Hinge teeth are part of the anatomical structure of the inner surface 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, includi ...

, i.e. the shell of a bivalve

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 ...

. Bivalves by definition have 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 fitting ...

s, which are joined together by a strong and flexible ligament situated on 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 ...

at the dorsal edge of the shell. In life, the shell needs to be able to open slightly to allow the foot and siphons to protrude, and then close again, without the valves moving out of alignment with one another. To make this possible, in most cases the two valves are articulated using an arrangement of structures known as hinge teeth (often referred to collectively as the "dentition"). Like the ligament, the hinge teeth are also situated along the hinge line of the shell, in most cases. In most families of bivalves, the two valves of the shell are almost perfectly symmetrical with one another along the hinge line, although the placement and shape of the teeth may differ slightly in the left valve and right valve in order for the two valves to articulate properly. Each related group of bivalves tends to have distinctive hinge teeth, and because of this, examining the arrangement of the hinge teeth in a bivalve shell is often essential for identification and classification. A formal terminology is used to describe the different types of dentition.

In taxonomy

The hinge teeth, or the lack thereof, is an important feature in identifying bivalves because the teeth are generally similar within the major taxonomic groups. Historically the hinge teeth have provided a convenient means by which to construct classification schemes and attempt to indicate the

phylogenetic 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 ...

relationships within the class

Bivalvia 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, biva ...

.Invertebrate Paleobiology on-line syllabus on Bivalves, by Dr. Burt Carter, Georgia Southwestern State University, at: http://itc.gsw.edu/faculty/bcarter/paleo/labs/moll/biv2.htm There are formal names for the various types of hinge tooth arrangements or ''dentition''.

Kinds of hinge teeth

Taxodont

The ''taxodont'' hinge shows either one or two rows of similar interlocking teeth on either side of the umbones, as is the case in the ark clams ( Arcidae), the bittersweets (

Glycymerididae Glycymerididae, often misspelled as Glycymeridae, common names dog cockles or bittersweets, is a worldwide family of salt water clams, marine bivalve mollusks in the order Arcida. They are related to the ark clams. This family contains 45 extan ...

), and the nut clams (

Nuculidae Nuculidae is a family of small saltwater clams in the order Nuculida. Species in this family are commonly known as nut clams. The nomenclature of the Western European species in this family is still uncertain. Their systematics has been based ma ...

). File:Arcidae hinge.jpg, Arcidae hinge File:Glycymerididae hinge.jpg,

hinge

Dysodont

The ''dysodont'' hinge shows a strong ligament along the hinge line, with weak teeth near the umbones, as in the marine mussels (

Mytilidae Mytilidae are a family of small to large marine and brackish-water bivalve molluscs in the order Mytilida. One of the genera, '' Limnoperna'', even inhabits freshwater environments. The order has only this one family which contains some 52 gener ...

). File:Mytilidae hinge.jpg,

hinge

Isodont

The ''isodont'' hinge has lateral tubercles and sockets on either side of a thick ligament which is referred to as 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 ...

. This arrangement is typical of the oysters (

Ostreidae The Ostreidae, the true oysters, include most species of molluscs commonly consumed as oysters. Pearl oysters are not true oysters, and belong to the order Pteriida. Like scallops, true oysters have a central adductor muscle, which means the ...

scallops Scallop () is a common name that encompasses various species of Marine (ocean), marine bivalve mollusc, mollusks in the Taxonomy (biology), taxonomic Family (biology), family Pectinidae, the scallops. However, the common name "scallop" is also s ...

(

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 ...

), thorny oysters (

Spondylidae ''Spondylus'' is a genus of bivalve molluscs, the only genus in the family Spondylidae.MolluscaBase (2019). MolluscaBase. Spondylus Linnaeus, 1758. Accessed through: World Register of Marine Species at: http://www.marinespecies.org/aphia.php?p=t ...

), and kittens paws (

Plicatulidae The Plicatulidae are a family of saltwater clams, marine bivalve mollusks, known commonly as kitten's paws or kittenpaws.Abbott, R.T. & Morris, P.A. ''A Field Guide to Shells: Atlantic and Gulf Coasts and the West Indies.'' New York: Houghton Mif ...

). File:Ostreidae hinge.jpg,

hinge File:Pectinidae hinge.jpg,

hinge File:Spondylidae hinge.jpg,

hinge File:Plicatulidae hinge.jpg,

hinge

Crurae

The hinge has lamellar ridges on or near the hinge plate, and these function as hinge teeth. This arrangement is characteristic of the

Pandoridae Pandoridae is a taxonomic family of small saltwater clams, marine bivalves in the order Anomalodesmata Anomalodesmata is an superorder of saltwater clams, marine bivalve molluscs. This grouping was formerly recognised as a taxonomic subcl ...

, the jingle shells

Anomiidae Anomiidae is a family of saltwater clams, marine bivalve molluscs related to scallops and oysters, and known as anomiids. It contains seven genera. The family is known by several common names, including jingle shells, mermaid's toenails, and sa ...

, and

Dimyidae Dimyidae is a family of extremely flattened, small (<1 cm), pleurothetic, relatively rare marine

hinge

Schizodont

The ''schizodont'' hinge has reverse "V" shaped scissurate teeth, and often an elongated lateral tooth. This arrangement is found in most

Unionidae The Unionidae are a family of freshwater mussels, the largest in the order Unionida, the bivalve molluscs sometimes known as river mussels, or simply as unionids. The range of distribution for this family is world-wide. It is at its most divers ...

freshwater mussels. File:Unionidae hinge.jpg,

hinge

Pachyodont

The ''pachyodont'' hinge has large obscure tubercules with corresponding pits on the opposite valve. This arrangement is characteristic of the

Chamidae Chamidae, common name the jewel boxes or jewel box clams, is a taxonomic family of saltwater clams, a group of marine bivalve mollusks in the order Venerida.Abbott, R.T. & Morris, P.A. ''A Field Guide to Shells: Atlantic and Gulf Coasts and the ...

, the jewel boxes. File:Chamidae hinge.jpg,

hinge

Heterodont

The ''heterodont'' hinge has two to three wedge-shaped cardinal teeth set in the center near the umbones, and generally also has elongated lateral teeth on the anterior and posterior margins (on one or both sides). This arrangement is characteristic of the venus clams (

Veneridae The Veneridae or venerids, common name: Venus clams, are a very large family of minute to large, saltwater clams, marine bivalve molluscs. Over 500 living species of venerid bivalves are known, most of which are edible, and many of which are ex ...

), the cockles (

Cardiidae A cockle is an edible marine bivalve mollusc. Although many small edible bivalves are loosely called cockles, true cockles are species in the family Cardiidae. MolluscaBase eds. (2022). MolluscaBase. Cardiidae Lamarck, 1809. Accessed through: W ...

) and several other important groups. File:Veneridae close up heterodont hinge.jpg,

: '' Pitar'' hinge File:Veneridae hinge.jpg, Veneridae: '' Tivela'' hinge File:Veneridae hinge 3.jpg, Veneridae: '' Dosina'' hinge File:Cardiidae hinge.jpg,

hinge

Desmodont

In the ''desmodont'' hinge, also known as an ''asthenodont'' hinge, the hinge consists of a large ligamentous

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 chondrophore) which replaces the cardinal teeth, as in the soft-shell clams (

Myidae Myidae, common name the softshell clams, is a taxonomic family of marine bivalve molluscs in the order Myida. Genera Genera within the family Myidae include: * '' Cryptomya'' ( Conrad, 1848) **'' Cryptomya californica'' ( Conrad, 1837) – Cali ...

). File:Myidae hinge.jpg, ''

'' hinge File:Pholadidae hinge.jpg, ''

Pholadidae Pholadidae, known as piddocks or angelwings, are a family of bivalve molluscs similar to a clam. Background Piddocks are unique in that each side of their shells is divided into 2 or 3 separate sections. Furthermore, one of the piddock's shells ...

'' hinge

Anodont

The ''anodont'' hinge is characterized by a strong ligament (or a series of transverse ligamental grooves as in the ''Isognomonidae''), however, true teeth are absent in adults as is the case in the pen shells (

Pinnidae The Pinnidae are a taxonomic family of large saltwater clams sometimes known as pen shells. They are marine bivalve molluscs in the order Pteriida. Shell description The shells of bivalves in this family are fragile and have a long and triang ...

), tree oysters (

Isognomonidae Isognomonidae is a family (biology), family of medium-sized to large saltwater clams. They are pearl oysters, Marine (ocean), marine bivalve molluscs in the superfamily PterioideaMolluscaBase eds. (2021). MolluscaBase. Isognomonidae Woodring, 192 ...

), the pearl oysters (

Pteriidae Pteriidae, also called the feather oysters, is a family of medium-sized to large saltwater clams. They are pearl oysters, marine bivalve molluscs in the order Pteriida. Some of the species in this family are important economically as the sou ...

), and some freshwater mussels such as the genus '' Anodonta''. File:Pinnidae hinge.jpg,

hinge File:Pteriidae hinge.jpg,

hinge File:Isognomonidae hinge.jpg,

hinge

References

{{Bivalve anatomy Mollusc shells