Mollusks: Snails and other weird animals!
There are over 50,000 known species of Mollusks, which makes them second only to the
Arthropods in invertebrate phylum size. Among the Mollusks are some of the most well
known of invertebrate sea creatures, like snails, clams, mussels, squid, and octopods.
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." The head
contains the sense organs and "brain," while the visceral mass contains the internal
organs. The "foot" is the muscular lower part of the body, which is in contact with the
substrate. 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.
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
Mollusks have well developed body organs (nervous system, circulatory system,
respiratory system, etc.) but lack body segmentation.
There are 3 main classes of Mollusks: Gastropoda, Bivalvia and Cephalopoda. Others
including, Monoplacophora, Aplacophora and Scaphapoda are rare and/or extremely
deep-water creatures, and will not be discussed.
The Class Gastropoda
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. The snails, limpets and abalones have a shell while the slugs and nudibranchs
do not. There are a small number of land Gastropods such as some snails, slugs, etc.
Do
you
see
my
eyes?
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!"
The Class Bivalvia
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).
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.
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.
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.
The Class Cephalopoda
Squid, octopods, 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. Thus, the body does not connect directly to the arms.
The octopods do not have shells at all, and the squid have a small chitinized internal shell.
Cephalopods have the most well developed nervous systems of all mollusks, as well as
the most well developed eye.
Cephalopods have extremely 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.
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.
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.
Our educational film Beneath the South Pacific has some very interesting stuff about mollusks.
Check out the O.R.G. Educational Films!
More Wonders of the Seas....