Youth Science Lessons – Invertebrates – Lesson 2 Mollusk

Based on diversity and species richness, at 80,000 species (maybe more) mollusk are one of the most successful animal phyla on the planet. These creatures are bilateral/coelomates meaning they have a distinct head region and a coelomic cavity. Having a head allows for the presence of a brain with a variety of senses to stimulate it. One of the most intelligent animals on the planet, the octopus, is a mollusk. Coelomic cavity allow for internal organs, such as a heart, gills, kidneys, and others – this allows for increase function and size. The largest invertebrate on the planet is the giant squid (60 feet or more), and they are mollusks as well.

The mollusk shell design is one of the most successful in the animal kingdom.
Photo: Flickr

What sets mollusk apart from other invertebrate phyla is the presence of a calcium carbonate (CaCO3) shell. There coiled and colorful forms are beautiful but also very useful in defense against predators. They can produce these because of a thin layer of skin called the mantle. The genetics of each species dictates what the shell will look like and how large they will grow.


The anatomy of a mollusk.
Photo: FWC




The majority of the mollusk belong to a class called Gastropoda. It is estimated that there are at least 60,000 species (75% of the phylum). These are the snails and slugs. Snails produce a single shell that coils around a stem called the columella. It whorls either in a right, or left, direction leaving a large opening called the aperture. The snail lives within literally attached to their shell. They will extend a large fleshy foot from the aperture to “slug” their way across the environment in search of food. Their head will also extend having eyes, antenna, and other receptive organs to stimulate a pretty advanced nervous system providing information to a simple brain. A tube called a siphon is also extended to draw water into the mantle cavity where the gills are oxygenated.

This crown conch displays its siphon as it crawls slowly across the sand.
Photo: Franklin County Extension.

They have a mouth that possess a tooth like structure called a radula. In some snails the radula is modified into a tongue like structure with numerous rasping teeth embedded on it. With this they scrape algae off of rocks and shells, even the glass of your aquarium as a source of food. Some will graze on large algae almost like a caterpillar. In others the radula is more like a drill bit and used to drill into other shell covered mollusk and thus are predators. Some further will use their radula like a venomous dart and kill resting fish and consume those.


The shape, size, and color of this class of mollusk is amazing. Collecting seashells has been a popular hobby for centuries. Some, such as the horse conch (Triplofusus papillosus) can reach over a foot long. Others, such as the olive nerite (Nertina reclivata) are extremenly tiny, about the size of a small pearl. Some have elongated shells with sharp pointed apexes. Others are spherical balls, like small jewels.


Some species have males and females, others are hermaphrodites. Most lay their eggs in an egg case which they deposit in the environment. Many of these egg cases are beautiful geometric designs in themselves.


There are slugs in this class. Slugs differ from snails in that their shells are excreted within the body instead of outside. This would seem unusual in that it would not afford any protection at all. But it does allow for easier movement not having to carry that large heavy shell around. They defend themselves by producing a poison within the skin and, for many, brightly colored bodies to alert potential predators that they are in fact poisonous. Some sea slugs will extend their gills to the environment externally (instead of protected in the mantle cavity) for more efficient oxygenation. They are exposed to predation here but, again, they are poisonous. Those that do this are called nudibranchs (“naked gills”) and are very popular in the aquarium trade.


Another class of mollusk takes a different approach. Instead of having a single coiled shell with an aperture to extend their body out of, they are COMPLETELY covered in shell – no opening at all. To do this they secrete two shells, joined at a point called an umbo. The two shells extend posteriorly and flare wider. They are connected by a muscle called the adductor muscle which the mollusk uses to open and close the shell. These are known as bivalves.

Coquina clams are common burrowing bivalves on local beaches.
Photo: Flickr

The Class Bivalvia consist of four major groups: oysters, clams, mussels, and scallops. They are all bottom dwellers (benthic) and feed on plankton. They have two siphons which they extend out when the valves are partially opened. One draws water in to provide food and oxygen. The other expels water releasing carbon dioxide and waste. Sometimes they will draw in sand while feeding/breathing. Most of the time the sand grain is expelled. But once in a while the sand grain becomes lodged inside. Irritating to the bivalve, they will secrete the nacre (the shiny beautiful layer of shell) over the sand grain forming pearls. Some of the largest pearls come from the Giant Pacific Clam (Tridacna gigas) but are ugly and worthless to the jewelry trade. The valuable ones come from the Pacific Pearl Oyster (Pinctada sp.).


Two groups of bivalves cannot move at all, they are sessile. These would include the oysters and mussels. Oysters cement themselves to rocks, pilings, boats, crab traps, but most often to themselves. Several will cement together into what is called an oyster clump and oyster clumps will form oyster reefs. These oyster reefs are like coral reefs, providing habitat for an abundance of marine life. Oysters can filter as much as 50 gallons of seawater a day while feeding, but this also helps keep the water clarity good so that seagrasses and other species benefit. They are also a favorite food item for humans. Oystermen harvest them using large tongs and sell them by the bushel.


Mussels on the other hand attached to the substrate using numerous stringy byssal threads. They will use these to latch onto plants, rocks, and oysters. Mussels too are a popular seafood item.


Clams are more mobile, though they are not really good at it. They typically bury in the sediment, extending their siphons to feed and breath. When feeding is not good, or it is time to breed, they will extend their long fleshy foot by slowly opening the two valves using their adductor muscle. The large foot will “push” against the substrate flipping them along to new hunting grounds – or whatever they are looking for. Clams are the most diverse group of bivalves and their shells, like those of the gastropods, are popular in the seashell trade. They are also popular in the seafood markets as well. When eating clams, you are eating the large fleshy foot.


The most mobile of the group are the scallops. These bivalves are closely associated with seagrass beds. There they filter feed in the grasses, observing the environment with their ice blue eyes. Any sign of trouble will stimulate them to “swim”. To do this, they quickly expand and contract their adductor muscle making the two valves “clap” together and lift them off the bottom “swimming”. They continue to do this while “swimming” until they reach a safer place. Scallops, like the other bivalves, are a popular seafood item with humans.

A young mollusk – the veliger.
Photo: University of California Irvine

As you might guess, reproduction in bivalves is external and mass spawning is the rule. Certain environmental factors stimulate different species. Males and females will fill the water column with gametes in hope that some will meet and fertilize each other. Fertilized gametes become swimming microscopic plankton called veligers. These move with the tides and currents, swimming with their cilia, searching for new places to live and grow. When they do find such a place they will settle out as small bivalves we call spat. The spat grow into adults and the process begins again.


One last class of mollusk we will talk about are the Cephalopods. These are truly amazing mollusk. At first you would not recognize them as such because the three of the four types have no external shell, but they are mollusk. They do have a mantle and they can secrete a CaCO3 shell.

The elongated, shell-less body of a squid does not look like your typical mollusk.
Photo: California Sea Grant

For one of them, the Nautilus, the shell is external. It is single and coiled but differs from gastropods in that (a) it is chambered inside, and (b) the animal who made it (the Nautilus) is a squid-like animal. Cephalopods or soft bodied animals whose giant foot is divided into arms and tentacles. They feed not by using a radula or filter feeding with their siphon but by grabbing their prey with their arms and tentacles pulling it to their mouth where a parrot-like beak crushes it, sometimes (as in octopus) injecting a venom. These are big time predators feeding on things like crustaceans and fish. And, with the lack of a heavy external shell, can swim very fast and easily chase down prey.

The chambered nautilus.
Photo: NOAA

The Nautilus is an ancient animal who has been plowing the seas since time before the dinosaurs, about 500 million years. Today they are found in the Indo-Pacific region and at depths of about 2000 feet. They do come near the sunlit waters to breed but rarely seen by humans in the wild. Their shells are famous. As mentioned before, they differ from snail shells in that they are chambered (hence their common name the chambered nautilus) and the squid-like animal lives in the largest/last chamber near the opening. Their eyes and 90 tentacles extend out into the environment – never being pulled in like snails. There is a tube connecting the animal to the inner chambers called a siphuncle. This siphuncle is used to exchange gas inside the chambers allowing the animal float up and down in the water column like a hot air balloon. Something snails cannot do. One species of Indo-Pacific octopus produces a paper like shell that resembles a nautilus shell in which it deposits its eggs and carries them around for protection. This octopus has the genus name Argonaut and it was because of this that the University of West Florida chose to use the nautilus shell as one of their logos. The Greek looking warrior mascot comes from the story Jason and the Argonauts.


The other three cephalopods do not have external shells and are very good swimmers. The cuttlefish is squid-looking creature that is shaped more like a football than a long skinny squid. They are found in tropical and temperature shallow waters around the globe but are not found in North or South America. They are extremely intelligent and are amazing with the variety of color patterns they can produce. Sometimes called the “ocean chameleon” the patterns and colors they can produce is second to none and has been the focus of a lot of research. They produce these amazing patterns and colors using specialized cells called chromatophores. These cells are filled with pigment and by contracting and relaxing connecting muscles in patterns that the cuttlefish controls, they can make themselves appear like moving waves or a piece of coral. The patterns and colors are controlled by the animal and the speed with which they can blend, and change shows how sophisticated their nervous system really is. They also have iridocytes, which are cells that can enhance the shine, or flash of the colors. They are truly amazing to watch.


The internal shell of a cuttlefish looks like a chalky surfboard and is called a cuttlebone. It provides support for the animal (much like a backbone) and is popular in the pet trade for bird cages. Many tropical birds need something to file their beaks on and a cuttlebone works very well. Speaking of beaks, the cuttlefish has one and they are predators of fish and crustaceans.

The basic body of the squid.
Photo: NOAA

Squid are more familiar to us. We have heard stories of giant squid that attack ships and eat pirates (the Kraken) but actually – they do exist. The giant squid (Architeuthis) can reach lengths of 60 feet – maybe larger. The largest ever recorded was 43 feet (and weighed over a ton) but many believe they may reach greater lengths because of sucker marks on sperm whales. Sperm whales actively hunt the giant squid and when encountered the squid will wrap its long tentacles around the snout of the whale to keep from being eaten. Sometimes this works, sometimes it does not. But the diameter of sucker marks on the snouts of sperm whales, compared to squid with recorded lengths, suggest they may get larger than 40 feet. These creatures terrified ancient sailors because there were stories of them grabbing ships. Though it does not happen very often, it has. Some scientists think that giant squid near the surface during the evenings may confuse the hull of a ship with a surfaced whale and go on the attack. In actual encounters it appears the squid realize the mistake and disappear quickly – but probably still shakes the nerves of sailors who were on watch and saw this – leading to the great sailor yarns (stories) back at the pub on shore.


Most squid are smaller, but their size still has a wide range. Bay squid (Lolliguncula) are small, less than a foot, and are often used as bait. They are quite common in shrimp trawls and I have captured them in Bayou Texar. The longfin squid (Loligo) is more about a foot long and common in the Gulf. These are often used for bill fishing. The Humboldt squid (Dosidiscus) is a larger deep-water squid found off the coast of California that appears near the surface at night hunting fish. They can reach six feet long. There are deep ocean squid, bioluminescent squid, and the “Vampire Squid” (Vampyroteuthis) of the deep sea, which can invert itself in defense.


Squid, like cuttlefish, are very fast swimming predators. They too lack an external shell but instead of a chalk surfboard internal one, theirs is more like a piece of plastic and is called a pen. They have large eyes, eight arms, but also two long extending tentacles that can explode to grab prey who think they are at a safe distance – again – amazing to watch. They have chromatophores and can change colors and shapes, and – as mentioned – may can produce bioluminescent lights in the deep ocean. They too are a popular mollusk in the seafood markets. They are sold as calamari.

Image courtesy of Lewis and Clark Legacy 2001

The last cephalopod/mollusk is one of the most amazing – the octopus. This cephalopod lacks all signs of a shell – internally and externally. It is just a big wad of gelatinous flesh that can squeeze through the smallest holes. Octopus are modest in size, when compared to squid, but there are some large ones. The Giant Pacific Octopus (Enteroctopus dofleini) can reach an arm span of 20 feet and weigh over 100 pounds. Most are much smaller. The common species found here in the Gulf is Octopus vulgaris who reach a size of an arm spam of about 4 feet and weigh up to 20 pounds.


These are amazingly smart animals. They hide in selected dens during the daylight hours and hunt crustaceans at night. In aquarium they will feed on fish and once, at a west coast aquarium, a giant octopus placed in a shark tank began eating the sharks! It is thought that in the ocean they are not capable of catching them, but in an aquarium – all bets are off. Their ability to change colors, patterns, and shapes equals the other cephalopods. Studies are ongoing on how they use their colors to express moods – basically communicate to each other – and maybe to us. In the program My Octopus Teacher, and the book, Soul of an Octopus many references to the attempts of octopus trying to communicate with colors are mentioned. Most octopus’ species do not live very long – one, maybe two years. At the end of their lives, they will breed. The smaller males will provide a tube of sperm called a spermatophore to the female, who will in turn fertilize her eggs. Once the eggs are produced (usually attached to the wall of her den) she will use her siphon to blow water over them keeping them oxygenated. Once the small octopus hatch – she is done and dies (the male dying shortly after he is finished as well).


Numerous studies have been done over the years testing the intelligence of these animals. Can they navigate a maze? Can they recognize themselves in a mirror? Can they teach other octopus how to solve problems? In most cases passing with flying colors. Studies continue and amazement about them does as well.


All mollusk are amazing and marine biologists can spend several lifetimes studying their habits, colors, and behaviors and still not get enough of them.



  1. If you can visit the beach collect seashells. Separate them into gastropods and bivalves. Which were more common on your walk? Do you have an idea why? Do you think if you were to repeat the walk you would find the same situation?
  2. Similarly, use a shell identification book and identify what type of shells you have. Which is the most common? Do you have an idea why?
  3. Visit a pet store, public aquarium, or shell store. Explore the diversity and variety of different mollusk/shells you find. Do you have a favorite? If so, go home and read about them. Learn where they live, what they eat, and how big they get.
  4. Mollusk do live on land (at least snails and slugs – gastropods – do). It is cold this time of year, but maybe by looking under rocks, pots, and logs, you may find some. If you do, are they all the same kind? Do they have a favorite hiding place? What do you think they eat?
  5. Watch a video of cuttlefish changing color online. I think you will be amazed.
  6. If you can find it, watch the film My Octopus Teacher. I think you will be amazed.
  7. Find videos of the Giant squid online. I think you will be amazed.
  8. Find videos of squid feeding. Yep, I think you will be amazed at how quickly they can extend their long tentacles and grab the prey.
  9. You can visit a local bait and tackle shop and find small bay squid for bait. IF YOU WANT – there are guides online on how to dissect squid. IF YOU WANT you can conduct a squid dissection at home. I know dissections are not for everyone.
  10. There is a good book by Sy Montgomery entitled The Soul of an Octopus. It is an interesting story of her encounters with octopus at a public aquarium. You might enjoy it.

Posted: February 11, 2021

Category: Coasts & Marine, Natural Resources
Tags: Florida Sea Grant, Invertebrates, Mollusk, Youth Science Lessons

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