The Pteriomorphia comprise a subclass of saltwater clams,

marine Marine is an adjective meaning of or pertaining to the sea or ocean. Marine or marines may refer to: Ocean * Maritime (disambiguation) * Marine art * Marine biology * Marine debris * Marine habitats * Marine life * Marine pollution Military ...

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 ...

mollusc 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 est ...

s. It contains several major orders, including the Arcida, Ostreida,

Pectinida Pectinida is a taxonomic order of large and medium-sized saltwater clams, marine bivalve molluscs, commonly known as scallops and their allies. It is believed that they began evolutionarily in the late Middle Ordovician epoch; many species, of ...

Limida The Limidae or file shells are members of the only family of bivalve molluscs in the order Limida. The family includes 130 living species, assigned to 10 genera. Widely distributed in all seas from shallow to deep waters, the species are usually ...

Mytilida Mytilida is an order of marine bivalve molluscs, commonly known as true mussels. There is one extant superfamily, the Mytiloidea, with a single extant family, the Mytilidae. Species in the order Mytilida are found worldwide, but they are more ...

, and Pteriida. It also contains some

extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

and probably basal families, such as the Evyanidae, Colpomyidae,

Bakevelliidae Bakevelliidae is an extinct family of prehistoric bivalves that lived from the Late Mississippian until the Middle Eocene.Cassianellidae Cassianellidae is a small extinct family of prehistoric bivalves which lived from the Middle Triassic, Ladinian stage through the Late Triassic Norian stage.

, and Lithiotidae. This subclass of molluscs has

lamellibranch 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 ...

gill A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they ar ...

s, and is

epibenthic Benthos (), also known as benthon, is the community of organisms that live on, in, or near the bottom of a sea, river, lake, or stream, also known as the benthic zone.substrate using a

byssus A byssus () is a bundle of filaments secreted by many species of bivalve mollusc that function to attach the mollusc to a solid surface. Species from several families of clams have a byssus, including pen shells ( Pinnidae), true mussels (Mytili ...

. The foot is reduced. The

mantle A mantle is a piece of clothing, a type of cloak. Several other meanings are derived from that. Mantle may refer to: *Mantle (clothing), a cloak-like garment worn mainly by women as fashionable outerwear **Mantle (vesture), an Eastern Orthodox ve ...

margins are not fused. Gills are usually large and assist in feeding. This group includes the well known

mussel Mussel () is the common name used for members of several families of bivalve molluscs, from saltwater and freshwater habitats. These groups have in common a shell whose outline is elongated and asymmetrical compared with other edible clams, which ...

scallop 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 ...

pen shell 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 triangu ...

s, and

oyster 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 ...

s. It also includes the only members of 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 ...

to have rudimentary eyes.

Photoreceptors

Pteriomorphian bivalves possess five types of photoreceptors, each evolving independently and each associated with different clades within Pteriomorphia. There are cap eyespots, pigmented cups,

compound eyes A compound eye is a visual organ found in arthropods such as insects and crustaceans. It may consist of thousands of ommatidia, which are tiny independent photoreception units that consist of a cornea, lens, and photoreceptor cells which distin ...

, concave mirror eyes, and invaginated eyes, each having evolved independently. The primary purpose of pteriomorphian eyes is to detect and respond to predators. As such, pteriomorphia respond to the presence of a shadow by retracting their

siphon A siphon (from grc, σίφων, síphōn, "pipe, tube", also spelled nonetymologically syphon) is any of a wide variety of devices that involve the flow of liquids through tubes. In a narrower sense, the word refers particularly to a tube in a ...

, adduction, digging, or some combination of the three. Beyond this shadow response, however, pteriomorphia typically do not respond to other visual stimuli. Pteriomorphia have much higher rates of eye loss than eye gain and studying eye loss and gain can yield insights into the mechanisms behind convergent evolution and the evolution and regression of complex traits. Eyes evolved exclusively in epifaunal lineages, and have been lost in some lineages that adopted infaunal and semi-infaunal lifestyles, suggesting a correlation between eye loss and adoption of infaunal or semi-infaunal lifestyles. Additionally, eyes in

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 ...

exhibit a reduction in functionality as habitat depth increases, ending in the complete absence of eyes in deep sea species.

Phylogeny

The

cladogram A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to ...

is based on molecular phylogeny using mitochondrial (12S, 16S) and nuclear (18S, 28S, and H3) gene markers by Yaron Malkowsky and Annette Klussmann-Kolb in 2012.

2010 Taxonomy

In 2010 a new proposed classification system for the Bivalvia was published by Bieler, Carter & Coan revising the classification of the Bivalvia, including the subclass Pteriomorphia. However, the following taxonomy represents the current accepted arrangement of this subclass according to the World Register of Marine Species Subclass: Pteriomorphia

Order: Arcida

(Ark shells and bittersweet shells) *Superfamily: Arcoidea **Family: Arcidae **Family: Cucullaeidae **Family: Glycymerididae **Family: Noetiidae **Family: Parallelodontidae *Superfamily: Limopsoidea **Family: Limopsidae **Family: Philobryidae

Order: Ostreida

(True oysters and their allies) *Superfamily: Ostreoidea **Family: Gryphaeidae, the foam oysters or honeycomb oysters **Family: Ostreidae, the true oysters

Order:
Pectinida Pectinida is a taxonomic order of large and medium-sized saltwater clams, marine bivalve molluscs, commonly known as scallops and their allies. It is believed that they began evolutionarily in the late Middle Ordovician epoch; many species, of ...

(Scallops and their allies) *Superfamily:

Anomioidea The Anomioidea are a superfamily of marine bivalve molluscs that include two families, the Anomiidae and the Placunidae, the jingle shells and saddle shells. They are mainly sessile bivalves that superficially resemble true oysters (partially ...

**Family:

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 ...

, the jingle shells and saddle oysters **Family: Placunidae *Superfamily: Plicatuloidea **Family:

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 ...

, the kittenpaws *Superfamily: Dimyoidea **Family: Dimyidae, the dimyarian oysters *Superfamily:

Pectinoidea The Pectinoidea are a superfamily of marine bivalve molluscs, including the scallops and spiny oysters.Gofas, S. (2012). Pectinoidea. Accessed through: World Register of Marine Species at http://www.marinespecies.org/aphia.php?p=taxdetails&id=15 ...

**Family: Entoliidae, the entoliids **Family:

, the scallops **Family: Propeamussiidae, the mud scallops **Family:

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 ...

, the thorny oysters

Order:
Limida The Limidae or file shells are members of the only family of bivalve molluscs in the order Limida. The family includes 130 living species, assigned to 10 genera. Widely distributed in all seas from shallow to deep waters, the species are usually ...

(File shells and their allies) *Superfamily: Limoidea **Family:

Limidae The Limidae or file shells are members of the only family of bivalve molluscs in the order Limida. The family includes 130 living species, assigned to 10 genera. Widely distributed in all seas from shallow to deep waters, the species are usually ...

, the file shells

Order:
Mytilida Mytilida is an order of marine bivalve molluscs, commonly known as true mussels. There is one extant superfamily, the Mytiloidea, with a single extant family, the Mytilidae. Species in the order Mytilida are found worldwide, but they are more ...

(Saltwater mussels) *Superfamily:

Mytiloidea Mytiloidea are a superfamily of small to large saltwater mussels, marine bivalve molluscs in the order Mytilida. ''Mytiloidea'' contains two families, Mysideiellidae, which is known only from fossils and Mytilidae Mytilidae are a family of ...

**Family:

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 ...

, the sea mussels

Order: Pteriida

(Winged oysters and their allies) *Superfamily: Pinnoidea **Family: Pinnidae, the pen shells *Superfamily: Pterioidea **Family: Malleidae, the hammer oysters **Family: Pteriidae, the feather oysters **Family: Pulvinitidae, the pulvinitids

Fossil orders

* Cyrtodontida† * Praecardiida†

References

{{Taxonbar, from=Q131403 Bivalve taxonomy Mollusc subclasses