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Bivalvia
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Bivalves are molluscs belonging to the class Bivalvia. They have two-part shells, and typically both valves are symmetrical along the hinge line. The class has 30,000 species, including scallops, clams, oysters and mussels. Other names for the class include Bivalva, Pelecypoda, and Lamellibranchia.
Bivalves are exclusively aquatic; they include both marine and freshwater forms. However some, for instance the mussels, can survive out of water for short periods by closing their valves.
Bivalves are unique among the molluscs for lacking a radula; they feed by siphoning and filtering large particles from water.
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Bivalves are molluscs belonging to the class Bivalvia. They have two-part shells, and typically both valves are symmetrical along the hinge line. The class has 30,000 species, including scallops, clams, oysters and mussels. Other names for the class include Bivalva, Pelecypoda, and Lamellibranchia.
Bivalves are exclusively aquatic; they include both marine and freshwater forms. However some, for instance the mussels, can survive out of water for short periods by closing their valves.
Bivalves are unique among the molluscs for lacking a radula; they feed by siphoning and filtering large particles from water. Some bivalves are epifaunal: that is, they attach themselves to surfaces in the water, by means of a byssus or organic cementation. Others are infaunal: they bury themselves in sand or other sediments; these forms typically have a strong digging foot. Some bivalves, such as scallops, can swim.
Systematics
Due to classification schemes using single organ systems to draw phylogenies, as well as numerous naming schemes, many conflicts have arisen. More recent systems combine multiple organ systems, fossil records, as well as molecular data to draw more robust phylogenies.
The systematic layout presented here is according to Newell's 1965 classification based on hinge teeth morphology. There exists no robust phylogeny, and due to the plethora of fossil lineages, DNA sequence data is only of limited use should the subclasses turn out to be paraphyletic. The monophyly of the Anomalodesmata is especially disputed, but this is of less consequence as that group does not include higher-level prehistoric taxa. It is, however, currently accepted that Anomalodesmata resides within the subclass Heterodonta.
Subclass Palaeotaxodonta
Subclass Cryptodonta
Subclass Pteriomorphia (oysters, mussels, etc)
Subclass Paleoheterodonta
- †Trigonioida
- Unionoida (typical freshwater mussels)
- †Modiomorpha
Subclass Heterodonta (typical clams, cockles, rudists, etc)
Subclass Anomalodesmata
There also exists an alternative systematic scheme according to gill morphology (Franc 1960). This distinguishes between Protobranchia, Filibranchia, and Eulamellibranchia. The first corresponds to Newell's Palaeotaxodonta + Cryptodonta, the second to his Pteriomorphia, and the last contains all other groups. In addition, Franc separated the Septibranchia from his eulamellibranchs, but this would seem to make the latter paraphyletic.
Anatomy
The shapes of bivalve shells vary greatly - some are rounded and globular, others are flattened and plate-like, while still others, have become greatly elongated in order 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, allowing the animal to dig tunnels through wood.
Nervous system
The sedentary habit of the bivalves has led to the development of a simpler nervous system than in other molluscs - so simple, in fact, that there is no brain. In all but the simplest forms the neural ganglia are united into two cerebropleural ganglia 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.
Senses
The sensory organs of bivalves are not well developed, and are largely a function of the posterior mantle margins. The organs are usually tentacles and most are typically mechanoreceptors and chemoreceptors.
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 shadows falling on the animal.
In the septibranchs the inhalant siphon is surrounded by vibration-sensitive tentacles for detecting prey.
Statocysts within the organism help the bivalve to sense its orientation, which can then be corrected if need be.
Muscles
The muscular system is composed of the posterior and anterior adductor muscles, although the anterior may be reduced or even lost in some species.
The paired anterior and posterior pedal retractor muscles operate the animal's foot. In some bivalves, such as oysters and scallops, these retractors are absent.
Circulation
Bivalves have an open circulatory system that bathes the organs in hemolymph.
Mantle and shell
In bivalves the mantle, a thin membrane surrounding the body, secretes the main shell valves, ligament and hinge teeth, the mantle lobes secreting the valves and the mantle crest the other parts. 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 purposes.
The shell is composed of two calcareous valves, which are made of either calcite (as with, e.g. 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 shell halves are held together at the animal's dorsum by the ligament, which is composed of the tensilium and resilium. The ligament opens the shells.
Reproduction
The sexes are usually separate, but some hermaphroditism is known. Bivalves practice external fertilisation.
Typically the marine bivalve will start life as a trochophore, later becoming a veliger. Freshwater bivalves 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 become serious pests of fish.
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 astounding mimic 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 shoal member or potential mate. Whatever they see, they approach for a closer look and this is when the female mussel releases huge numbers of larvae from her gills, dousing the inquisitive fish with her tiny, parasitic babies. The 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.
Behaviour
The radical structure of the bivalves affects 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 such as mussels, drying out. 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 the Oystercatcher.
Feeding
Most bivalves are filter feeders (although some have taken up scavenging and predation), extracting organic matter from the sea in which they live. 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).
Feeding types
There are four feeding types, defind by their gill structure.
The Protobranchs use their ctenidia solely for respiration, and the labial palps catch their food. The filibranchs and lamellibranchs trap the food with a mucous coating on the ctenidia; the filibranchs and lamellibranchs are differentiated by the way the ctenidia are joined. Finally, the septibranchs possess a septum across the mantle cavity, which pumps in food.
Movement
Razor shells (Ensis spp.) can dig themselves into the sand with great speed to escape predation. Scallops, and file clams (Lima spp.), can swim to escape an enemy, 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.
Defensive secretions
The file shells (Limidae) can produce a noxious secretion when threatened, and the fan shells of the same family have a unique, acid-producing organ.
Comparison with brachiopods
Bivalves are laterally combined and have a shell composed of two valves. The valved shell makes them 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 upper and lower 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; indeed, by the end-Permian extinction, bivalves were undergoing a huge radiation in numbers while brachiopods (along with around 95% of all species) were devastated.
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 fossil strata. In fact, these taxa even appeared in textbooks as an example of replacement by competition. Evidence adduced for this included use of an energetically-efficient ligament-muscle system for opening valves, requiring less food to subsist. Lately, however, this interpretation of the interaction between brachiopods and bivalves has been largely exploded. It seems instead that the reason for the prominence of bivalves over brachiopods has to do with chance disparities in their response to extinction events.
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