Brachiopod
Brachiopods, phylum Brachiopoda, are a phylum of animals that have hard "valves" 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 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 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 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 and podos. They are often known as "lamp shells", since the curved shells of the class Terebratulida resemble pottery oil-lamps.
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. Brachiopods were highly diverse during the Paleozoic era, when their diversity exceeded that of bivalves. Their diversity was strongly affected by the end-Capitanian 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. Today, there are around 400 living species of brachiopods, in comparison to around 9,200 species of bivalves. Brachiopods now live mainly in cold water and low light.
Among brachiopods, only the lingulids have been fished commercially, on a very small scale.
Anatomy
Shell structure and function
Modern brachiopods range from long, and most species are about. Magellania venosa is the largest extant species. The largest brachiopods known—Gigantoproductus and Titanaria, reaching in width—occurred in the upper part of the Lower Carboniferous. Brachiopods have two valves, which cover the dorsal and ventral surface of the animal, unlike bivalve molluscs whose shells cover the lateral surfaces. The valves are unequal in size and structure, with each having its own symmetrical form rather than the two being mirror images of each other. The formation of brachiopod shells during ontogeny builds on a set of conserved genes, including homeobox genes, that are also used to form the shells of molluscs.The brachial valve is usually smaller and bears brachia on its inner surface. These brachia are the origin of the phylum's name, and support the lophophore, used for feeding and respiration. The pedicle valve is usually larger, and near the hinge it has an opening for the stalk-like pedicle through which most brachiopods attach themselves to the substrate. The brachial and pedicle valves are often called the dorsal and ventral valves, respectively, but some paleontologists regard the terms "dorsal" and "ventral" as irrelevant since they believe that the "ventral" valve was formed by a folding of the upper surface under the body. The ventral valve actually lies above the dorsal valve when most brachiopods are oriented in life position. In many living articulate brachiopod species, both valves are convex, the surfaces often bearing growth lines and/or other ornamentation. However, inarticulate lingulids, which burrow into the seabed, have valves that are smoother, flatter and of similar size and shape.
Articulate brachiopods have a tooth and socket arrangement by which the pedicle and brachial valves hinge, locking the valves against lateral displacement. Inarticulate brachiopods have no matching teeth and sockets; their valves are held together only by muscles.
All brachiopods have adductor muscles that are set on the inside of the pedicle valve and which close the valves by pulling on the part of the brachial valve ahead of the hinge. These muscles have both "quick" fibers that close the valves in emergencies and "catch" fibers that are slower but can keep the valves closed for long periods. Articulate brachiopods open the valves by means of abductor muscles, also known as diductors, which lie further to the rear and pull on the part of the brachial valve behind the hinge. Inarticulate brachiopods use a different opening mechanism, in which muscles reduce the length of the coelom and make it bulge outwards, pushing the valves apart. Both classes open the valves to an angle of about 10 degrees. The more complex set of muscles employed by inarticulate brachiopods can also operate the valves as scissors, a mechanism that lingulids use to burrow.
Each valve consists of three layers, an outer periostracum made of organic compounds and two biomineralized layers. Articulate brachiopods have an outermost periostracum made of proteins, a "primary layer" of calcite under that, and innermost a mixture of proteins and calcite. Inarticulate brachiopod shells have a similar sequence of layers, but their composition is different from that of articulated brachiopods and also varies among the classes of inarticulate brachiopods. The Terebratulida are an example of brachiopods with a punctate shell structure; the mineralized layers are perforated by tiny open canals of living tissue, extensions of the mantle called caeca, which almost reach the outside of the primary layer. These shells can contain half of the animal's living tissue. Impunctate shells are solid without any tissue inside them. Pseudopunctate shells have tubercles formed from deformations unfurling along calcite rods. They are only known from fossil forms, and were originally mistaken for calcified punctate structures.
Lingulids and discinids, which have pedicles, have a matrix of glycosaminoglycans, in which other materials are embedded: chitin in the periostracum; apatite containing calcium phosphate in the primary biomineralized layer; and a complex mixture in the innermost layer, containing collagen and other proteins, chitinophosphate and apatite. Craniids, which have no pedicle and cement themselves directly to hard surfaces, have a periostracum of chitin and mineralized layers of calcite. Shell growth can be described as holoperipheral, mixoperipheral, or hemiperipheral. In holoperipheral growth, distinctive of craniids, new material is added at an equal rate all around the margin. In mixoperipheral growth, found in many living and extinct articulates, new material is added to the posterior region of the shell with an anterior trend, growing towards the other shell. Hemiperipheral growth, found in lingulids, is similar to mixoperipheral growth but occurs in mostly a flat plate with the shell growing forwards and outwards.
Mantle
Brachiopods, as with molluscs, have an epithelial mantle which secretes and lines the shell, and encloses the internal organs. The brachiopod body occupies only about one-third of the internal space inside the shell, nearest the hinge. The rest of the space is lined with the mantle lobes, extensions that enclose a water-filled space in which sits the lophophore. The coelom extends into each lobe as a network of canals, which carry nutrients to the edges of the mantle.Relatively new cells in a groove on the edges of the mantle secrete material that extends the periostracum. These cells are gradually displaced to the underside of the mantle by more recent cells in the groove, and switch to secreting the mineralized material of the shell valves. In other words, on the edge of the valve the periostracum is extended first, and then reinforced by extension of the mineralized layers under the periostracum. In most species the edge of the mantle also bears movable bristles, often called chaetae or setae, that may help defend the animals and may act as sensors. In some brachiopods groups of chaetae help to channel the flow of water into and out of the mantle cavity.
In most brachiopods, diverticula of the mantle penetrate through the mineralized layers of the valves into the periostraca. The function of these diverticula is uncertain and it is suggested that they may be storage chambers for chemicals such as glycogen, may secrete repellents to deter organisms that stick to the shell or may help in respiration. Experiments show that a brachiopod's oxygen consumption drops if petroleum jelly is smeared on the shell, clogging the diverticula.