Bivalvia

| image = Haeckel Acephala.jpg | image_width = 250px | image_caption = "Acephala", from Ernst Haeckel's Kunstformen der Natur (1904) | regnum = Animalia | phylum = Mollusca | classis = Bivalvia | classis_authority = Linnaeus, 1758 | subdivision_ranks = Subclasses | subdivision = Anomalosdesmata
Cryptodonta
Heterodonta
Paleoheterodonta
Palaeotaxodonta
Pteriomorphia
see text }} , England]] and attached mytilid bivalves in a Jurassic limestone (Matmor Formation) in southern Israel]] ; an extinct pectenoid bivalve from the Logan Formation of Wooster, Ohio (external mold)]]

Bivalvia is a class of marine and freshwater mollusks known for some time as Pelecypoda, but now commonly referred to simply as bivalves. As with Gastropoda and Cephalopoda, the term Pelecypoda is in reference to the animal itself while Bivalvia simply describes the shell. Other names for the class include Acephala, Bivalva, and Lamellibranchia. The class contains some 30,000 species, including scallops, clams, oysters and mussels.

Bivalves have a shell consisting of two asymmetrically rounded halves called valves that are mirror images of each other, 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.

Bivalves are unique among the molluscs, having lost their odontophore and radula in their transition to filter feeding.

Some bivalves are epifaunal; they attach to surfaces. Others are infaunal; they bury themselves in sediment. These forms typically have a strong digging foot. Some bivalves such as scallops can swim.

The term bivalve is derived from the Latin bis, meaning 'two', and valvae, meaning leaves of a door Other bivalved animals include brachiopods, ostracodes, and conchostrachans.

Taxonomy

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 systematic layout presented here follows Norman D. Newell's 1965 classification based on hinge tooth morphology:

{| class="wikitable" |- ! Subclass ! Order |- | Palaeotaxodonta | *Nuculoida |- | Cryptodonta | †Praecardioida Solemyoida |- | Pteriomorphia | Arcoida (ark shells) †Cyrtodontoida

Limoida (file shells)

Mytiloida (true mussels)

Ostreoida (oysters, formerly included in Pterioida)

†Praecardioida

Pterioida (pearl oysters, pen shells) |- | Paleoheterodonta | †Trigonioida

Unionoida (freshwater mussels)

†Modiomorpha |- | Heterodonta | †Cycloconchidae

†Hippuritoida

†Lyrodesmatidae

Myoida (most "soft shell calms" razor clams)

†Redoniidae

Veneroida (most "hard shell calms", cockles, etc) |- | Anomalodesmata | Pholadomyoida |}

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. This distinguishes between Protobranchia, Filibranchia, and Eulamellibranchia. The first corresponds to Newells 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.

In May 2010 a new taxonomy of the Bivalvia was published in the journal Malacologia. In this classification 324 families were recognized as valid, 214 of which are known exclusively as fossils and 110 families occur in the Recent with or without a fossil record. This publication consisted of two parts :

Nomenclator of Bivalve Names of the Family-Group and above

Classification of Bivalve Families (under the redaction of Rüdiger Bieler, Joseph G. Carter and Eugene V. Coan)

Anatomy

anatomy]] Margaritifera margaritifera]] , 2: growth lines, 3: ligament, 4: umbo]]

, Tridacna gigas]] 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.

Nervous system

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.

Senses

The sensory organs of bivalves are not well developed and are largely a function of the posterior mantle margins. The organs are usually tentacle mechanoreceptors or 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 a shadow falling over 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 and correct its orientation.

Muscles

The muscular system is composed of the posterior and anterior adductor muscles, although the anterior muscles 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 and respiration

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.

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 shell.

Digestive system

Modes of feeding

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.

Behaviour

bivalves with their siphons visible]] 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 defense 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.

Feeding

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).

Feeding types

There are four feeding types, defined by their gill structure:

Protobranchs use their ctenidia solely for respiration, and the labial palps to feed

Septibranchs possess a septum across the mantle cavity which pumps in food.

Filibranchs and lamellibranchs trap food with a mucous coating on the ctenidia; the filibranchs and lamellibranchs are differentiated by the way the ctenidia are joined

Movement

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.

Defensive secretions

The file shells can produce a noxious secretion when threatened, and the fan shells of the same family have a unique, acid-producing organ.

Comparison with brachiopods

, a bivalve with taxodont dentition from the Pliocene of Cyprus]] 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.

References

External links

Marine Bivalve Shells of the British Isles – Online Identification Guide

Museum of Paleontology - Palaeontology from the University of California, Berkeley

Bioerosion website at The College of Wooster

A Database of Cambodian Marine Bivalves

Category:Fossils