Did you know that snails are related to squid?
Coming up!
Jonathan explores the weird world of molluscs!
Welcome to Jonathan Bird’s Blue World!
There are around 85,000 known species of Molluscs, a very diverse phylum of animals.
Among the Molluscs are some of the most well-known sea creatures, like snails, clams, mussels,
squid, and octopods.
Although one might not see an obvious physical relationship between a snail and a cuttlefish,
they are remarkably similar in their internal construction, clearly sharing a common ancestor
hundreds of millions of years ago.
In general, molluscs have 3 body regions: a head, a visceral mass, and a "foot."
The head contains the sense organs and the "brain," while the visceral mass (basically
the body) contains the internal organs.
The "foot" is the muscular lower part of the body in contact with the substrate.
Most, but not all molluscs, have a shell.
Molluscs also have an extension of the body wall called the mantle.
This portion of the animal's anatomy is responsible for secreting the shell in a manner similar
to the way human fingersnails grow.
Many molluscs have a kind of tongue called a radula, 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.
About 70% of the molluscs are contained within the class Gastropoda, meaning "stomach-foot".
These are the familiar snails, limpets, and nudibranchs, known for crawling on their stomachs.
There are also a small number of land Gastropods such as some snails, and slugs.
The snails, limpets and abalones have a shell, while the slugs and nudibranchs do not.
The Waved Whelk, common in the north Atlantic, is a perfect example of a classic gastropod,
crawling along on a foot with primitive eyes on stalks, and a proboscis to smell for food.
The hard shell is a nearly indestructible portable home that makes the whelk, and most
other snails, pretty difficult for predators to eat.
Some snails have taken the shell to a higher level.
The Murex snail’s shell has added defenses.
Most snails are scavengers, just cruising around looking for dead stuff to eat.
But some snails, like the so-called cone shells, have become predators, hunting their food
with venomous harpoons.
The beautiful shells of many gastropods have made them targets for collectors, so it may
be hard to believe but even snails can be overfished and endangered.
A nudibranch has no shell for protection, but instead has fleshy projections on its
back called cerata that contain stinging cells stolen from the local hydroid population.
The cerata also significantly increase the animal’s surface area, thereby functioning
as gills to absorb oxygen from the water.
The term nudibranch, meaning “naked gills,” is a reference to the cerata.
Other nudibranchs actually have a tuft of naked gills that don’t sting.
For protection, these species secret a toxic slime that makes them unappealing.
Looking at a scallop, it’s hard to imagine how it’s possible for this animal to be
related to a snail.
But scallops, oysters, mussels and clams are all bivalves, another class of molluscs.
The name bivalve means "two-shells" and these animals live exclusively in water—about
80% of them in seawater.
But the zebra mussel is a rather infamous freshwater bivalve, known for its invasiveness
in U.S. waterways and lakes.
The bivalve may either burrow through the bottom, or attach itself to the substrate
with glue-like strings called "byssal threads."
Some bivalves, like scallops do not attach themselves because they prefer to be able
to swim away with a squirt of water forced from the mantle, for example to escape from
attacking sea stars.
Most bivalves feed by filtering organic particles from water.
Some of the giant clams found in the Pacific have symbiotic algae in their skin tissue,
and can utilize the light of the sun to make food though photosynthesis, although they
still filter feed as well.
If you think clams don’t look much like their cousins the snails, this one is going
to blow your mind.
Cephalopods are also molluscs!
The class cephalopoda means “head-foot” because the head is connected to the feet,
technically called arms.
The body is out in front.
If people were built like this, we would look a lot different.
“Hey what’s up?
My head is where my butt should be!”
Squid, octopods, cuttlefish and nautiluses are all cephalopods.
Perhaps the most obvious difference between most cephalopods and other molluscs is the
apparent lack of a shell.
The octopods don’t have shells at all, and the squid have only a tiny internal shell.
Nautiluses--the most primitive of the group--are the only Cephalopods with an external shell.
Even though cephalopods don’t look anything like bivalves or gastropods, they have very
similar internal construction, which makes it clear to biologists that they have common
ancestors with the gastropods and bivalves.
Hence they’re molluscs.
Cephalopods have the most well-developed nervous systems of all molluscs, as well as the most
advanced eyes.
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 origin than the
eyes of mammals, yet it turned out to function in almost the same exact way.
Cephalopods, therefore, have good eyesight.
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.
All octopods have a salivary gland that secretes venom to subdue prey.
The venom of the Blue Ringed Octopus is so powerful that the bite of this octopus is
often lethal to a human.
Fortunately, it takes a lot of provocation to get an octopus--any octopus--to bite a
person.
While the octopus has eight sucker-equipped arms, the squid and cuttlefish actually have
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.
There are four more classes of less common molluscs, most of which live in deep water
and most people will never see.
The Phylum Mollusca is one of the most diverse-looking groups of animals in the world.
It’s hard to believe that something like a clam is closely related to something as
outlandish as a cuttlefish.
Sometimes truth really is stranger than fiction when it comes to the amazing life in the Blue
World.
Hey Everyone!
Have you subscribed to our extras Channel BlueWorld_plus?
It’s full of great behind-the-scenes and additional fun content!
Check it out now!
Molluscs!
Why can’t I say it?