Mussels and Clams (Bivalvia)
obtained from “The Living World of Molluscs”
by Robert Nordsieck and Martina Eleveld
Introduction
Walking along the beaches in France it is impossible to miss the
vast beds of blue mussels. Thousands of these blue black mussels
settle where the sea meets the coast. The beach walker's eye
passing over the mussel bed seems not to be able to find anything
living in there, but if you approach closer, the builder of this large
construction can be discovered. It is a mollusc, that has adapted in
a fascinating way to life between sea and land, between high and
low tides. And as it has, it fulfills a vital task in the coastal
ecosystem.
Mussels seem to have nothing in common with other molluscs. Compared to a snail crawling its way, but
especially to a squid shooting through the water like an arrow, mussels seem to have stopped at a low point of
evolution. Taking a closer look at a living mussel it becomes obvious, though, that what seems like inability to
move anywhere is the result of an evolution towards not necessarily having to. Mussels, in contrary to all other
molluscs live exclusively on filtration. From the surrounding water they not only take oxygen to breathe, but also
food. This nutrition method proved to be so successful that mussels not only managed to distribute into almost
all parts of the sea, no matter in which climate zone, but also into the ever changing salt-less waters of rivers
and ponds on the continents.
Soft body
A bivalve's soft body is protected on both sides by the mantle lobes. The space within is called the mantle
cavity. Usually both mantle lobes are partly grown together, leaving three openings, two, through which water
enters and leaves the cavity, providing the mussel with food and oxygen, and a third opening for the foot to
leave the shell. Usually the foot is the only part of the mussel's soft body to be seen outside of the shell.
On its rim the mantle is laid in three folds, which each have different tasks: The fold furthest outside contains
cells producing shell and shell skin (periostracum), the middle one has got sensory tasks, and the innermost
fold regulates the water flow into and from the mantle cavity.
Because most bivalve species rarely move from their place there was no need for evolution to provide them
with a head to concentrate sense organs such as eyes. (That is why CUVIER in 1798 named bivalves Acephala the headless molluscs).
There are, though, bivalve species that can swim and do so by flapping their shell halves, such as scallops
(Pecten and Lima). The mantle rim of those bivalves is full of simple eyes (ocelles). The mantle rim of giant
clams is also inhabited by symbiotic algae (zooxanthelles),that are protected by the mollusc and in exchange
provide it with nutrients produced by photosynthesis.
In the many bivalve species digging and drilling in the ground, the respiratory openings of the mantle rim are
elongated to form tubes, which are then called siphons. So the bivalve, though dug deeply into the ground, is
provided sufficiently with water and food. As there are two siphons, they are distinguished as ingestive (leading
in) and egestive (leading out) siphon. They may be grown together to form one double tube, and in erected
state may be longer than the bivalve itself.
Foot
A bivalve's foot is adapted to how it lives and moves. So there are different types of feet in bivalves, from beamshaped, tongue-shaped or worm-shaped feet. Swimming mussels, as well as completely sessile mussels, often
have a largely reduced foot.
Some species, such as Mytilus, the blue mussel, Arca, the ark shell, Pecten, the pilgrim's scallop and Pinna, the
pen shell, have got a byssus gland at their foot's end, producing a thread that hardens in water and can be used
to fasten the mussel to the ground. Later the thread can be severed (Mytilus) or thrown off (Pinctada).
Bivalves generally are known as creatures that almost never move. That is not entirely accurate. Many mussel
species that are absolutely sessile as adults, may move around as juveniles. Even the adults, though usually
sessile, may move, when necessary. To do so, the foot is inserted into the ground and then pumped up with
blood, so that it serves as an anchor, after which the mussel can pull body and shell over the ground. The same
way mussels dig themselves in the ground.
Some bivalve species, are able to swim through open water on their own. The bivalve jerkily presses together
the two shell valves and ejects the water contained within. With this bivalve-type rocket propulsion the mollusc
is then propelled backwards. Those bivalves usually have simple eyes on their mantle rim, so they can collect
information about light and shadow in their surroundings. Tentacles also on the mantle rim feel their way around
in the mollusc's environment.
Shell
The generally accepted systematic name (in contrary to some others) of all mussels, clams and scallops Bivalvia - the two-valve molluscs - refers to the most important character of all bivalves, that separates them
from all other molluscs: The shell is separated into two pieces.
Both shell halves are connected on one side by an elastic band, the ligament, which in relaxed state opens the
shell, Against the ligament's resistance the bivalve's closing muscle closes the shell, a weaker muscle
performing the actual closing movement and a very strong locking muscle afterwards holding the shell closed.
The hinge where the ligament holds the shell together may be armed with teeth that hold the shell halves in
position. Those hinge teeth are also different in different groups and are therefore used for identification.
A bivalve's shell halves usually protect all of its soft body.
The main part of a bivalve's shell is made from aragonite, a mineral of calcium carbonate in prism-shaped
crystals. This layer is made of a very hard substance but very susceptible to chemical corrosion. The solution of
this problem is an external skin of organic matter, mechanically very weak, but chemically very resistant. This is
called the shell-skin or Periostracum.
In most mussel species there is also a shell layer below the ostracum, which is called hypostracum (hypo
meaning "under"). Its thin plates of aragonite reflect the light in many colours. That is why this layer has been
used in the manufacture of jewellery perhaps since bivalves have been discovered in the dawn of mankind.
One special capability of the mantle cells producing this shell layer, though, has given it its name: Foreign
objects caught between the mantle and the shell, are encased in the irising matter this shell layer consists of.
What comes into existence is - a pearl. So the colloquial name of a mussel's hypostracum is mother-of-pearl.
There are many bivalve species that produce pearls, several of them in the sea (such as Pinctada) and some in
fresh water (Margaritifera in Central Europe and Northern America, Hyriopsis and Cristaria in East Asia).
When the amount of pearls found in nature is not sufficient to meet the market's demands, pearls can be
cultivated. Foreign objects, such as grains of sand, are artificially inserted into a mussel. Those mussels are
kept on artificial mussel beds, same as blue mussels and oysters cultivated in France. But similar aqua-cultures
are built on coasts everywhere else in the world.
Pearls from the sea though are much more frequently used than the rare fresh water pearls. Those, on the
other hand, being so rare, have to a certain extent led to the extermination of large numbers of fresh water pearl
mussels.
It is not only because of the pearls they produce, that bivalves have always been economically important to
man. Palaeolithic mussel shell heaps (also referred to by the Danish name Køkkenmøddinger for a kitchen
waste heap) are witnesses from stone age times to man collecting bivalve molluscs, mainly mussels and clams,
for food.