Metazoa		Bivalvia
MOLLUSCA		Bivalvia

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Bivalvia

MOLLUSCA 
BASAL MOLLUSCA
|--Diasoma
|  |--Rostroconchia  
|  `--BIVALVIA  
|     |--Protobranchia  
|     `--HETERODONTA  
`--CYRTOSOMA
   |--CEPHALOPODA
   `--GASTROPODA 
      |--Eogastropoda  
      `--APOGASTROPODA
         |--Caenogastropoda
         `--HETEROBRANCHIA
            |--Opisthobranchia
            `--PULMONATA

Mytilus edulis, the common Mussel

Introduction

The Bivalves - clams, oysters, scallops, mussels, and their kin - are among the most modified group of mollusks, although some fossil groups seem to have resembled them. As the name indicates, all bivalves are characterized by a two-valved shell, which protects the soft animal inside. The valves are joined by a hinge on the dorsal side of the animal along a line parallel to the length of the body. The axis of symmetry is perpendicular to that of the other great group of bivalved marine filter-feeders, the brachiopods. In bivalves, the two shells are identical in profile (although in some types this is not the case) but one valve is on the right side of the animal, and the other on the left.

All bivalves are aquatic, with the majority of types living in shallow marine waters. There is however a single group of freshwater forms, the Unionoidea. Most are quite sluggish and sit on the bottom, or burrow into the sand and mud. and some can bore through wood and even rock. A few types, such as the common scallops and file shells can swim through the water for short distances by clapping the valves together strongly.

Physiology

In Bivalves the head and radula have completely disappeared. The bivalve body consists mostly of a muscular foot in the anterior, and a series of gills (ctenidia) amid a pair of siphons (if present) in the center and posterior. Two symmetrical mantle flaps enclose the whole body and secrete the paired shell valves. The valves can be tightly closed by retraction of the adductor muscles, typically situated at each end of the animal (anterior and posterior). Bivalves have no diductor muscle system for opening the valves; instead they usually use an elastic ligament - an elastic connecting strip between the two valves - that automatically opens the shell when the adductors relax. Along the hinge-line are interlocking teeth that hold the shells in place and developed to prevent fore and aft displacement of the valves.

The paired gills consist of two plate-like flaps hanging into the mantle cavity on each side of the foot. These have tiny hairs called cilia that draw a current of water into the mantle cavity. In most species, as the water passes between the gill filaments, food particles are strained out and transferred by sticky mucus to the mouth.

In more specialized burrowing forms, the mantle margins are fused and the entrance and exit of the water current is governed by the inhalant and exhalant siphons. The line of attachment of the mantle to the shell shows as a scar line (called the pallial line, see illustration) on the shell, and the indentation of the line left by the free lobe of the siphonal area is called the pallial sinus.  Macpherson & Gabriel (1962).

Despite the limitation imposed on them by their morphology, bivalves are a very diverse group, that have adopted to a large number of lifestyles. Modern bivalves can be grouped into a number of morphoecological categories (Stanley 1970): infaunal shallow burrowing; infaunal deep burrowing; epifaunal, attached by byssus threads to the substratum; epifaunal, cemented to the rock; free-lying; swimming; and boring and cavity-dwelling. Some of these are shown in the following diagram.

The shape and general morphology of bivalve shells directly reflects their mode of life. Often unrelated groups will adopt similar forms and lifestyles. From an understanding of the ways in which modern bivalves are adapted to particular modes of life one can make reasonable inferences on how extinct bivalves lived.  Clarkson (1993).  

Relationships and Evolutionary History

Pojetaia runnegari,
Botomian Age (Cambrian Epoch 2) of south-east Gondwana (South Australia)

Despite their specialized nature, Bivalves are among the earliest mollusks to appear, with both Fordilla and Pojetaia dating from the Tommotian. Fordilla continues through to the end of the Botomian, and Pojetaia to the middle of the Middle Cambrian.  Pojeta (2000), vide Schneider (2001).  Both belong to, or are the sister taxa of, the most primitive bivalve lineage, the Palaeotaxodonta.  Runnegar & Pojeta (1992) have shown that the Fordilla had the same shell microstructure as the palaeotaxodont Pojetaia and placed them in the same family. The former genus, previously considered an isofilibranch, is thus now placed in or near the Palaeotaxodonta. These very early forms seem to have disappeared by the end of the Middle Cambrian, so that true bivalves are unknown in the fossil record until the Early Ordovician, when a new diversity of Palaeotaxodonts appeared. The other great clade of bivalves, the Autobranchia or Autolamellibranchiata, do not appear until the Middle Ordovician, and are clearly derived from the Palaeotaxodonta, making the latter group paraphyletic.  This is confirmed by molecular analysis.  Giribet & Wheeler (2002).  

During the early Paleozoic, epifaunal and primitive infaunal siphonate suspension feeding was the most common bivalve life mode. Infaunal siphon feeders appeared in the middle and late Paleozoic, but they gained prominence after the Permo-Triassic extinctions. The infaunal and epifaunal forms increased in diversity throughout the Mesozoic until the Cretaceous-Tertiary extinction event, when the epifaunal suspension feeders were decimated. During the Cenozoic, the infaunal siphonate clams continued to diversify. The labial palp deposit feeders and the mucus-tube builders evolved early in the Paleozoic and remained relatively unaffected by the extinctions and diversifications of the remainder of the class.  Clarkson (1993).

Systematics and Classification

Many diverse schemes have been developed for the classification of bivalves. For a long time, experts used only one characteristic feature, such as shell morphology and hinge type, or type of gills. This led to a number of disparate systems, which are tabulated in the following diagram:

The most popular and widely adapted system was that of Newell in the Treatise on Invertebrate Paleontology Moore [ed.] (1969) (right-hand column in above table), which employs a system based on general shell shape, microstructures and hinge configuration. Features such as hinge, dentition, mineralogy, and shell structure and composition do not appear to be characters of much adaptive significance among bivalves, and hence change slowly and can be used for defining major taxonomic groups.

The classification proposed by the Treatise gives the following subclasses and orders (note - the following is based on Moore (1969), Clarkson (1979: 150-151), and Barnes (1980: 430-431).

Subclass Palaeotaxodonta (Cambrian Epoch 2? to Recent)
The palaeotaxodonta are the simplest and most primitive of the bivalves. These were the first bivalves to appear in the fossil record, and can be considered ancestral to all other groups. They have a protobranch gill structure, an equivalved aragonite shell and taxodont dentition. These include the labial palp-feeding nuculoids, which are small clams found today in deep water and fine sediment.
Order Nuculoida

Subclass Cryptodonta (Middle Ordovician to Recent)
Infaunal, with an aragonite shell. Common in the Palaeozoic, with Solemya as the only recent representative. Protobranchiate gills (in Solemya at least). Nowadays included with the Palaeotaxodonta
Order Solemyoida (Largely dysodont (no hinge teeth) - Devonian to Recent)
Order Praecardioida (Taxodont dentition. Palaeozoic only - Ordovician to Carboniferous.)

Subclass Pteriomorpha (Middle Ordovician to Recent)
A very diverse group of normally byssate forms (some are instead cemented to the substratum), with variable musculature and dentition. Shells may be calcite, aragonite or both. Many inequivalved, some equivalved. Variable dentition. Foot commonly reduced, absent in some. Mantle mostly unfused and lacking siphons. Gills fillibranch or eulamellibranch. Most have auricles of some sort at the hinge margin, but convergence and parallelism have made the determination of more definitive shell characters impossible.
Order Arcoida (Fillibranch gills (in living forms at least); circular to trapezoidal equivalved shells - Late Ordovician to Recent. The "Arc Shells".)
Order Mytiloida (Equivalve and very inequilateral. With or without well developed siphons. Gills filibranchiate or eulamellibranchiate - Devonian to Recent. Mussels and their relatives)
Order Pterioida ( Adults fixed by byssus or cementation. fillibranch or eulamellibranch gills; Ordovician to Recent. Include pectinaceans (scallops and their kin), ostreaceans, and the fossil inoceramids; the latter sometimes growing to large size; some inoceramids were over a meter in length)

Subclass Palaeoheterodonta (Middle Ordovician to Recent)
A mostly Palaeozoic group with equivalved aragonite shells, usually with only a few hinge teeth.
Order Modiomorphoida (Palaeozoic only - modern researchers [Schneider 2001 p.1124] tend to consider this a grade rather than a clade - the ancestors of higher bivalves)
Order Unionoida (freshwater clams; the only non-marine bivalves - Devonian to Recent)
Order Trigonioida (Bivalves with large trigonal and well developed schizodont teeth. Abundant during the Mesozoic; now represented only by Neotrigonia)

Subclass Heterodonta (Triassic to Recent)
Heterodont hinge structure, eulamellibranch gill structure; almost always aragonitic shells, Includes most of the extant bivalves. Adapted to varied modes of life, especially infaunal siphon feeding (the most notable exception being the mucus-tube building lucinaceans [order Veneroida], which represent a primitive/ancestral lineage). The hinge structure may degenerate into a desmodont condition.
Order Veneroida (mostly active equivalve heterodonts with true heterodont teeth)
Order Myoida (Thin-shelled burrowers and borers, very inequivalve, hinge degenerate, with one cardinal tooth on each valve; siphons well developed - Carboniferous to Recent)
Order Hippuritoida (The Rudists; large, thick-shelled, aberrant forms with pachydont dentition. Later forms very inequivalve, coral-like. Very common during the later Mesozoic, when they built tropical reefs. The largest forms have shells up to two meters long. The recent Chama was at one time included here on the basis of its dentition, but is now considered to be only distantly related at best. Middle Silurian to Latest Cretaceous )

Subclass Anomalodesmata (Middle Ordovician to Recent)
Burrowing or boring forms, very modified, with aragonitic shells and desmodont dentition. Gills eulamellibranchiate or, in the Superfamily Poromyoidea septbranch.
Order Pholadomyoida

Recently a number of cladistic analyses - based on neontological, paleontological, and more recently molecular evidence, have proposed alternative classifications. Some of these are shown as follows.

When represented in table form these recent classifications give a very confusing picture.

Is it any wonder people prefer the arrangement in the Treatise!

For more phylogenies see the Cladogram page

Links

Links

Class Bivalvia - short intro with some images

Bivalvia - good page, detailed intro

Bivalves - another good intro

Mollusca - Class Bivalvia - short intro

Bivalvia