Fossil range: early Cambrian–Recent
|"Acephala", from Ernst Haeckel's Kunstformen der Natur (1904)|
Bivalves are marine and freshwater molluscs belonging to the class Bivalvia. Other names for the class include Acephala, Bivalva, Pelecypoda, and Lamellibranchia. The class contains 30,000 species, including scallops, clams, oysters and mussels.
Bivalves have a shell consisting of two rounded plates called valves joined at one edge by a flexible ligament called the hinge. The shell is typically bilaterally symmetrical, with the hinge lying in the sagittal plane.
No consensus exists on bivalve phylogeny. Many conflicts exist due to taxonomies based on single organ systems and conflicting naming schemes. More recent taxonomies use multiple organ systems, fossil records, as well as molecular phylogenetics to draw more robust phylogenies. Due to the numerous fossil lineages, DNA sequence data is of limited use should the subclasses turn out to be paraphyletic.
The monophyly of the Anomalodesmata is disputed, but this is of less consequence as that group does not include higher-level prehistoric taxa. The standard view now is that Anomalodesmata resides within the subclass Heterodonta.  
An alternative systematic scheme exists according to gill morphology (Franc 1960). This distinguishes between Protobranchia, Filibranchia, and Eulamellibranchia. The first corresponds to Newell's Palaeotaxodonta and Cryptodonta, the second to his Pteriomorphia, with the last corresponding to all other groups. In addition, Franc separated the Septibranchia from his eulamellibranchs, but this would seem to make the latter paraphyletic.
Bivalve shells vary greatly in shape; some are globular, others flattened, while others are elongated to aid burrowing. The shipworms of the family Teredinidae have greatly elongated bodies, but the shell valves are much reduced and restricted to the anterior end of the body, where they function as burrowing organs that permit the animal to dig tunnels through wood.
The sedentary habit of the bivalves has led to the development of a simpler nervous system than in other molluscs; they have no brain. In all but the simplest forms the neural ganglia are united into two cerebropleural ganglia on either side of the oesophagus. The pedal ganglia, controlling the foot, are at its base, and the visceral ganglia (which can be quite large in swimming bivalves) under the posterior adductor muscle. These ganglia are both connected to the cerebropleural ganglia by nerve fibres. There may also be siphonal ganglia in bivalves with a long siphon.
Scallops have complex eyes with a lens and retina, but most other bivalves have much simpler eyes, if any. There are also light-sensitive cells in all bivalves that can detect a shadow falling over the animal.
Many bivalves possess a number of tentacles, which have chemoreceptor cells to taste the water, as well as being sensitive to touch. These are typically found near the siphons, but in some species may fringe the entire mantle cavity.
Another notable sensory organ found in bivalves is the osphradium, a patch of sensory cells located below the posterior adductor muscle. It may serve to taste the water, or measure its turbidity, but it is probably not homologous with the structure of the same name found in snails and slugs.
In the septibranchs the inhalant siphon is surrounded by vibration-sensitive tentacles for detecting prey.
Bivalves have an open circulatory system that bathes the organs in hemolymph. The heart has three chambers; two auricles receiving blood from the gills, and a single ventricle. The ventricle is muscular and pumps hemolymph into the aorta, and through this to the rest of the body. Many bivalves have only a single aorta, but most also have a second, usually smaller, aorta serving the hind parts of the animal.
Oxygen is absorbed into the hemolymph in the gills, which hang down into the mantle cavity, and also assist in filtering food particles from the water. The wall of the mantle cavity is a secondary respiratory surface, and is well supplied with capillaries. Some species, however, have no gills, with the mantle cavity being the only location of gas exchange. Bivalves adapted to tidal environments can survive for several hours out of water by closing their shells and keeping the mantle cavity filled with water.
In bivalves the mantle forms a thin membrane surrounding the body which secretes the valves, ligament and hinge teeth. The mantle lobes secrete the valves and the mantle crest secretes the ligament and hinge teeth. The mantle is attached to the shell by the mantle retractor muscles at the pallial line. In some bivalves the mantle edges fuse to form siphons, which take in and expel water for suspension feeding.
The shell is composed of two calcareous valves, which are made of either calcite (as with oysters) or both calcite and aragonite, usually with the aragonite forming an inner layer (as with the pterioida). The outermost layer is the periostracum, composed of a horny organic substance. This forms the familiar coloured layer on the shell.
The shell is added to in two ways; at the open edge and by a gradual thickening throughout the animal's life.
The majority of bivalves are filter feeders, using their gills to capture particulate food from the water. In almost all species, the water current enters the shell from the posterior ventral surface of the animal, and then passes upwards through the gills in a U-shape, so that it exits just above the intake. In burrowing species, there may be elongated siphons stretching from the body to the surface, one each for the inhalant and exhalant streams of water.
The gills of filter-feeding bivalves have become highly modified to increase their ability to capture food. For example, the cilia on the gills, which originally served to remove unwanted sediment, are adapted to capture food particles, and transport them in a steady stream of mucus to the mouth. The filaments of the gills are also much longer than those in more primitive bivalves, and are folded over to create a groove through which food can be transported. The structure of the gills varies considerably, and can serve as a useful means for classifying bivalves into groups.
Some bivalves feed by scraping detritus from the bottom, and this may be the primitive mode of feeding for the group, before the gills became adapted for filter feeding. These primitive bivalves hold onto the substratum with a pair of tentacles at the edge of the mouth, each of which has a single palp, or flap. The tentacles are covered in mucus, which traps the food particles, and transports them back to the palps using cilia. The palps then serve to sort the particles, ejecting those that are too large to be digestible.
A few bivalves, such as Poromya, are carnivorous, eating much larger prey than the tiny phytoplankton consumed by the filter feeders. In these animals, the gills are relatively small, and form a perforated barrier separating the main mantle cavity from a smaller chamber through which the water is exhaled. Muscles pump water through the cavity, sucking in small crustaceans and worms. The prey are then seized in the palps and consumed.
The digestive tract of typical bivalves consists of an esophagus, stomach, and intestine. A number of digestive glands open into the stomach, often via a pair of diverticula; these secrete enzymes to digest food in the stomach, but also include cells that phagocytose food particles, and digest them intracellularly.
In the filter feeding bivalves, an elongated rod of solidified mucus referred to as the crystalline style projects into the stomach from an associated sac. Cilia in the sac cause the style to rotate, winding in a stream of food-containing mucus from the mouth, and churning the stomach contents. This constant motion propels food particles into a sorting region at the rear of the stomach, which distributes smaller particles into the digestive glands, and heavier particles into the intestine.
Like most other molluscs, the excretory organs of bivalves are nephridia. There are two nephridia, each consisting of a long, glandular tube, which opens into the body cavity just beneath the heart, and a bladder. Waste is voided from the bladders through a pair of openings near the front of the upper part mantle cavity, where it can easily be washed away in the stream of exhalant water.
The sexes are usually separate, but some hermaphroditism is known. Bivalves practice external fertilization. The gonads are located close to the intestines, and either open into the nephridia, or through a separate pore into the mantle cavity.
Typically bivalves start life as a trochophore, later becoming a veliger. Freshwater bivalves of the Unionoida have a different life cycle: they become a glochidium, which attaches to any firm surface to avoid the danger of being swept downsteam. Glochidia can be serious pests of fish if they lodge in the fish gills.
Some of the species in the freshwater mussel family, Unionidae, commonly known as pocketbook mussels have evolved a remarkable reproductive strategy. The edge of the female's body that protrudes from the valves of the shell develops into an imitation of a small fish complete with markings and false eyes. This decoy moves in the current and attracts the attention of real fish. Some fish see the decoy as prey, while others see a conspecific. Whatever they see, they approach for a closer look and the mussel releases huge numbers of larvae from her gills, dousing the inquisitive fish with her tiny, parasitic young. These glochidia larvae are drawn into the fish's gills where they attach and trigger a tissue response that forms a small cyst in which the young mussel resides. It feeds by breaking down and digesting the tissue of the fish within the cyst.
The radical structure of the bivalves reflects their behaviour in several ways. The most significant is the use of the closely-fitting valves as a defence against predation and, in intertidal species, against desiccation. The entire animal can be contained within the shell, which is held shut by the powerful adductor muscles. This defence is difficult to overcome except by specialist predators such as sea stars and oystercatchers.
Most bivalves are filter feeders although some have taken up scavenging and predation. Nephridia remove the waste material. Buried bivalves feed by extending a siphon to the surface (indicated by the presence of a pallial sinus, the size of which is proportional to the burrowing depth, and represented by their hinge teeth).
There are four feeding types, defined by their gill structure:
Razor shells can dig themselves into the sand with great speed to escape predation. Scallops, and file clams can swim to escape a predator, clapping their valves together to create a jet of water. Cockles can use their foot to leap from danger. However these methods can quickly exhaust the animal. In the razor shells the siphons can break off only to grow back later.
Bivalves are superficially similar to brachiopods, but the construction of the shell is completely different in the two groups. In brachiopods, the two valves are on the dorsal and ventral surfaces of the body, while in bivalves, they are on the left and right sides.
Bivalves appeared late in the Cambrian explosion and came to dominate over brachiopods during the Palaeozoic. By the Permian-Triassic extinction event bivalves were undergoing a huge radiation while brachiopods were devastated, losing 95% of their diversity.
It had long been considered that bivalves are better adapted to aquatic life than the brachiopods were, causing brachiopods to be out-competed and relegated to minor niches in later strata. These taxa appeared in textbooks as an example of replacement by competition. Evidence included the use of an energetically-efficient ligament-muscle system for opening valves, requiring less food to subsist. However the prominence of bivalves over brachiopods might instead be due to chance disparities in their response to extinction events.
Fossil range: Cambrian - Recent
|"Acephala" from Ernst Haeckel's Kunstformen der Natur, 1904|
Bivalves is a huge class of mollusks. They usually have a shell that is composed of two parts. Both parts are more or less symmetrical. There are over 30.000 species of bivalves. All of them live in the water, some of them in the seas, others in fresh water. All are filter feeders. Like the other shelled mollusks, the gastropods, bivalves produce a limy shell out of calcium.Bilvales are also known as clams or pelecypods.
A bivalve inhales (breathes in) water that has plankton and other things floating in it. Anything that is small enough to fit inside the hole of its incurrent siphon, a tube through which a bivalve can suck in water, enters the bivalve. When the floating material comes in, it gets stuck in slimy mucus that is on the surface of the bivalve's gills. The food is moved down to the mouth, which is on the other side of the siphon. Food is digested in the bivalve's stomach and intestine, and everything that is not digested goes out through the excurrent siphon (another tube that pushes waste out). Bivalves take in lots of bacteria and other unhealthy or unclean things, removing them from the water.
Giant clam or Tridacna
A giant clam