelling of the word itself. Should it be types of molluscs or types of mollusks? Well, it depends, but both are correct.

When we define animals of the phylum Mollusca, there are two different spellings. Mollusk is generally preferred in American English, while mollusc is more commonly used in British English. This distinction is similar to other English language variations, such as the difference between 'color' and 'colour'.

Mollusks are an incredibly diverse group of invertebrate animals, and finding common characteristics among all of them can be slightly tricky. Therefore, here we present some of the most common characteristics among mollusks, but bear in mind, there are some exceptions:

A mollusk body is divided into four main areas:

• Mantle: refers to the dorsal surface of the body that can secrete protection. This protection has a chitinous and protein origin that subsequently creates calcareous deposits, spicules, or a shell. Animals that do not have a shell generally have chemical defenses to deter predators.
• Locomotor foot: is ciliated, muscular, and includes mucous glands. This 'foot' carries several pairs of dorsoventral muscles that serve to retract the foot and attach it to the mantle. The foot's structure can vary significantly among different mollusk classes, adapting to functions like crawling or burrowing.
• Cephalic zone: includes the brain, mouth, and other sensory organs. This area is often highly developed in predatory mollusks, such as cephalopods, which possess advanced nervous systems and acute sensory capabilities.
• Paleal cavity: where the osphradium (olfactory organs), body orifices (anus), and gills are located. In aquatic mollusks, the gills play a crucial role in respiration and filter feeding.

Mollusk digestive system some characteristic features:

• Stomach: they follow extracellular digestion. This means that the digestible particles are selected by the digestive gland (hepatopancreas), and the rest pass into the intestine and form stool.
• Radula: is an organ located inside a mollusk’s mouth, this membrane is formed in the shape of a tooth, supported by the odontophore (mass of cartilaginous consistency) and moved by complex musculature. The chitinous teeth that the radula presents tear food. Old and worn-out teeth fall out, and new ones form in the radular sac. Many solenogastres (mollusk subclass) lack a radula. Additionally, bivalves, a class of marine and freshwater mollusks, have no radula.

When it comes to a mollusk's circulatory system, only the heart and the nearest organs have vessels. A mollusk heart is divided into two atria and a ventricle. They carry metanephrids which collaborate with the ultrafiltration heart, producing primary urine that reabsorbs the nephrids, also regulating the amount of water in the body. A mollusk’s reproductive system has two gonads in front of the pericardium. Gametes are evacuated to the paleal cavity, mostly attached to the nephrids. Mollusks can be classified as dioecious or hermaphroditic, with some species capable of changing sex during their lifespan.

For more, we recommend reading our article where we list and discuss the major groups of invertebrate animals.

Mollusk classification is divided into eight classes, all of them living. Mollusk classification includes:

• Class Caudofoveata: includes mollusks shaped like worms. They have no shell, but their body is covered with calcareous and aragonitic spicules. They live buried in the ground with their heads down, often in deep-sea environments.
• Class Solenogastres: are very similar to the previous class, so much so that they were previously included in the same group. They also carry a worm shape, but instead of living buried in the ground, they move freely in the ocean and feed on cnidaria. They also have calcareous and aragonitic spicules.
• Class Monoplacophora: includes very primitive mollusks. Their body is covered by a single shell, resembling clams, but they have a muscular foot like snails. These mollusks are considered living fossils as they retain many ancestral traits.
• Class Polyplacophora: at first glance, this group of mollusks is similar to a type of crustacean. Its body is covered by a set of plates reinforced with magnetite. They also have a crawling muscular foot and a radula. Polyplacophorans are commonly found in intertidal zones, clinging to rocks.
• Class Scaphopoda: these mollusks have a very elongated body like their shell, which is shaped like a horn. These are also known as fang shells. This is one of the best-known types of marine mollusks, often found in sandy or muddy substrates.
• Class Bivalve: are aquatic mollusks whose body is enclosed between two leaflets or shells. These two leaflets can close thanks to set muscles and ligaments. The most well-known types of bivalve mollusks include clams, mussels, or oysters. Bivalves are essential filter feeders in aquatic ecosystems, playing a vital role in maintaining water quality.
• Class Gastropoda: gastropods include snails and slugs, both terrestrial and marine species. They have a well-differentiated cephalic area, a muscular foot that serves to crawl and/or swim, as well as a shell on the back. This shell may be absent in some species. Gastropods are the most diverse class of mollusks, with members found in nearly every habitat.
• Class Cephalopoda: this class is made up of octopuses, cuttlefish, squid, and nautilus, and, surprisingly, all carry a shell. The most obvious is that of the nautilus, whose shell is external. Sepias and squid have an internal shell that varies in size. The octopus shell is almost vestigial, with only two thin calcareous strands present inside the body. Another important characteristic of cephalopods is that their muscular foot (present in mollusks) are tentacles. Cephalopods can carry between 8 to more than 90 tentacles, depending on the species. They are known for their intelligence, complex behaviors, and ability to change color.

Mollusks without a shell

Three types of mollusks without a shell include banana slugs, squid, and octopi. These mollusks often rely on other means of protection, such as camouflage, speed, or toxic secretions.

For more, you may be interested in reading our article where we list the most common types of seashells.

Now that we’ve covered Mollusca characteristics and classification of mollusks, let’s take a look at both marine mollusks and terrestrial mollusk types. These examples illustrate the vast diversity found within this phylum, showcasing the wide range of adaptations that mollusks have developed to thrive in various environments.

This species of glisten worm is a mollusk without a shell. This type of mollusk belongs to the class Caudofoveata. This marine mollusk species is tropically distributed in the Pacific Ocean. It can be found at depths of 50 meters to more than 1800 meters. These depths provide a stable environment with little competition from other organisms.

The Neomenia carinata is another vermiform mollusk without a shell but belongs to the Solenogastres family. This mollusk type can be found in a depth range of between 10 and 565 meters and lives freely in the Atlantic Ocean, on the coasts of Portugal. Their presence in these waters is often indicative of the health of the marine ecosystem.

The sea cockroach is a kind of endemic Polyplacophora marine mollusk from Mexico. It lives on the rocky substrate in intertidal zones. It is a large species, able to reach 7.5 centimeters in length. The chiton's ability to cling to rocks with its muscular foot allows it to withstand strong waves and currents.

Image: https://www.researchgate.net

It is a species of Scaphopoda marine mollusk with a tubular shell or fang-shaped shell. This white mollusk lives in sandy and muddy substrates at shallow depths in intertidal zones. They can be found along the Atlantic and Mediterranean coasts. These mollusks play an important role in the ecosystem by aerating the sediment as they burrow.

Image: http://www.aphotomarine.com

This Bivalve species, also referred to as the abrupt wedge shell or wedge clam, is a small marine mollusk that lives on the Atlantic and Mediterranean coasts. They are also considered edible mollusks and are greatly appreciated in the local culinary culture. Wedge clams can live in the subtidal areas of about 20 meters deep. Their presence is often an indicator of sandy beach health.

Oysters are one of the types of Bivalve marine mollusks of the order Ostreoida. This species can measure up to 11 centimeters and produce pearls. Oysters are distributed from Norway to Morocco and the Mediterranean. In addition, they are cultivated in aquaculture and are a popular edible mollusk. Their role in filtering water makes them crucial to maintaining healthy marine environments.

The common snail is a species of Gastropod terrestrial mollusk that follows pulmonary respiration. This means that they have no gills and live on the earth’s surface. Garden snails require a lot of moisture, and if they do not receive it, they hide in their shells to avoid drying out. These snails are often found in gardens and agricultural areas, where they can be both beneficial and pests.

For more, read about animals with exoskeleton.

The common octopus is a Cephalopod marine mollusk that lives in the Atlantic Ocean and the Mediterranean Sea. They can measure up to about a meter in length and can change color thanks to chromatophores in their body. Octopuses are also a common edible mollusk, popular in gastronomy. Their intelligence and problem-solving skills are subjects of scientific research.

Looking for more? Here are some additional mollusk types:

• Scutopus robustus
• Scutopus ventrolineatus
• Laevipilina cachuchensis
• Laevipilina Rolani
• Tonicella lineata
• West Indian fuzzy chiton (Acanthopleura granulata)
• Ditrupa arietina
• Freshwater pearl mussel (Margaritifera margaritifera)
• Cockscomb pearl mussel (Cristaria plicata)
• Iberus gualtieranus alonensis
• Iberus gualtieranus gualtieranus
• Giant African snail (Achatina fulica)
• Common cuttlefish (Sepia officinalis)
• Giant squid (Architeuthis dux)
• Giant Pacific octopus (Enteroctopus dofleini)
• Palau Nautilus (Nautilus belauensis)

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