Animals in the phylum Mollusca, also known as molluscs or mollusks, are some of the most diverse invertebrates in the animal kingdom. There there are approximately 85,000 species of molluscs recognized today. There are seven taxonomic classes or seven main types of molluscs.
However two of these classes; Aplacophora and Monoplacophora are very cryptic and mysterious, meaning there isn’t much information on them! To keep things simple, we will cover five out the seven groups.
What is a mollusc?
While the different classes of molluscs are all very different, there are three main characteristics shared amongst nearly all species in this group. Some molluscs you may be familiar with include animals like slugs, snails, clams and even octopuses.
These characteristics include:
- The presence of a mantle or head, where there is a relatively large cavity which is important for respiration and excretion.
- Next, nearly all molluscs have a radula which is almost like a specialized tongue that is equipped with rows of small teeth that are used for scraping and drilling during feeding.
- Finally, this group of invertebrates have a nervous system that consists of two nerve cords and a neural ring that surrounds the gut or stomach, meaning that food must pass through their brain!
Most molluscs are found in marine environments, however there are many species that are found in freshwater and even on land.
The 5 main types of molluscs
1. Gastropods
Number of species: ~70,000
The gastropods are the largest group of molluscs and can be found in saltwater, freshwater and terrestrial environments. Most people are familiar with gastropods, as this group includes both slugs and snails. In fact, you likely have a few gastropods in your backyard! Snails and slugs may even be in your garden playing the role of decomposers.
As most people know, snails have a hard, protective shell while slugs lack this structure. However, both have a fairly obvious head with 2-4 sensory tentacles or eye stalks. Additionally, animals in this group have a muscular foot that they use to get from point A to point B. Although, their “feet” do not look like a typical foot and are located on the underside of their body.
Most of the important organs in the body of a gastropod are asymmetrical, and are typically skewed to the left side of the body.
2. Bivalves
Number of species: ~20,000
Several species of bivalves are common menu items at most seafood restaurants. If you have tried clams, oysters, scallops, or mussels then you have in fact eaten a bivalve!
Bivalves are different from other types of molluscs for a number of reasons. However, the biggest differing characteristic is the lack of a head and a radula. These invertebrates have two shells that are joined together by a strong but flexible ligament that allows them to open and close their shell. They tend to have bilateral symmetry, with both shells being near if not totally identical.
Most shells have a shiny finish on the inside with a more subtle appearance on the outside. Bivalves can be found in salt and fresh water where they bury themselves in the sand and sediment or anchor themselves to rocks or other structures.
3. Chitons
Number of species: ~1,000
Chitons are small marine molluscs that can greatly differ in appearance from species to species. These invertebrates create a strong suction to rocks or other hard surfaces and can be nearly impossible to remove. This makes them well suited to live in rocky-intertidal zones where they are exposed to changing tides and waves.
One thing all chitons have in common is a shell made up of eight segments or shell valves. These segments are hard and provide protection, but also allow for them to move easily.
Additionally, their segmented shells allow for them to crawl up into a ball- almost like an armadillo! This tactic protects them when/if they become detached from a rock.
4. Cephalopods
Number of species: ~900
While you may not be familiar with the term cephalopod, you are likely familiar with the types of animals that fall into this group. Cephalopods include octopuses, squid, cuttlefish and nautiluses.
Most invertebrates have a reputation of being simple, unintelligent beings but this is not the case for many species of cephalopod. In fact, octopuses are incredibly smart and actually have multiple brains.
Animals in this group have bilateral symmetry and often have large heads, sometimes referred to as mantles. Additionally, they have tentacles or arms that they use for getting around, manipulating objects, and grabbing their prey. Another interesting trait of cephalopods is their ability to expel a dark liquid, also known as ink when threatened as way to create a distraction in order to escape.
5. Scaphopods
Number of species: ~500
Scaphopods, more commonly known as tusk shells are interesting looking molluscs and get their common name from their tusk-like appearance. They are somewhat mysterious and spend their lives deep down in sandy substrates.
Scaphopods are strictly marine molluscs and are therefore only found in saltwater. They feed on microscopic, single-celled organisms and occasionally marine vegetation. These mollusks have the largest radula relative to body size of any other mollusc.
12 Examples of Molluscs
1. Common periwinkle (Littorina littorea)
Type: Gastropo
Common periwinkles are small snails that can be found in the rocky intertidal zone of the Atlantic Ocean. These gastropods are grazers and will spend much of their time slowly passing over algae covered rocks munching away. These snails are also sold as seafood snacks in coastal pubs and restaurants.
2. Blue-ringed octopus (Hapalochlaena spp.)
Type: Cephalopo
Small but deadly, the Blue-ringed octopus is a highly venomous species of octopus that can actually be considered as one of the most dangerous animals in the world. These cephalopods are native to the tropical temperatures of coral reefs found in the Pacific Ocean.
3. Lined chiton (Tonicella lineata)
Type: Chito
The Lined chiton is a colorful species of chiton that is found in the North Pacific. They are typically found anchored to rocks, however they have also been found on the undersides of vessels. They feed on algae and their biggest predators are Sea stars.
4. Straight tusk shell (Rhabdus rectius)
Type: Scaphopo
The Straight tusk shell can be found along the coast of central California. They prefer shallow silt or soft sand to burrow in, and will even eat sediment. This species, along with other types of tusk shells have been used by indigenous people to make jewelry.
(See diagram about for example of tusk shell)
5. Chambered nautilus (Nautilus pompilius)
Type: Cephalopo
Chambered nautiluses are an incredible species of mollusc. They have chambers within their spiraled shells that they use to adjust their buoyancy, which allows them to quickly descend to evade predators. They are capable of diving down to depths of 2000 feet! These cephalopods can be found in the South Pacific.
6. Sea hare (Aplysia spp.)
Type: Gastropo
Sea hares are not hares (rabbits) at all but a type of gastropod! They likely get their name from their large eye stalks that could resemble bunny ears. These strange marine, almost sea-cucumber looking molluscs can also expel clouds of ink to dissuade predators. There are many different species, however they tend to be found in shallow, tropical waters.
7. Cuttlefish (Order Sepiida)
Type: Cephalopo
There are numerous species of cuttlefish found in warm, coastal waters. Cuttlefish are seemingly a mix between octopuses and squids, and are actually considered as some of the most intelligent invertebrates in the world! They are strictly carnivorous and eat other types of molluscs (even other cuttlefish!), fish, worms and crustaceans.
8. Banana slug (Ariolimax spp.)
Type: Gastropo
Unsurprisingly, Banana slugs are often bright yellow and resemble that of a banana. They are terrestrial molluscs and are common throughout the west coast of the United States and up into British Columbia, Canada and even as far north as Alaska. They specialize in decomposing, making them great to have in your yard to break down dead leaves and other vegetation.
9. Pacific Geoduck (Panopea generosa)
Type: Bivalv
Certainly the strangest looking mollusc on this list, the Pacific geoduck (pronounced gooey-duck) is the largest species of burrowing clam. This species is also capable of living incredible lifespans and may grow to be as old as 140-168 years! Geoducks are native to the coast of the Pacific Northwest and western Canada.
10. Pearl oysters (Pinctada spp.)
Type: Bivalv
In addition to being popular at seafood restaurants, oysters are also well-known for their ability to produce pearls. There are several species of Pearl oysters, however only a handful are harvested for their pearls. This group of oysters tends to do well in warmer, tropical waters throughout the world. It can take up to two years for an oyster to grow a pearl!
11. Horse conch (Triplofusus papillosus)
Type: Gastropo
The Horse conch is an impressive species of snail given that it is one of the largest gastropods in the world. They have shells that can be nearly two feet in length! Unlike many other species of snail, they are strictly carnivorous and feed on other gastropods. Horse conchs are native to the coastal regions of the Atlantic from South Carolina down to Mexico.
12. Giant clam (Tridacna spp.)
Type: Bivalv
Unsurprisingly, Giant clams are very large clams. They can grow to be over 4 feet and weigh hundreds of pounds! Historically, Giant clams have been feared by divers and sailors as a “killer clam”, however they are harmless and eat microscopic organisms and algae through filter feeding. They can be found on coral reefs in the Indo-Pacific.
