Phylum Mollusca, known as mollusks, are a group of invertebrate animals that include slugs, snails, mussels, octopus, bivalves, gastropods, etc. This phylum is attributed to 85,000 species with tens of thousands of extinct Mollusca species.
Mollusks is one of the most fascinating and diverse animal groups on our planet. Although most people would recognize mollusks, it is also true that they may not know all their characteristics. If you would like to know more are Phylum Mollusca (mollusks) and see some examples, you have come to the right place.
What Are the General Characteristics of Phylum Mollusca?
In the first place, what characterizes Phylum Mollusca is their habitat. Most mollusks live in the sea. But that is not the case with all of them. Some of them live in freshwater and, others just in damp soil.
Some freshwater mollusks live in streams, rivers, springs, canals, and even in underground streams in caves. These are known as lotic mollusks because they live in flowing water. Others live in still water, such as ditches, ponds, and lakes; they are known as lentic mollusks.
The main kinds of freshwater mollusks are snails (gastropods), and freshwater clams and mussels (bivalves).
But most mollusks have the sea as their natural habitat.
“My parents would drive us to Florida every spring in this big old, rusy Suburban, and we’d collect stuff on the beach for our aquarium back in Ohio; we had this big saltwater aquarium back in Ohio. Every time we found anything, any molluskm my mom would bring out the guidebook and quiz us on what it was, so that stuff was built early.” — Anthony Doerr
Mollusks are also characterized by having bodies that are un-segmented and have a distinct head, a visceral hump, and a muscular foot. There are some exceptions, such as the mollusk known as Neopilina, who has a segmented body, unlike most other mollusks.
Another important characteristic of mollusks is that they have a specific symmetry: bilateral. Only in some rare cases, such as the Pila, are mollusks asymmetrical when they reach adulthood.
Their shells that are secreted by mantle are another of the salient characteristics of mollusks. Their material is calcium carbonate. Although most mollusks have external shells, some of them such as the cuttlefish, the squid, or the slug have internal shells. Some mollusks, notably the octopus, do not have shells at all.
The three main body parts of a mollusk are as follows:
- Head-foot: this is the body part that is in charge of both sensing and locomoting.
- Visceral mass: this is the body part that contains all the organ systems.
- Mantle: this is the body part that covers the whole visceral mass and it is also what secretes the shell for those mollusks that have one.
Most mollusks have a fourth body part: the radula, which is covered in teeth and they use for feeding.
How Many Kinds of Mollusks Are There?
Examples of animals in the phylum Mollusca. Images licensed under CC0.
There are many kinds of mollusks but the four main classes are the class Bivalvia, the class Polyplacophora, the class Gastropoda, and the class Cephalopoda.
“Ocean acidification is often referred to as osteoporosis of the oceans because as acidity rises, shell building creatures such as lobster, oyster, crab, shrimp and coral are unable to extract the calcium carbonate from the water that they need to build their shells and are thus unable to survive.” — Philippe Cousteau, Jr.
Let us now look at each and every class in more detail.
The class Bivalvia is characterized by having a hinged shell. Its shell has two halves: one to the right and one to the left. Both parts of the shell cover the mollusk’s visceral mass.
But this class of mollusks also has other characteristics. For example, they have feet that go beyond their shells. They use their feet for locomotion purposes. But one thing that this kind of mollusk does not have is a radula, so they have no teeth.
Examples of Bivalvia mollusks include clams, mussels, and oysters.
The second class I want to discuss is the one known as Polyplacophora, sometimes also referred to as chitons. What characterizes these mollusks is that their bodies have an elliptical shape and are covered with a shell. Their shells have eight different plates.
The third class of mollusks is the class Gastropoda. Its main characteristic is that its visceral mass is covered by a shape with a spiral shape.
These mollusks have a distinct head and, usually, at least one pair of tentacles (sometimes two pairs). Their foot is characterized by being long and flat.
The main examples of this kind of mollusks include slugs and snails.
Finally, the fourth class of mollusks is the one known as Cephalopoda. The main feature that distinguishes these mollusks from the rest is that they have a rather prominent head in which they host a set of complex eyes. Also, the Cephalopoda mollusks have eight, nine, ten or even more tentacles all of them surrounding the mouth.
“We split from our common ancestor with the octopus half a billion years ago. And yet, you can make friends with an octopus.” — Sy Montgomery
The Cephalopoda mollusks have shells but in some of them, the shell can be external, whereas others have internal shells.
Important examples of Cephalopoda mollusks include the octopus and the squid.
How Important Are Phylum Mollusca?
Mollusks are more present in marine environments much more than in any other kind. This is so much so that specialists estimate that as much as 23 percent of all the sea species on the planet are Phylum Mollusca (mollusks).
But Phylum Mollusca is hugely diverse. Specialists have been able to describe as many as 75,000 different species. It is also likely that the actual number of species on the planet is larger than that. This makes them one of the most diverse phylum of animals in the world.
How did mollusks get their name? Well, the word “Mollusca” from which the English world mollusk is derived means “soft body.” Phylum Mollusca got their name because the first species that were observed and classified by scientists did not possess shells. But since those very early observations, we now know that many of them actually have shells.
We saw some of the main classes of mollusks earlier but the reality is that there are many more classes and that every class can also be divided into many other subclasses. The world of Phylum Mollusca is complex in part because they live in many different environments, although mostly marine environments but, also, as we saw earlier also in freshwater and other environments.
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