As of late in this series, we have been covering all the animals in the clade Spiralia outside
of the traditional Lophotrochozoa classification.
Now it’s time to delve into Lophotrochozoa, and we’re beginning with the second most
biodiverse animal phylum, the mollusks.
All told, there are at least 85,000 extant species of mollusks.
They are an incredibly diverse group of animals that include the snails, slugs, sea butterflies,
clams, scallops, mussels, squids, oysters, octopuses, nautiluses, argonauts, and cuttlefish.
Some, like the filter-feeding bivalves, are incredibly ecologically important organisms
that as adults, lack brains and are often sessile, or nonmoving.
Others, like the octopuses, are widely accepted to be the most intelligent invertebrates on
the planet, and have complex methods of communication that science is just beginning to decipher.
Mollusks range in size from the relatively simple, nearly microscopic, shell-less solenogastres,
to the several species of giant squid and the singular species of colossal squid, both
of which can grow to sizes over 12 meters, or 39 feet, and weigh as much as 700 kilograms,
or 1,500 pounds.
Other mollusks, like the giant clams, can weight up to 200 kilograms, or 440 pounds,
but these massive creatures are in the minority.
Over 80% of mollusks are less than 10 centimeters long as adults.
Most mollusks are marine.
Only the bivalves, like freshwater mussels, and the gastropods, like snails and slugs,
have moved into freshwater environments.
In fact, snails and slugs are the only terrestrial mollusks, and due to their need for relatively
high humidity, their terrestrial range is a bit limited.
However, like all species of mollusk, they still play crucial roles in the ecosystem.
Not only are mollusks essential to the ecosystem, they’re also essential to the world economy.
In fact, mollusk aquaculture, where the animals, almost always bivalves, are raised under controlled
conditions throughout part or all their lifecycle, kind of like aquatic farming, was a $441 million
dollar industry in the United States alone in 2018, and mollusks caught in the wild accounted
for more than $1 billion dollars in 2019, though the sustainability of this practice
has been called into question.
Suffice to say, mollusks are important, and most of us have probably interacted with them
before.
In addition to an occasional terrestrial gastropod, if you’ve ever gone shell collecting, you’ve
seen a mollusk shell.
Nearly all of the shells that wash up on the beach come from once living mollusks.
To be more specific, every sea shell you’ve ever held is actually the corpse of a mollusk.
You see, mollusks can’t just leave their shells when they get too big.
Unlike the arthropods, such as bugs and crustaceans, they can’t “shed” their shells, they
continue to grow with them their entire lives.
Their shells are literally part of their bodies, secreted from the mantle.
The only way they are separated from their shells is in death.
However, not all mollusks have shells.
In fact, the very name of their phylum “Mollusca” was originally derived from the Latin “mollis”
meaning “soft” as a reference to their bodies, which are generally divided between
the muscular “head-foot” portion used for locomotion, feeding, neurological control
and environmental sensing; and the protected “visceral mass” portion which houses the
majority of the other organs and organ systems.
Protecting the visceral mass is the mantle, which is comprised of two folds of skin that
are outgrowths of the dorsal wall.
It is the outermost layer of the mantle that is responsible for secreting the characteristic
mollusk shell.
The head/foot and visceral mass are easy to tell apart on a snail, but they are sometimes
confused on other mollusks, like the cephalopods.
For example, take a look at this picture of the common octopus, scientific name octopus
vulgaris.
The large area behind the eyes, which is actually the visceral mass, kind of looks like it houses
a huge brain.
But that area actually houses the majority of the octopus’s organs, somewhat like the
human torso.
The “head” is actually right here, and the mouth isn’t up here, it’s on the octopus’s
ventral end, down here.
Let’s now go over general mollusk anatomy and physiology in eight sections.
These will be as follows: One, the head foot region.
Two, the radula.
Three, foot adaptations.
Four, visceral mass.
Five, shell composition.
Six, reproduction.
Seven, general characteristics.
And eight, molluscan cladistics.
First let’s look a bit closer at the head-foot region.
Like the common octopus, many, but not all mollusks, have well-developed head regions
which contain their mouths and other specialized structures like tentacles and photoreceptors.
Some mollusks, like the scallops, have up to 200 eyes that dilate around their shells
and contract.
Others like the chitons, have hundreds of tiny eyes less than 1 millimeter across called
ocelli on each shell plate, complete with retina and lenses.
Still others, like the snails and slugs, have vesicular or “bubble eyes” that sit atop
their eyestalks.
And then there’s the pinhole eye of the nautilus, which contains a water filled cavity,
and the famously complex “camera-type” cephalopod eye that remarkably resembles a
vertebrate eye.
Though, unlike the vertebrate eye, a cephalopod eye is focused through movement, much like
the lens of a camera or telescope, rather than changing shape as the lens in the vertebrate
eye does.
Moving on, in addition to complex photoreceptors, there is a unique organ known as a radula
that is a rasping, protractible, tongue-like organ found in all mollusks except the bivalves.
Snails use it to scrape up food particles as they glide along a surface, and cephalopods
use it to grind off pieces of their prey.
In the cephalopods, the radula is located within the buccal bulb behind the sharp beak.
Mollusk radula are covered in up to 250,000 teeth that rasp food, and carry it like a
conveyor belt in a continuous stream towards the digestive tract.
Since the radula is often in use, it wears away anteriorly and is continually replaced
by new secretions posteriorly.
Each species of mollusk has a different pattern, morphology, and number of teeth on their radula.
Some, like the venomous predatory cone snails, have modified radula that function as a hypodermic
needle for capturing prey.
In addition to highly modified radula, mollusks also show a wide range of foot adaptations
that are usually adapted for locomotion or attachment.
There are the attachment discs of the limpets, the laterally compressed “hatchet foot”
of bivalves, and the siphon for jet propulsion in the cephalopods.
In snails the foot is controlled through body hydrology, that is, engorgement with blood.
Moving on, let’s take a closer look at the visceral mass.
First, let’s look at the mantle and mantle cavity, then the shell, and finally the internal
structure and function.
The mollusk mantle is a sheath of skin, which extends from the visceral mass and hangs down
each side of the body, protecting the animal’s soft parts.
This forms the mantle cavity that houses the respiratory organs, which aids in gas exchange,
and protects the digestive, excretory, and reproductive systems that all empty into the
mantle cavity.
The mollusk shell, when present, is made up of three layers.
First there is the periostracum, the organic outer layer.
This is resistant to damage and provides most shell coloration.
Then there is the prismatic, middle layer which is made of densely packed prisms of
calcium carbonate.
And finally the continually excreted nacreous, inner layer, made of mother of pearl in some
mollusks.
In terms of reproduction, most mollusks are dioecious, though some are hermaphroditic.
Some species, like the cuttlefish and leopard slugs, have elaborate courtship rituals while
others, like many bivalves simply release large quantities of sperm and egg into the
water column.
With these details covered let’s quickly summarize the most common characteristics
of phylum Mollusca.
One, mollusks have bodies separated into two parts, the head-foot and the visceral mass.
Two, mollusks have mantle cavities that function in excretion, gas exchange, and reproduction.
Three, mollusks are triploblastic, bilaterally symmetrical coelomates with reduced coeloms.
Squid and octopuses are sometimes referred to as “radially symmetrical,” but they’re
not.
They’re bilateral, you can only separate them with one line of symmetry, just as with
all mollusks.
Four, many mollusks have a free swimming trochophore larva stage that emerges from their eggs.
Five, mollusks have an open circulatory system, except for the cephalopods.
And six, mollusks have radula, except for the bivalves.
Finally, let’s briefly look at molluscan cladistics.
Mollusks are generally split into eight classes.
The shell-less Caudofoveatans and Solenogastres; the eight or seven-plated Polyplacophorans
known as “Chitons”; the single shelled Monoplacophorans; the land, sea, and freshwater
dwelling Gastropods; the marine “two-shelled” Bivalves; the tusk shells of class Scaphopoda;
and finally the astoundingly intelligent and mobile Cephalopods.
Let’s now move forward and start working our way through these classes so we can get
a better sense of the incredibly diversity in this phylum.