(Acephala Cuvier 1798, Pelecypoda Goldfuss 1820, Lamellibranchia Blainville 1824)
Blue mussels (Mytilus edulis). Picture: Ron Offermans.
Walking along the beaches of Brittany 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. its 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 fulfils 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.
Bivalves exclusively live in water, only it has happened several times in evolution that bivalve groups managed to adapt to life in fresh water, which is why today there are several fresh water mussel groups that are not closer related to each other than all being bivalves. One of these are the large fresh water mussels also called naiads (Unionacea).
River mussels (Unio crassus) on a creek's floor. Picture: S. Hochwald 
The main characteristic of mussels can easily be seen from outside: A mussel shell is divided into two shell valves (hence the class name Bivalvia). Usually they are large enough to cover and protect all of a mussel's body. The two valves of a mussel shell are held together on one side by an elastic band called the ligament. In a relaxed state it opens the shell. Antagonists to the ligament are the mussel's strong muscles. The mollusc has to keep the shell closed by active muscle action. To make this easier for the mollusc, its closing musculature consists in fact of two muscles: One relatively weak muscle for the actual closing motion and one very strong locking muscle to keep the shell shut. So it is quite easy to stop a muscle from closing its shell, but very hard to open it with the mollusc holding tight from inside.
An experiment with blue mussels. In the left glass there
are no mussels, in right one mussels filter the water.
Mussels breathe exclusively with gills. In different bivalve groups the gill construction can be so different, that the gill construction type is a major part of bivalve systematics. The gills not only serve for respiration, but also for feeding the mussel. Though there are some few species actively collecting food around their place, mussels generally sieve food particles out of the water current caused by respiratory action. Ingestible particles are separated from indigestible ones, the latter swept out by the same water current leaving the mussel. As those particles are clustered inside the mussel, they become to heavy to float and after leaving the mussel fall to the ground and are added to the sediment. So the mussel has got an important part in clearing the water. Blue mussels for example can filtrate up to 5 litres of water per hour, oysters even manage 25 litres.
Most bivalve species have got separate sexes, though there are some hermaphroditic groups. Especially bivalves living in colonies, such as oysters (Ostrea) can change from females into males, when the number of males in the colony is not sufficient to ensure the population's number. While fertilisation takes place in the surrounding water among marine bivalves and some of the fresh water groups, there are also fresh water species (the already mentioned naiads or Unionaceans), among which fertilisation takes place inside the female mussel's mantle cavity. They then develop past a parasitic larval stage, the Glochidia, that have to infect a fish to be able to develop into young mussels. In contrary to those, most marine bivalves' development happens past a free-floating larval stage, comparable to a trochophora, and later a veliger type of larva.
Schematic internal composition of a bivalve, one shell valve removed. Colours see
Body construction of a Bivalve. Source: Biodidac, further editing: R. Nordsieck.
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).
Model of a pilgrim's scallop (Pecten jacobaeus) from the Vienna
Natural History Museum. [RN]
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.
That way, on one hand, soft shelled clams (Mya arenaria), living dug deeply into the soil of the wadden sea, feed themselves and breathe over the siphons. If they are flushed out of the ground, they will die. The blue mussel on the other hand always lives on the soil surface and therefore has no elongate siphons. If it is covered with soil, it will die.
The gills in a bivalve's mantle cavity serve not only respiration, but also nutrition. Their composition may be different and thus is used to divide the Bivalvia class into sub-classes:
A freshly opened oyster (Ostrea edulis).
As in most molluscs, the bivalves' blood circulation is mainly open. The heart has three chambers, two antechambers (atria) and one heart chamber (ventricle).
Nutrition and Respiration
Respiration and nutrition in a blue mussel. Source: Aquascope.
The primordial bivalves from the Protobranchia subclass actively collect food like protozoa, larvae, eggs and detritus from the surrounding substrate. The food, collected by tentacles, is then transported to the mouth in a ciliate groove. Other than that, most other, more highly developed, bivalves feed by filtering their respiration water. Cilia located in the mantle cavity produce a water current from the ingestive siphon into the mantle cavity and out through the egestive siphon. Digestible particles are collected and transported to the mouth opening.
Feeding by filtration mussels get into contact with a very large amount of water, including the matter dissolved in it. That makes them especially susceptible to harmful substances in the water. So bivalves especially are harmed by the growing extent, to which water is polluted by industrial wastes and agricultural fertilisers. For man it is especially important, that mussels accumulate harmful substances they filter from the water. So the poisons, that are introduced into the water by industry, might be found again in the mussels and oysters we eat.
A clam digs itself into the ground using its foot. (see text).
A bivalve's foot is adapted to how it lives and moves. So there are different types of feet in bivalves, from beam-shaped, 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).
Blue mussels also use their byssus thread to defend themselves by tying down attacking sea snails, such as the netted dog whelk (Hinia reticulata). Against a full grown common whelk (Buccinum undatum), though, this defence will not help.
Swimming flame shell (Lima hians). Picture: Erling Svensen.
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 (see picture above).
Like Rick Stein, the famous sea food chef, had to discover when he tried to pull a razor clam out of the soil in an episode of "Food Heroes of Britain - Another Helping", such a clam can give somewhat of a struggle!
Blue mussels (Mytilus) also use their byssus threads to move themselves over the ground: The byssus is thrown out until it catches ground, then the mussel gradually shortens the thread, thus moving itself towards the point of attachment.
There are also some bivalve groups (Nut shell, Nucula; Dog cockle, Glycymeris, Tellin, Tellina; Venus clam, Venus), that possess a real crawling foot, like gastropods do.
Some bivalve species, such as the pilgrim's scallop (Pecten) (see above), and the flame shell (Lima) 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 surrounding. Tentacles also on the mantle rim feel their way around in the mollusc's environment (see picture above).
Reproduction and Development
A thick shelled river mussel's (Unio crassus)
glochidia in a minnow's gills (Phoxinus phoxi-
nus). Picture: Susanne Hochwald .
Most bivalves have got separate sexes. That means, there are female as well as male individuals. Fertilisation and subsequent larval development take place in free water, the larvae of the veliger type floating with the plankton. Among some bivalve species, such as the giant clam (Tridacna gigas) the release of sperm cells and egg cells is coordinated hormonally.
After development of the larva over various stages, the young mussel develops through a metamorphosis. The young mussel now tries to find a suitable place to settle for adult life. Among mussel colonies, as in blue mussels (Mytilus) and oysters (Ostrea), the young usually stay near the colony and settle not only on the ground but also on other mussels. That way, mussel beds, like those described in the beginning from Brittany, come into existence.
Fresh water bivalves have developed very different methods of reproduction and development.
Glochidium of a river mussel (U. crassus).
Picture: Susanne Hochwald 
Like their gastropod relatives, fresh water bivalves also had to adapt to the living conditions in rivers, lakes and ponds, always changing and unstable, other than the relatively stable and constant conditions of life in the vast space of the sea.
The large fresh water mussels or naiads (Unionacea), among those the large common river and pond mussels, develop past a parasitical larval stage, called a glochidium. For further development, this glochidium depends on the infection of a fish, usually of a certain species, by fastening itself on the fishes' gills.
In contrary to those, the much smaller species of tiny pea mussels (Pisidium) and fingernail clams (Sphaerium) are hermaphrodites, which deliver living larvae.
The zebra mussel (Dreissena polymorpha) finally develops, like the marine bivalves, past a veliger-like planktontic larval stage, one more reason for the vast distribution that gave this mussel species the by-name "wandering mussel".
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.
Hinge and ligament of a river mussel (Unio tumidus).
Source: M. Kohl: European Unionaceans.
Because of its high variability the shell form is also the most important identification criterion in bivalves. The shell can be oval, elliptical, shaped like a wedge or like a sheath. Both shell halves can be almost symmetric, as among bivalves usually sitting on their narrow side, such as river mussels, or, very different from each other, as in bivalves usually sitting on one broad side, such as an oyster (Ostrea) or a scallop (Pecten).
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.
Shell halves of the swan mussel (Anodonta cygnea).
Source: M. Kohl: European Unionaceans.
A bivalve's shell halves usually protect all of its soft body. Calciferous cells in the folds of the mantle rim attach further calcium carbonate substance in rings around the shell's rim. Those rings are also called umbones. They are the reason a bivalve shell keeps on growing with its inhabitant.
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, because it is around (peri) the shell layer (ostracum).
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 Brittany. 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.