# Bivalvia
**Bivalvia** is a class of mollusks whose soft body is enclosed in a two-part hinged shell made of two valves; it includes the clams, oysters, mussels, and scallops. Like fish, bivalves breathe through gills, and as filter feeders they gather food through those same gills; many species also use a pointed, retractable foot to dig into sediment[1](https://oceanservice.noaa.gov/facts/bivalve.html). An internal organ called the mantle secretes calcium carbonate so that as the animal grows, the outer shell provides a roomier home[1](https://oceanservice.noaa.gov/facts/bivalve.html). The group, estimated at about 9,200 species, inhabits virtually the entire world ocean, from the tropics to the sub-zero Arctic and from the deep sea to sandy and rocky shorelines; a few live around Pacific hydrothermal vents below 13,000 feet (4,000 m)[1](https://oceanservice.noaa.gov/facts/bivalve.html).
| Key fact | Detail |
|---|---|
| Body plan | Two hard, usually bowl-shaped valves enclosing the soft body, joined by an elastic hinge; the mantle secretes the calcium carbonate shell[1](https://oceanservice.noaa.gov/facts/bivalve.html) • [2](https://www.bgs.ac.uk/discovering-geology/fossils-and-geological-time/bivalves/) |
| Species diversity | About 9,200 species, from shorelines to hydrothermal vents below 4,000 m[1](https://oceanservice.noaa.gov/facts/bivalve.html) |
| Feeding | Most filter waterborne food particles through the gills; some extract nutrients directly from sediment[1](https://oceanservice.noaa.gov/facts/bivalve.html) • [2](https://www.bgs.ac.uk/discovering-geology/fossils-and-geological-time/bivalves/) |
| Major lineages | Six monophyletic lineages of modern Bivalvia: Protobranchia, Pteriomorphia, Palaeoheterodonta, Archiheterodonta, Anomalodesmata, and Imparidentia[3](https://doi.org/10.1071/IS13010) |
| Fossil record | First appear in the Early Cambrian (Fordilla); over 500 million years of history[4](https://www.digitalatlasofancientlife.org/learn/mollusca/bivalvia/evolutionary-history/) • [2](https://www.bgs.ac.uk/discovering-geology/fossils-and-geological-time/bivalves/) |
| Economic value | U.S. commercial bivalve harvesting was valued at about $1 billion in 2011 (153.6 million pounds)[1](https://oceanservice.noaa.gov/facts/bivalve.html) |
| Ecosystem services | Global non-food bivalve aquaculture services worth $6.47 billion ($2.95–9.99 billion) per year[5](https://aquaculture.ec.europa.eu/knowledge-base/scientific-papers-publications/global-review-ecosystem-services-provided-bivalve) |
## Shell and anatomy
The valves are layers of crystals of the minerals calcite or aragonite; different crystal-layer arrangements confer particular strengths that help bivalves adapt to varied environments[2](https://www.bgs.ac.uk/discovering-geology/fossils-and-geological-time/bivalves/). A hinge of interlocking teeth and sockets along the inside edge of each valve guides opening and closing, and muscle attachment scars, teeth, and sockets are important for classification[2](https://www.bgs.ac.uk/discovering-geology/fossils-and-geological-time/bivalves/). Shell shape reflects mode of life: streamlined burrowing forms contrast with the irregular oysters that encrust hard surfaces, the bowl-shaped Jurassic oyster *[Gryphaea](https://www.edgepedia.org/gryphaea)* rested on soft mud, and scallops such as *Pecten* escaped predators by rapid flapping of their fan-like shells[2](https://www.bgs.ac.uk/discovering-geology/fossils-and-geological-time/bivalves/).
Feeding and breathing are coupled in most bivalves: water is drawn over the gills, which both extract oxygen and trap food particles[1](https://oceanservice.noaa.gov/facts/bivalve.html). In the Mesozoic, the evolution of extendable siphons, tubes of soft tissue, allowed bivalves to burrow more deeply while keeping their food supply accessible; a shell feature called the pallial sinus stored the retracted siphons when danger threatened[2](https://www.bgs.ac.uk/discovering-geology/fossils-and-geological-time/bivalves/).
## Classification and phylogeny
Traditional bivalve classification relied on prominent morphological features such as gill structure, stomach morphology, hinge teeth, and siphons; molecular data now play a much larger role, and the major features of the bivalve Tree of Life have stabilized in recent years[6](https://www.digitalatlasofancientlife.org/learn/mollusca/bivalvia/classification/). The Bivalve Assembling the Tree of Life project combined molecular data with novel morphological characters and recognized six major monophyletic lineages of modern Bivalvia: [Protobranchia](https://www.edgepedia.org/protobranchia), [Pteriomorphia](https://www.edgepedia.org/pteriomorphia), Palaeoheterodonta, Archiheterodonta, Anomalodesmata, and Imparidentia[3](https://doi.org/10.1071/IS13010). A later family-level study using five genes sampled 219 species spanning 93, or 82%, of the 113 currently accepted bivalve families, and supported the monophyly of Autobranchia, Pteriomorphia, Heteroconchia, Palaeoheterodonta, Heterodonta, Archiheterodonta, Euheterodonta, Anomalodesmata, Imparidentia, and Neoheterodontei[7](https://www.sciencedirect.com/science/article/pii/S1055790316303104).
**Resolution is incomplete.** The same family-level study did not recover monophyly of Bivalvia or Protobranchia, as in most previous Sanger-based approaches, and found that almost 20% of bivalve families are not monophyletic[7](https://www.sciencedirect.com/science/article/pii/S1055790316303104). A transcriptomic (RNA-seq) phylogenomic analysis of 31 bivalves provided a well-resolved backbone addressing long-standing questions about the monophyly of Protobranchia and Heterodonta and resolving the positions of [Palaeoheterodonta](https://www.edgepedia.org/palaeoheterodonta), Archiheterodonta, and Anomalodesmata[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC4308999/). Combining fossil and living taxa in one phylogeny remains difficult because many fossil groups have distinctive morphologies and fossil taxa lack soft bodies and molecules[6](https://www.digitalatlasofancientlife.org/learn/mollusca/bivalvia/classification/).
Protobranchia, the lineage sister to the remaining bivalves, are deposit feeders with a wide frilly foot used for shallow plowing through soft sediment, with highest diversity in the deep sea; many constituent lineages are deep-sea endemics, which has long made collecting and preserving genetic material difficult[6](https://www.digitalatlasofancientlife.org/learn/mollusca/bivalvia/classification/) • [9](https://pubmed.ncbi.nlm.nih.gov/23742885/).
## Evolutionary history
The oldest known bivalves were tiny Early Cambrian forms, currently placed in the order Fordillida without modern representatives; *Fordilla* is characterized by small size, an anterior adductor muscle larger than the posterior, and a single hinge tooth in each valve[4](https://www.digitalatlasofancientlife.org/learn/mollusca/bivalvia/evolutionary-history/). No bivalve fossils are known between the Middle Cambrian and the Early Ordovician, when body size increased markedly and forms exploded into a variety of habits, including epifaunal and shallow infaunal (living buried within seafloor sediment) species[4](https://www.digitalatlasofancientlife.org/learn/mollusca/bivalvia/evolutionary-history/). The Great Ordovician Biodiversification Event established all six major bivalve clades still known today[10](https://www.vliz.be/imisdocs/publications/ocrd/415719.pdf).
**The Mesozoic transformation.** After the mass extinction at the end of the Permian, brachiopods suffered a much more severe extinction than bivalves, which soon emerged in the Early Triassic as the dominant group of bottom-living marine invertebrates[4](https://www.digitalatlasofancientlife.org/learn/mollusca/bivalvia/evolutionary-history/). During the Mesozoic, bivalves diversified dramatically from shallow epicontinental seas to deep-sea seeps, and cementation of one valve to the substrate evolved several times independently, including in the true oysters (Ostreidae) in the middle Triassic[4](https://www.digitalatlasofancientlife.org/learn/mollusca/bivalvia/evolutionary-history/). Mesozoic bivalves included some of the most distinctive fossil invertebrates: inoceramids, flat pteriomorphs that dominated many sea floors and include some of the largest known bivalves, occasionally reaching 2–3 m in length; rudists, highly derived heterodonts that were the most important reef-formers from the Late Jurassic to the end of the [Cretaceous](https://www.edgepedia.org/cretaceous), some reaching 2 m; and coiled oysters such as *[Exogyra](https://www.edgepedia.org/exogyra)* and *Gryphaea*[4](https://www.digitalatlasofancientlife.org/learn/mollusca/bivalvia/evolutionary-history/).
The mass extinction at the end of the Cretaceous removed approximately 65% of bivalve genera and possibly up to 80% of species; epifaunal diversity declined relative to infaunal species, and diversity quickly increased from the survivors in the [Paleogene](https://www.edgepedia.org/paleogene) and continued expanding through the Cenozoic[4](https://www.digitalatlasofancientlife.org/learn/mollusca/bivalvia/evolutionary-history/). Today both infaunal and epifaunal marine bivalves show a strong latitudinal diversity gradient, the exception being the deposit-feeding protobranchs[4](https://www.digitalatlasofancientlife.org/learn/mollusca/bivalvia/evolutionary-history/).
## Ecology and human use
By filtering water and particulates, bivalves create substrates that provide nursery habitat for other species[5](https://aquaculture.ec.europa.eu/knowledge-base/scientific-papers-publications/global-review-ecosystem-services-provided-bivalve). Suspension-feeding bivalves remove phytoplankton and particulates from the water column, clarifying the water and transferring organic- and nutrient-rich material to the bottom[11](https://www.nationalacademies.org/read/12802/chapter/9). Measured across bivalves, tissue dry weight averages 44.9% carbon, 9.3% nitrogen, and 0.9% phosphorus, whereas shell averages 11.7% carbon, 0.3% nitrogen, and 0.04% phosphorus.[12](https://doi.org/10.1111/raq.12301) A global review valued non-food bivalve aquaculture ecosystem services at $6.47 billion per year, with much of the extra value in the use of shell as aggregate[5](https://aquaculture.ec.europa.eu/knowledge-base/scientific-papers-publications/global-review-ecosystem-services-provided-bivalve) • [12](https://doi.org/10.1111/raq.12301).
**Commercial harvesting** of bivalve mollusks in the United States was estimated by NOAA at about $1 billion in 2011, with a harvest weight of 153.6 million pounds[1](https://oceanservice.noaa.gov/facts/bivalve.html). Because bivalves accumulate contaminants from filtered water, depuration, controlled purification in clean water, is used to reduce pathogenic microorganisms before the animals are eaten[13](https://www.fao.org/4/i0201e/i0201e00.htm).
Freshwater bivalves tell a different story. Freshwater bivalves first appear in the fossil record during the Devonian, yet it was in the Triassic, when the Unionoida emerged, that non-marine bivalves diversified substantially; this order now comprises over 960 species and, because of habitat destruction and its specialized life cycle, ranks among Earth's most threatened animal groups.[4](https://www.digitalatlasofancientlife.org/learn/mollusca/bivalvia/evolutionary-history/)
## Fossils as geological tools
Bivalves indicate the environment in which their rocks formed, distinguishing marine, brackish, and freshwater deposits; abundant fossil oysters, for example, suggest shallow water or proximity to ancient shorelines[2](https://www.bgs.ac.uk/discovering-geology/fossils-and-geological-time/bivalves/). Inoceramid remains in the Late Cretaceous Chalk Group have been used to refine Chalk biostratigraphy and aid correlation[2](https://www.bgs.ac.uk/discovering-geology/fossils-and-geological-time/bivalves/). Their use for dating is limited, however, because environmental factors strongly influence bivalve distribution and their rates of evolution are rather slow[2](https://www.bgs.ac.uk/discovering-geology/fossils-and-geological-time/bivalves/).
## References
1. [What is a bivalve mollusk? NOAA National Ocean Service](https://oceanservice.noaa.gov/facts/bivalve.html)
2. [Bivalves, British Geological Survey](https://www.bgs.ac.uk/discovering-geology/fossils-and-geological-time/bivalves/)
3. [Investigating the Bivalve Tree of Life – an exemplar-based approach combining molecular and novel morphological characters, Invertebrate Systematics (Bieler et al., 2014)](https://doi.org/10.1071/IS13010)
4. [Evolutionary History of Bivalves, Digital Atlas of Ancient Life](https://www.digitalatlasofancientlife.org/learn/mollusca/bivalvia/evolutionary-history/)
5. [A global review of the ecosystem services provided by bivalve aquaculture, EU Aquaculture Knowledge Base](https://aquaculture.ec.europa.eu/knowledge-base/scientific-papers-publications/global-review-ecosystem-services-provided-bivalve)
6. [Bivalve Phylogeny and Classification, Digital Atlas of Ancient Life](https://www.digitalatlasofancientlife.org/learn/mollusca/bivalvia/classification/)
7. [A family-level Tree of Life for bivalves based on a Sanger-sequencing approach, Molecular Phylogenetics and Evolution (Combosch et al., 2017)](https://www.sciencedirect.com/science/article/pii/S1055790316303104)
8. [A phylogenetic backbone for Bivalvia: an RNA-seq approach, Proceedings of the Royal Society B (Giribet et al., 2015)](https://pmc.ncbi.nlm.nih.gov/articles/PMC4308999/)
9. [Into the deep: a phylogenetic approach to the bivalve subclass Protobranchia, PubMed](https://pubmed.ncbi.nlm.nih.gov/23742885/)
10. ["Winners" and "Losers" of the Bivalve Evolution, VLIZ](https://www.vliz.be/imisdocs/publications/ocrd/415719.pdf)
11. [Ecosystem Concepts for Sustainable Bivalve Mariculture, National Academies Press](https://www.nationalacademies.org/read/12802/chapter/9)
12. [A global review of the ecosystem services provided by bivalve aquaculture (Schatte Olivier et al., 2020), Reviews in Aquaculture](https://doi.org/10.1111/raq.12301)
13. [Bivalve depuration: fundamental and practical aspects, FAO Fisheries Technical Paper](https://www.fao.org/4/i0201e/i0201e00.htm)
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*Topic: Encyclopedia › Life and health › Animals › Invertebrates › Molluscs › Bivalves › Bivalves*
*Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 21, 2026 · Edited: Oct 15, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
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License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license