Bivalves

Bivalves are laterally compressed mollusks enclosed within a pair of hinged shells or valves. The valves are closed by the adductor muscles. The shell is opened by relaxing these muscles and water currents are drawn into the cavity. In the majority of bivalves the gill is modified for filter feeding, although the earliest bivalves may have been deposit feeders. Some bivalves retain this feeding strategy. Most forms are capable of limited movement and the foot can protrude into the sediment to enable them to burrow (Fig. 9.4). Bivalves are superficially similar to brachiopods, but have a very different internal anatomy and the shell is usually symmetric between the valves, rather than across the valve as in brachiopods (see Chapter 6).

Bivalves originated in the early Cambrian and have diversified, particularly after the end-Permian extinction, to exploit a wide range of aquatic environments. They are characteristic of Mesozoic and Cenozoic shallow water assemblages. Most bivalves are marine but there are some freshwater forms. As the shell forms of different species are adapted to the nature of the substrate, bivalves are very useful in paleoenvironmental reconstruction.

Tooth and sockets: located on the hinge plate. These structures fit into their inverse on the other valve to insure that the valves fit tightly and securely

Anterior adductor muscle scar: the adductor muscles actively hold the valves together. When the muscles relax the hinge ligament expands and the shell opens

Arca Morphology

Anterior

Pallial line: line that marks the attachment of the mantle to the shell

Anterior

Pallial line: line that marks the attachment of the mantle to the shell

Posterior adductor muscle scar

Pallial sinus: a deflection in the pallial line. This feature indicates the presence of a retractable siphon. Depth of burrowing correlates with depth of indentation

Posterior

(b) Umbo: earliest formed part of the shell

Growth line: each line marks the earlier position of the shell edge

Rib (costa): strengthens shell. In burrowing bivalves, ribs may help move sediment as burrowing proceeds

(c) Mantle: surrounds the mantle cavity. The outer part of the mantle secretes the shell

Mouth -

Mouth -

Exhalant siphon

Inhalant siphon

Foot: in burrowers the foot swells and the muscles contract, moving the animal through the sediment

Exhalant siphon

Inhalant siphon

Gill: modified for filter feeding. Cilia produce currents and trap food particles that are transported to the mouth

Fig. 9.4 Bivalve morphology: (a) general interior of a valve (right valve), (b) general exterior of a valve (left valve), and (c) internal morphology. Arrows show water currents in the mantle cavity.

Bivalve shells

Table 9.3 Types of bivalve dentition.

Bivalve shells are multilayered. Two phases comprise the shell - an organic matrix and a crystalline calcareous component, in the form of aragonite or calcite (CaCO3). Distinct shell structures have been identified that may be linked to the mode of life, for example burrowing bivalves possess a shell structure that is resistant to abrasion. Environmental factors such as water temperature may also affect shell mineralogy.

The two halves of the bivalve shell are joined by a ligament that holds the valves in tension, allowing the shell to open when the adductor muscles relax. Teeth and sockets in the hinge area of the valves interlock to insure a tight fit when the shell is closed. Several patterns of dentition exist in bivalves, and this is a useful characteristic in classification. The main types are shown in Table 9.3.

Evolutionary history

The earliest bivalves are known from Lower Cambrian rocks. These primitive forms are extremely small and may be descended from an unusual group of primitive mollusks called rostroconchs. Although superficially similar to bivalves, rostro-chonchs have a bilobed shell, without a functional hinge line. Strangely, no bivalves are recorded from the Middle and Upper Cambrian. A major period of diversification occurred in the early Ordovician. Groups arose with taxodont, dyso-dont, and heterodont hinges and a range of feeding strategies. Deposit feeders, byssally attached bivalves, and burrowers colonized marginal and nearshore environments. After this rapid radiation the group stabilized and bivalves were not a particularly diverse or abundant group during the Palaeozoic. Non-marine bivalves appeared in the Devonian and were abundant in the Carboniferous, particularly in deltaic environments.

During the early Mesozoic, bivalves underwent a second radiation. The presence of a muscular foot and the development of siphons, by the fusion of the posterior edge of the mantle, gave bivalves an evolutionary advantage over bra-chiopods, allowing them to successfully exploit an infaunal mode of life. Predation pressure increased in the Mesozoic and bivalves were able to feed effectively from the secure position of a burrow within the sediment. By colonizing new infaunal environments, bivalves expanded into the intertidal zone, burrowed deeper into the sediment, and developed mechanisms for boring into hard substrates.

Diversification of epifaunal bivalves also occurred in the Mesozoic. Large, strongly inequivalved, recumbent forms developed. These are groups in which one valve is large and

Dentition

Example

Taxodont

Numerous teeth in radial or subparallel arrangement

Arca

Dysodont

Small simple teeth at valve margin

Isodont

Very large teeth positioned either side of ligament pit

Schizodont

Large grooved teeth

Heterodont

Large teeth (termed cardinal teeth) flanked by smaller (lateral) teeth

Mytilus

Arca

Pecten Neotrigonia

Venus

Desmodont

Teeth reduced to ridges or absent

Venus

Lutraria forms a body cavity for the whole animal, while the other valve is much smaller, is often flattened, and functions as a lid. The most important of these groups were the rudist bivalves. They colonized carbonate shelves, adopting conical forms similar to corals or encrusting or lying on hard substrates. This group was short lived, originating in late Jurassic times and becoming extinct at the end of the Cretaceous.

Apart from the more bizarre groups, such as rudists, most bivalves survived the end-Cretaceous mass extinction. Bivalves have been ubiquitous in most shallow marine environments throughout the Cenozoic and they continue to be abundant in most present-day marine environments.

Bivalve ecology

Bivalves have adapted to live in a wide range of marine and freshwater environments. As shell shape is constrained by function, the mode of life of bivalves can be interpreted from the shell morphology. The major life habits of bivalves are: (i) burrowing in soft substrates; (ii) boring and cavity dwelling; (iii) attached (cemented or by byssus threads); (iv) unattached recumbant; and (v) intermittent swimming.

Infaunal bivalves

Bivalves that burrow in soft substrates tend to be equivalved and have a distinct pallial sinus (Fig. 9.5). Burrowing is achieved by the foot, which penetrates the sediment and swells. The muscles in the foot then contract, drawing the shell down through the sediment. Circular, heavily sculptured bivalves tend to burrow more slowly than smooth elongate forms, although the nature of the sediment also affects the speed of burrowing. Most burrowing bivalves are siphonate. During burrowing the siphons are closed. Some infaunal species are deposit feeders.

Sea water

Sea water

Venus

Siphonate shallow burrower. Animal rocks to penetrate the sediment. Pallial sinus is present but not pronounced

Coral Boring Bivalves

Venus

Siphonate shallow burrower. Animal rocks to penetrate the sediment. Pallial sinus is present but not pronounced

Ensis

Elongate bivalve living in tubular burrow. Shell thin and smooth with permanent posterior and anterior gapes. Although Ensis lives in shallow burrows it is capable of rapid deep burrowing when threatened

Soft to firm substrate

Deeply burrowing bivalve. Shell thin and elongate with permanent posterior and anterior gapes. Teeth reduced. Pronounced pallial sinus

Shallow infaunal

Bivalves living in shallow burrows usually have thick, equivalved shells with an approximately circular outline. Adductor muscles are of the same size and there is usually a pallial sinus

Some shallow burrowers have pronounced ridges on the exterior of the shell, and this may enable the bivalves to "saw" through the sediment. It also helps to anchor the animal in the mud or sand where it is burrowing

Deep infaunal

Deep burrowing bivalves tend to have thin, elongate shells without surface sculpture. Some shells have permanent posterior and anterior gapes. Teeth often reduced. Pallial sinus pronounced

Fig. 9.5 Burrowing bivalves.

Boring bivalves

Bivalves that bore into hard substrates typically have elongate, thin shells that are resistant to abrasion (Fig. 9.6). The shell edges are used to penetrate the material aided by acid secretions from the mantle. Some wood- or rock-boring bivalves have spines that scrape the substrate as the bivalve rotates. Some bivalves squat in borings made by other species. These bivalves grow to fit the cavity shape.

Burrowing Bivalve Shell

Fig. 9.6 Bivalves living in hard substrates.

A rock-boring bivalve. The bivalve has an elongate shell with a strongly ribbed, blade-like texture

Fig. 9.6 Bivalves living in hard substrates.

Epifaunal and swimming bivalves

Epifaunal bivalves exploit three living strategies: (i) attachment to the substrate by byssus threads; (ii) cementation to hard surfaces; and (iii) recumbent, free lying on the sediment surface stabilized by the shell morphology (Fig. 9.7).

Byssally attached bivalves secrete threads of collagen that adhere to the substrate. Typically, shells are elongate with a byssal gape, or notch. The anterior end of the shell and anterior muscle scar are usually reduced.

Cemented bivalves produce a calcareous fluid at the mantle margin that crystallizes, fixing the bivalve firmly to the substrate. Shells of cemented bivalves are usually inequivalved. Often there is only one large adductor muscle scar. Tropical cementers are usually covered in spines. As the bivalve grows it accommodates to the irregular surface on which it sits. This results in an irregular shell form that is characteristic of this life habit.

Recumbent, unattached bivalves have extremely asymmetric valves. Usually the lower valve is thickened, to increase stability, whilst the upper valve is reduced to a flat "lid".

Bivalves that are able to swim do so only intermittently. The shells of swimming valves tend to be thin, to reduce weight, although some forms have pronounced radial ribs. There is only one large adductor muscle scar. A single large muscle provides the strong contractions needed for swimming. Two pronounced "ears" are present, either side of the umbo, extending the hinge line. The umbonal angle in swimming bivalves is greater than that of byssally attached forms with a similar shape.

Pecten

Thin-shelled form with broadly symmetric valves. The radial ribs on the shell exterior are more common in very shallow swimming bivalves. Bivalves that swim below the wave base tend to be smooth

Mytilus

Byssally attached form. The shell is elongate and the posterior part is enlarged. Dentition is reduced or absent

Sea water

Mytilus

Byssally attached form. The shell is elongate and the posterior part is enlarged. Dentition is reduced or absent

Sea water

Crassostrea

Cementing bivalve, attached around the umbo. Shell thick and inequivalved. Adductor muscles are reduced to a single muscle

Gryphaea

An unattached, free-lying bivalve. The lower valve is strongly convex and thickened to enhance stability. Upper valve is reduced to a thin flat cover

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Responses

  • Hiewan
    Where are bivalve mollusks descended from?
    8 years ago
  • tomi
    Which phyllum of fossil come under the category infaunal?
    2 years ago
  • demi-lee
    What does pallial sinus indicates about life mode of bivalve?
    1 year ago
  • Amalda
    What is the mode of preservation for this pecten and venus bivalves?
    11 months ago
  • olga
    Are thick shelled bivalves nearshore dwellers?
    10 months ago
  • pandora whitfoot
    Do large teeth and sockets in bivalves help to keep a bivalve shell closed?
    9 months ago
  • theodore
    Which group of bivalves burrow through sand and mud?
    9 months ago
  • belladonna
    How to classify bivalvia?
    6 months ago
  • cipriano
    Are infaunal deposit feedING BIVALVES alive today?
    4 months ago

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