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 Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class (biology), class of marine and freshwater Mollusca, molluscs that have laterally compressed bodies enclosed by a shell consisting of two hing ...

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 A ligament is the fibrous connective tissue that connects bones to other bones. It is also known as ''articular ligament'', ''articular larua'', ''fibrous ligament'', or ''true ligament''. Other ligaments in the body include the: * Peritoneal l ...

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 ...

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, bival ...

.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), and the nut clams ( Nuculidae). File:Arcidae hinge.jpg, Arcidae hinge File:Glycymerididae hinge.jpg, Glycymerididae 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 in ...

. This arrangement is typical of the

oysters Oyster is the common name for a number of different families of salt-water bivalve molluscs that live in marine or brackish habitats. In some species, the valves are highly calcified, and many are somewhat irregular in shape. Many, but not all ...

( Ostreidae),

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), 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 Mi ...

). File:Ostreidae hinge.jpg, Ostreidae hinge File:Pectinidae hinge.jpg,

hinge File:Spondylidae hinge.jpg, Spondylidae 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. Genera and species Genera and species within the family Pandoridae include: * '' Clidiophora'' Carpenter, 1864 * '' Coania'' Valentich-Scot ...

, the jingle shells Anomiidae, and Dimyidae. File:Anomiidae hinge.jpg, Anomiidae 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, the jewel boxes. File:Chamidae hinge.jpg, Chamidae 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) and several other important groups. File:Veneridae close up heterodont hinge.jpg,

: '' Pitar'' hinge File:Veneridae hinge.jpg, Veneridae: ''

Tivela Tivela is a genus of saltwater clams, marine bivalve molluscs in the subfamily Meretricinae of the family Veneridae, the Venus clams. MolluscaBase eds. (2022). MolluscaBase. Trigona Megerle von Mühlfeld, 1811. Accessed through: World Register ...

'' hinge File:Veneridae hinge 3.jpg, Veneridae: '' Dosina'' hinge File:Cardiidae hinge.jpg, Cardiidae 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 l ...

(or chondrophore) which replaces the cardinal teeth, as in the soft-shell clams ( Myidae). File:Myidae hinge.jpg, '' Myidae'' 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), tree oysters (

Isognomonidae Isognomonidae is a family of medium-sized to large saltwater clams. They are pearl oysters, marine bivalve Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater mol ...

), the pearl oysters ( Pteriidae), and some freshwater mussels such as the genus ''

Anodonta ''Anodonta'' is a genus of freshwater mussels in the family Unionidae, the river mussels. Species Species in this genus include: * ''Anodonta anatina'' Linné, 1758 – duck mussel * ''Anodonta beringiana'' Middendorff, 1851 – Yukon fl ...

''. File:Pinnidae hinge.jpg, Pinnidae hinge File:Pteriidae hinge.jpg, Pteriidae hinge File:Isognomonidae hinge.jpg,

hinge

References

{{Bivalve anatomy Mollusc shells