A hinge ligament is a crucial part of the

anatomical Anatomy () is the branch of morphology concerned with the study of the internal 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 scien ...

structure of a

bivalve shell A bivalve shell is the enveloping exoskeleton or mollusc shell, shell of a bivalve mollusc, composed of two hinged halves or ''valve (mollusc), valves''. The two half-shells, called the "right valve" and "left valve", are joined by a ligament and ...

, i.e. the shell of a

bivalve Bivalvia () or bivalves, in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class (biology), class of aquatic animal, aquatic molluscs (marine and freshwater) that have laterally compressed soft bodies enclosed b ...

mollusk Mollusca is a phylum of protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant species of molluscs are recognized, making it the second-largest animal phylum after Arthropoda. The ...

. 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 Slurry, slurries) by opening, closing, or partially obstructing various passageways. Valves are technically Pip ...

s and these are joined 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, 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 residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

and

collagen Collagen () is the main structural protein in the extracellular matrix of the connective tissues of many animals. It is the most abundant protein in mammals, making up 25% to 35% of protein content. Amino acids are bound together to form a trip ...

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 and one of the three most common naturally occurring crystal forms of calcium carbonate (), the others being calcite and vaterite. It is formed by biological and physical processes, including precipitation fr ...

fibers and organic material, is lighter in color and often

iridescent Iridescence (also known as goniochromism) is the phenomenon of certain surfaces that appear gradually to change colour as the angle of view or the angle of illumination changes. Iridescence is caused by wave interference of light in microstruc ...

, 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, 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 A scanning electron microscope (SEM) is a type of electron microscope that produces images of a sample by scanning the surface with a focused beam of electrons. The electrons interact with atoms in the sample, producing various signals that ...

(SEM),

X-ray diffraction X-ray diffraction is a generic term for phenomena associated with changes in the direction of X-ray beams due to interactions with the electrons around atoms. It occurs due to elastic scattering, when there is no change in the energy of the waves. ...

(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 functio ...

(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 (ocean), marine bivalve mollusc, molluscs in the Taxonomy (biology), taxonomic Family (biology), family Pectinidae, the scallops. However, the common name "scallop" is also ...

) 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. 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 as palaeontology or palæontology, is the scientific study of the life of the past, mainly but not exclusively through the study of fossils. Paleontologists use fossils as a means to classify organisms, measure geolo ...

s and

malacologist Malacology, from Ancient Greek μαλακός (''malakós''), meaning "soft", and λόγος (''lógos''), meaning "study", is the branch of invertebrate zoology that deals with the study of the Mollusca (molluscs or mollusks), the second-largest ...

s as a means of inferring phylogenic

evolution Evolution is the change in the heritable Phenotypic trait, characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, re ...

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 bivalve shell, shell of certain bivalve mollusks. It is an internal ligament (bivalve), ligament, which holds the two valves together and is located in a pit or depression known as the resilifer. The resilium ...

and is attached to a resilifer 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