Brachiopod

Brachiopod Temporal range: Lower Cambrian–Recent PreꞒ Ꞓ O S D C P T J K Pg N
Lingula anatina, an inarticulate linguliform brachiopod
Terebratalia transversa, an articulate (rhynchonelliform) brachiopod
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Superphylum:	Lophotrochozoa
Clade:	Lophophorata
Clade:	Brachiozoa
Phylum:	Brachiopoda Duméril, 1806[1]
Subphyla and classes
See taxonomy
Diversity[2]
About 100 living genera About 5,000 fossil genera

Brachiopods (/ˈbrækioʊˌpɒd/), phylum Brachiopoda, are a phylum of animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection.

Two major categories are traditionally recognized, articulate and inarticulate brachiopods. The word "articulate" is used to describe the tooth-and-groove structures of the valve-hinge which is present in the articulate group, and absent from the inarticulate group. This is the leading diagnostic skeletal feature, by which the two main groups can be readily distinguished as fossils. Articulate brachiopods have toothed hinges and simple, vertically oriented opening and closing muscles. Conversely, inarticulate brachiopods have weak, untoothed hinges and a more complex system of vertical and oblique (diagonal) muscles used to keep the two valves aligned. In many brachiopods, a stalk-like pedicle projects from an opening near the hinge of one of the valves, known as the pedicle or ventral valve. The pedicle, when present, keeps the animal anchored to the seabed but clear of sediment which would obstruct the opening.

Brachiopod lifespans range from three to over thirty years. Ripe gametes (ova or sperm) float from the gonads into the main coelom and then exit into the mantle cavity. The larvae of inarticulate brachiopods are miniature adults, with lophophores (a feeding organ consisting of an array of tentacles) that enable the larvae to feed and swim for months until the animals become heavy enough to settle to the seabed. The planktonic larvae of articulate species do not resemble the adults, but rather look like blobs with yolk sacs, and remain among the plankton for only a few days before metamorphosing and leaving the water column.

Brachiopods live only in the sea, and most species avoid locations with strong currents or waves. The larvae of articulate species settle in quickly and form dense populations in well-defined areas while the larvae of inarticulate species swim for up to a month and have wide ranges. Fish and crustaceans seem to find brachiopod flesh distasteful and seldom attack them.

The word "brachiopod" is formed from the Ancient Greek words brachion ("arm") and podos ("foot").^[3] They are often known as "lamp shells", since the curved shells of the class Terebratulida resemble pottery oil-lamps.^[2]

Although superficially resembling bivalves, brachiopods are not particularly closely related, and evolved their two valved structure independently, an example of convergent evolution. Brachiopods are part of the broader group Lophophorata, alongside Bryozoa and Phoronida, with which they share the characteristic lophophores.

Brachiopods are thought to have evolved from "tommotiid" ancestors during the Early Cambrian.^[4] Brachiopods were highly diverse during the Paleozoic era, when their diversity exceeded that of bivalves.^[5] Their diversity was strongly affected by the end-Capitanian^[6] and end-Permian mass extinction events, from which their diversity would never recover to its former Paleozoic levels, with bivalves subsequently ascending to dominance in marine ecosystems.^[5] Today, there are around 400 living species of brachiopods,^[7] in comparison to around 9,200 species of bivalves.^[8] Brachiopods now live mainly in cold water and low light.

Among brachiopods, only the lingulids (Lingula sp.^[9]) have been fished commercially, on a very small scale.