A hinge ligament is a crucial part of the

anatomical Anatomy () is the branch of biology concerned with the study of the structure of organisms and their parts. Anatomy is a branch of natural science that deals with the structural organization of living things. It is an old science, having it ...

structure of a

bivalve shell A bivalve shell is part of the body, the exoskeleton or shell, of a bivalve mollusk. In life, the shell of this class of mollusks is composed of two hinged parts or ''valves''. Bivalves are very common in essentially all aquatic locales, including ...

, i.e. the shell of a

bivalve Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bival ...

mollusk Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is e ...

. The shell of a bivalve has two

valve A valve is a device or natural object that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening, closing, or partially obstructing various passageways. Valves are technically fittings ...

s and these are joined together by the ligament at the dorsal edge of the shell. The ligament is made of a strong, flexible and elastic, fibrous, proteinaceous material which is usually pale brown, dark brown or black in color. In life, the shell needs to be able to open a little (to allow the foot and siphons to protrude) and then close again. As well as connecting the two bivalve shells together at the

hinge line A hinge line is an imaginary longitudinal line along the dorsal edge of the shell of a bivalve mollusk where the two valves hinge or articulate. The hinge line can easily be perceived in these images of a mussel shell and an ark shell Ark cla ...

, the ligament also functions as a spring which automatically opens the valves when the adductor muscle or muscles (that close the valves) relax.

Composition

The ligament is an uncalcified elastic structure comprised in its most minimal state of two layers: a lamellar layer and a fibrous layer. The lamellar layer consists entirely of organic material (a

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...

and

collagen Collagen () is the main structural protein in the extracellular matrix found in the body's various connective tissues. As the main component of connective tissue, it is the most abundant protein in mammals, making up from 25% to 35% of the whole ...

matrix), is generally brown in color, and is elastic in response to both compressional and tensional stresses. The fibrous layer is made of

aragonite Aragonite is a carbonate mineral, one of the three most common naturally occurring crystal forms of calcium carbonate, (the other forms being the minerals calcite and vaterite). It is formed by biological and physical processes, including prec ...

fibers and organic material, is lighter in color and often

iridescent Iridescence (also known as goniochromism) is the phenomenon of certain surfaces that appear to gradually change color as the angle of view or the angle of illumination changes. Examples of iridescence include soap bubbles, feathers, butterfl ...

, and is elastic only under compressional stress.''A Theoretical Morphologic Analysis of Bivalve Ligaments'', Takao Ubukata, Paleobiology Vol. 29, No. 3 (Summer, 2003), pp. 369-380 The protein responsible for the elasticity of the ligament is

abductin Abductin is a naturally occurring elastomeric protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including ...

, which has enormous elastic resiliency: this resiliency is what causes the valves of the bivalve mollusk to open when the adductor muscles relax.Steven Vogel (2003) ''Comparative Biomechanics: Life's Physical World.'' Princeton: Princeton University Press. 580 p. at p. 304 Ligaments that are simple morphologically have a central fibrous layer between the anterior and posterior lamellar layers. Repetitive ligaments are morphologically more complex, and display additional, repeated layers. A recent study using scanning electron microscopy(SEM), X-ray diffraction (XRD), and

infrared spectroscopy Infrared spectroscopy (IR spectroscopy or vibrational spectroscopy) is the measurement of the interaction of infrared radiation with matter by absorption, emission, or reflection. It is used to study and identify chemical substances or function ...

(FTIR), found that some bivalve mollusks have a third type of fibrous layer in the ligament (located in the middle) which has a unique spring-like protein fiber (ca. 120 nm in diameter) structure, stretching continuously from the left to right valve.

Elastic opening of valves

When the adductor muscles of a bivalve mollusk contract, the valves close, which compresses the ligament. When the adductor muscles relax again, the elastic resiliency of the ligament reopens the shell. Scallops (

Pectinidae Scallop () is a common name that encompasses various species of marine bivalve mollusks in the taxonomic family Pectinidae, the scallops. However, the common name "scallop" is also sometimes applied to species in other closely related families ...

) swim through the water column by rapidly and repeatedly clapping (opening and closing) their valves. An interesting fact about scallops swimming in this manner is that they recover a greater percentage of the work (as defined by physics) performed through the elasticity of their abductin than do other bivalves (which are more sedentary clams).

Taxonomic use

The hinge ligament of a bivalve shell can be either internal, external, or both, and is an example of complex development.Evolution on the half shell – Assembling the Tree of Life: the Bivalve Mollusks, see http://www.bivatol.org/index.php?option=com_content&view=featured&Itemid=21 Various types of hinge ligaments have been found in living species (i.e. extant species), and the ligaments can be reconstructed in most fossil bivalves based on their sites of attachment on the shell. The taxonomic distribution of ligament types among families of bivalves has been used by

paleontologist Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (roughly 11,700 years before present). It includes the study of fossi ...

s and

malacologist Malacology is the branch of invertebrate zoology that deals with the study of the Mollusca (mollusks or molluscs), the second-largest phylum of animals in terms of described species after the arthropods. Mollusks include snails and slugs, clams, ...

s as a means of inferring

phylogenic In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...

evolution Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...

Hinge ligament types

External hinge ligaments may be described as having an "orientation" that is ''amphidetic'' (between the beaks), ''opisthodetic'' (behind/ posterior to the beaks), or, rarely, ''prosodetic'' (before the beaks). Then, there are four main "structural types": ''alivincular'' (a flattened, usually triangular area with a central fibrous layer and a peripheral lamellar layer), ''duplivincular'' (alternating bands of fibrous and lamellar layers forming chevrons on the cardinal area), ''parivincular'' (a single arched structure behind the beaks), and ''planivincular'' (a long ligament with a slight arch that extends behind the beaks). An internal ligament is usually called a

resilium In anatomy, a resilium is part of the shell of certain bivalve mollusks. It is an internal ligament, which holds the two valves together and is located in a pit or depression known as the resilifer. The resilium is part of the hinge mechanism i ...

and is attached to a

resilifer A resilifer is a part of the shell of certain bivalve mollusks. It is either a recess (a pit or groove of some sort) or a process, the function of which is the attachment of an internal ligament, which holds the two valves together. An internal ...

or chrondophore, which is a depression or pit inside the shell near the umbo.Huber, Markus (2010). Compendium of Bivalves. A Full-color Guide to 3'300 of the World's Marine Bivalves. A Status on Bivalvia after 250 Years of Research. Hackenheim: ConchBooks. pp. 901 pp. + CD. , at p. 59

References

General references

E.R. Trueman, ''General features of Bivalvia.'' In: Moore R.C., editor. Bivalvia. Ligament. In: Treatise on invertebrate paleontology. Vol. 2. Geological Society of America and University of Kansas Press; 1969. p. N58-N64. Part N - Mollusca, Bivalvia Vol. 6. T.R. Waller, ''The evolution of ligament systems in the Bivalvia.'' In: Morton B., editor. Proceedings of a Memorial Symposium in Honour of Sir Charles Maurice Yonge, Edinburgh, 1986. Hong Kong: Hong Kong University Press; 1990. p. 49-71. J. G. Carter, ''Evolutionary significance of shell microstructure in the Paleotaxodonta, Pteriomorphia and Isofilibranchia (Bivalvia: Mollusca).'' In: Carter J.G., editor. Skeletal biomineralization: patterns, processes, and evolutionary trends. New York: Van Nostrand Reinhold; 1990. p. 135-296. {{Bivalve anatomy Bivalve anatomy