Mollusks: From Tiny Snails to Giant Squid!

Amazing Fact: With over 50,000 species, mollusks are the second largest group of invertebrates on Earth! From tiny snails to giant squid, this diverse phylum includes some of the ocean’s most well-known creatures: clams, mussels, octopuses, and everything in between.

What Makes a Mollusk?

Although one might not see an obvious physical relationship between a snail and a squid, they are remarkably similar in construction. In general, mollusks have 3 body regions: a head, a visceral mass, and a “foot.”

  • Head – Contains the sense organs and “brain”
  • Visceral mass – Contains the internal organs
  • Foot – The muscular lower part of the body in contact with the substrate

The Mantle & Shell

Mollusks usually have a shell (although some do not). Mollusks also have an extension of the body wall called the mantle. This portion of the animal’s anatomy is responsible for secreting the shell. The mantle encloses the mantle cavity which contains the ctenidia (gills), anus and excretory pores.

The Radula

Many mollusks have a radula, a tongue of sorts, which is rough like sandpaper and is used to rasp away at food. The radula is made of a hard material called chitin, the same material of which Arthropod exoskeletons are made.

In addition, some Mollusk radulae are impregnated with magnetite to give them superior wear characteristics. Some chiton teeth are so hard they can etch glass!

Body Systems

Mollusks have well developed body organs (nervous system, circulatory system, respiratory system, etc.) but lack body segmentation.

Class Gastropoda: Stomach-Footed

The class Gastropoda (meaning “stomach-foot”) contains about 70% of the Molluscan species—around 35,000! These are the familiar snails, limpets, nudibranchs and abalones.

Queen conch marine snail crawling across the sandy seafloor

Shelled Gastropods

The snails, limpets and abalones have shells. The conch (pronounced “konk”) is a big snail that can weigh more than a pound! Two eyes on stalks peek out from under the shell, and the points on the shell protect it from other animals.

Queen conch marine snail crawling across the sandy seafloor

Shell-Less Gastropods: Nudibranchs

The slugs and nudibranchs do not have shells. The word “nudibranch” means naked gill. These creatures often display brilliant coloration.

Nobody is sure why some nudibranchs are so brightly colored. Some believe that the coloration is aposematic, meaning that it tells other animals “Danger! I don’t taste good!”

Mollusk Classes

Gastropoda

35,000+ species – Snails, slugs, nudibranchs, abalones

Bivalvia

15,000 species – Clams, mussels, oysters, scallops

Cephalopoda

800+ species – Squid, octopuses, cuttlefish, nautilus

Polyplacophora

1,000+ species – Chitons

Mollusk Gallery

Big Gastropod: The conch (pronounced "konk") is a big snail. There are two eyes on stalks peeking out from under the shell. The points on the shell protect it from other animals. This conch weighs more than a pound!

Big Gastropod: The conch (pronounced “konk”) is a big snail. There are two eyes on stalks peeking out from under the shell. The points on the shell protect it from other animals. This conch weighs more than a pound!

Big Gastropod: The conch (pronounced “konk”) is a big snail. There are two eyes on stalks peeking out from under the shell. The points on the shell protect it from other animals. This conch weighs more than a pound!

Colorful Gastropod: This is a nudibranch, basically a snail without a shell. The word "nudibranch" means naked gill. The tuft of yellow on the right side is the animal's gill cluster. Nobody is sure why some nudibranchs are so brightly colored. Some beleive that the coloration is aposematic, meaning that it tells other animals "Danger! I don't taste good!"

Colorful Gastropod: This is a nudibranch, basically a snail without a shell. The word “nudibranch” means naked gill. The tuft of yellow on the right side is the animal’s gill cluster. Nobody is sure why some nudibranchs are so brightly colored. Some beleive that the coloration is aposematic, meaning that it tells other animals “Danger! I don’t taste good!”

Colorful Gastropod: This is a nudibranch, basically a snail without a shell. The word “nudibranch” means naked gill. The tuft of yellow on the right side is the animal’s gill cluster. Nobody is sure why some nudibranchs are so brightly colored. Some beleive that the coloration is aposematic, meaning that it tells other animals “Danger! I don’t taste good!”

A chiton is stuck to a rock. It may not look like much, but getting it off the rock is nearly impossibel!

A chiton is stuck to a rock. It may not look like much, but getting it off the rock is nearly impossibel!

A chiton is stuck to a rock. It may not look like much, but getting it off the rock is nearly impossibel!

This is the scallop that most people never see. This bivalve has hundreds of tiny eyes to keep a lookout for predators (those are the tiny black dots around the opening). The animal feeds by filtering food from the water.

This is the scallop that most people never see. This bivalve has hundreds of tiny eyes to keep a lookout for predators (those are the tiny black dots around the opening). The animal feeds by filtering food from the water.

This is the scallop that most people never see. This bivalve has hundreds of tiny eyes to keep a lookout for predators (those are the tiny black dots around the opening). The animal feeds by filtering food from the water.

This is the Caribbean Reef squid, an animal capable of amazing color changes. Using chromatophores in its skin, the squid can go from white to blue to gold in the blink of an eye. It can jet off at high speed by squirting water though its nozzle, or it can hover in one place with its fins.

This is the Caribbean Reef squid, an animal capable of amazing color changes. Using chromatophores in its skin, the squid can go from white to blue to gold in the blink of an eye. It can jet off at high speed by squirting water though its nozzle, or it can hover in one place with its fins.

This is the Caribbean Reef squid, an animal capable of amazing color changes. Using chromatophores in its skin, the squid can go from white to blue to gold in the blink of an eye. It can jet off at high speed by squirting water though its nozzle, or it can hover in one place with its fins.

The Common Caribbean octopus on the prowl at night, looking for food.

The Common Caribbean octopus on the prowl at night, looking for food.

The Common Caribbean octopus on the prowl at night, looking for food.

Giant Clams live in the tropical Indo-Pacific and reach several hundred pounds in weight. They have symbiotic algae in their mantle tissue that harvests the sun's energy so they need strong sunlight to survive.

Giant Clams live in the tropical Indo-Pacific and reach several hundred pounds in weight. They have symbiotic algae in their mantle tissue that harvests the sun’s energy so they need strong sunlight to survive.

Giant Clams live in the tropical Indo-Pacific and reach several hundred pounds in weight. They have symbiotic algae in their mantle tissue that harvests the sun’s energy so they need strong sunlight to survive.

Class Polyplacophora: The Chitons

Chiton mollusk showing eight overlapping shell plates attached to a rock surface

Members of the class Polyplacophora (meaning “many-plated”) are more commonly called the chitons (pronounced KIE-tuns). These creatures have eight plates and outwardly resemble the “pill-bugs” common under logs and rocks (they are not related, however).

Rock Scrapers

Chitons crawl along rocks and forage for food (mostly algae) using their radulae to scrape it off of the substrate. Magnetite is used for hardening the teeth of the radula in chitons, so they can scrape coralline algae off of rocks. These teeth are so hard that they can etch glass!

These creatures are extremely slow moving. In a year, a chiton may not move more than ten feet. They can detect the presence of light with primitive eyes embedded in the shell plates. Getting a chiton off a rock is nearly impossible!

Class Bivalvia: Two Shells

Close-up of an underwater scallop showing its black eyes along the shell edge

The bivalves (meaning “two-shells”) are perhaps the most well known mollusks simply because of their history as a source of food. Clams, mussels, oysters and scallops are all bivalves. There are about 15,000 known species of bivalve, with about 80% of them being marine (the rest are found in fresh water).

Lifestyle & Feeding

The bivalve may either burrow through the bottom, or attach itself to the substrate with glue-like strings called byssal threads. Some scallops do not attach themselves because they prefer to be able to swim away with a squirt of water forced from the mantle.

Most bivalves feed by filtering organic particles from water, and therefore do not have a radula. The gills are used in feeding by means of a mucous coating which traps food particles as water passes through them.

Scallops have hundreds of tiny eyes (the tiny black dots around the opening) to keep a lookout for predators while they filter feed.

Pearl Production

The shell is generated by the mantle from the inside. Pearls are made by clams, oysters and mussels when a grain of sand or other small irritant becomes painfully stuck in the mantle of the creature.

The bivalve coats the irritant with the same material which is secreted to produce the inner lining of the shell. This makes the irritant smooth, and theoretically, less painful to the bivalve. Although many people think of pearls as coming only from oysters, most bivalves can produce pearls, as well as some snails, like the conch.

Giant Clams

Some of the giant clams found in the Pacific have symbiotic algae in their mantle tissue, and can utilize the light of the sun to make food, although they still filter feed as well. These massive bivalves live in the tropical Indo-Pacific and reach several hundred pounds in weight. They need strong sunlight to survive.

Class Cephalopoda: Head-Footed

Reef octopus resting on coral displaying mottled colours and spread arms

Although we usually think of Mollusks as benthic (bottom dwelling), the cephalopods have taken to a nektonic (swimming), rather than benthic, existence in the ocean.

Squid, octopuses, cuttlefish and nautiluses are all members of the class Cephalopoda, meaning “head-footed.” This term stems from the way a cephalopod’s body is constructed. The “feet” (usually called arms or mistakenly called tentacles) are attached to the part of the body containing the eyes (the “head”) while the rest of the body is out in front of the head.

Shells? What Shells?

The cephalopods may not seem very closely related to the other mollusks, but physiologically, they are similar in internal construction. Perhaps the most obvious difference between most cephalopods and other mollusks is the apparent lack of a shell.

  • Octopuses do not have shells at all
  • Squid have a small chitinized internal shell
  • Nautiluses are the only Cephalopods with an external shell (found in the South Pacific and Indian oceans)

Superior Vision

Cephalopods have the most well developed nervous systems of all mollusks, as well as the most well developed eye. The cephalopod eye is one of the most notable examples of convergent evolution in all of the animal world, because this eye evolved from a completely different direction than the mammalian eye, yet it turned out to function in almost the exact same way. Cephalopods, therefore, have extremely good eyesight.

Hunting & Feeding

This eyesight is well suited for finding prey. The cephalopod then grasps the prey firmly with its arms and eats the prey with a mouth located between the arms. Cephalopods also have a beak, very similar to that of a parrot, used to help bite into prey.

The Blue-Ringed octopus of the South Pacific has a salivary gland which secretes a venom to subdue prey. The venom is so powerful that the bite of this octopus is almost always lethal to a human. Fortunately, it takes a lot of provocation to get an octopus—any octopus—to bite a person.

Arms vs. Tentacles

While the octopus has eight sucker-equipped arms, the squid actually has ten. Eight of the squid arms are of the same length, while the other two are extra long, and are used for helping to grab prey. These two additional arms are called the tentacles.

Caribbean Reef Squid are capable of amazing color changes. Using chromatophores in their skin, the squid can go from white to blue to gold in the blink of an eye. They can jet off at high speed by squirting water through their nozzle, or hover in one place with their fins.

Octopuses or Octopi?

Many people get confused about the proper plural for the word octopus. The word derives from Greek, not Latin. The correct plural is “octopuses,” believe it or not! However, to avoid confusion and arguments, many scientists prefer to use “octopods,” which is the proper plural of the taxonomic order Octopoda, containing the eight-armed cephalopods.

Shared Characteristics

Cephalopods do share many characteristic molluscan traits with the rest of the creatures in the phylum, such as:

  • The presence of a mantle and mantle cavity
  • A radula in the mouth
  • A U-shaped digestive tract (useful for a creature in a conical shell, but not necessary for a Cephalopod)

Mollusk Classes

Gastropoda

35,000+ species – Snails, slugs, nudibranchs, abalones

Bivalvia

15,000 species – Clams, mussels, oysters, scallops

Cephalopoda

800+ species – Squid, octopuses, cuttlefish, nautilus

Polyplacophora

1,000+ species – Chitons

Monoplacophora

~30 species – Deep-sea limpets (living fossils)

Aplacophora

~320 species – Solenogasters, caudofoveates (worm-like mollusks)

Scaphopoda

~600 species – Tusk shells

Mollusk Gallery

Big Gastropod: The conch (pronounced "konk") is a big snail. There are two eyes on stalks peeking out from under the shell. The points on the shell protect it from other animals. This conch weighs more than a pound!

Big Gastropod: The conch (pronounced “konk”) is a big snail. There are two eyes on stalks peeking out from under the shell. The points on the shell protect it from other animals. This conch weighs more than a pound!

Big Gastropod: The conch (pronounced “konk”) is a big snail. There are two eyes on stalks peeking out from under the shell. The points on the shell protect it from other animals. This conch weighs more than a pound!

Colorful Gastropod: This is a nudibranch, basically a snail without a shell. The word "nudibranch" means naked gill. The tuft of yellow on the right side is the animal's gill cluster. Nobody is sure why some nudibranchs are so brightly colored. Some beleive that the coloration is aposematic, meaning that it tells other animals "Danger! I don't taste good!"

Colorful Gastropod: This is a nudibranch, basically a snail without a shell. The word “nudibranch” means naked gill. The tuft of yellow on the right side is the animal’s gill cluster. Nobody is sure why some nudibranchs are so brightly colored. Some beleive that the coloration is aposematic, meaning that it tells other animals “Danger! I don’t taste good!”

Colorful Gastropod: This is a nudibranch, basically a snail without a shell. The word “nudibranch” means naked gill. The tuft of yellow on the right side is the animal’s gill cluster. Nobody is sure why some nudibranchs are so brightly colored. Some beleive that the coloration is aposematic, meaning that it tells other animals “Danger! I don’t taste good!”

A chiton is stuck to a rock. It may not look like much, but getting it off the rock is nearly impossibel!

A chiton is stuck to a rock. It may not look like much, but getting it off the rock is nearly impossibel!

A chiton is stuck to a rock. It may not look like much, but getting it off the rock is nearly impossibel!

This is the scallop that most people never see. This bivalve has hundreds of tiny eyes to keep a lookout for predators (those are the tiny black dots around the opening). The animal feeds by filtering food from the water.

This is the scallop that most people never see. This bivalve has hundreds of tiny eyes to keep a lookout for predators (those are the tiny black dots around the opening). The animal feeds by filtering food from the water.

This is the scallop that most people never see. This bivalve has hundreds of tiny eyes to keep a lookout for predators (those are the tiny black dots around the opening). The animal feeds by filtering food from the water.

This is the Caribbean Reef squid, an animal capable of amazing color changes. Using chromatophores in its skin, the squid can go from white to blue to gold in the blink of an eye. It can jet off at high speed by squirting water though its nozzle, or it can hover in one place with its fins.

This is the Caribbean Reef squid, an animal capable of amazing color changes. Using chromatophores in its skin, the squid can go from white to blue to gold in the blink of an eye. It can jet off at high speed by squirting water though its nozzle, or it can hover in one place with its fins.

This is the Caribbean Reef squid, an animal capable of amazing color changes. Using chromatophores in its skin, the squid can go from white to blue to gold in the blink of an eye. It can jet off at high speed by squirting water though its nozzle, or it can hover in one place with its fins.

The Common Caribbean octopus on the prowl at night, looking for food.

The Common Caribbean octopus on the prowl at night, looking for food.

The Common Caribbean octopus on the prowl at night, looking for food.

Giant Clams live in the tropical Indo-Pacific and reach several hundred pounds in weight. They have symbiotic algae in their mantle tissue that harvests the sun's energy so they need strong sunlight to survive.

Giant Clams live in the tropical Indo-Pacific and reach several hundred pounds in weight. They have symbiotic algae in their mantle tissue that harvests the sun’s energy so they need strong sunlight to survive.

Giant Clams live in the tropical Indo-Pacific and reach several hundred pounds in weight. They have symbiotic algae in their mantle tissue that harvests the sun’s energy so they need strong sunlight to survive.

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