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Brachiopods (from Latin brachium, arm + New Latin -poda, foot) are a small phylum of benthic invertebrates. Also known as lamp shells (or lampshells), "brachs" or Brachiopoda, they are sessile, two-valved, marine animals with an external morphology superficially resembling bivalve to which they are not closely related. Approximately 99 percent of all brachiopod species are documented solely from the fossil record.

Description

(poss overview here)

Name

The scientific name "brachiopod" is formed from the Ancient Greek words βραχίων ("arm") and πούς ("foot"). They are often known as "lamp shells", since the curved shells of the class Rhynchonellida look rather like pottery oil-lamps.

Distinguishing features

(to be completed)

Shells and their mechanisms

Modern brachiopods range from long, and most species are about . Each has two valves (shell sections) which are biomineralized. The brachial valve bears on its inner surface the brachia ("arms") from which the phylum gets its name, and which supports the lophophore, used for filtering and respiration. The other is known as the pedicle valve, as its inner surface bears the stalk-like pedicle by which most brachiopods attach themselves to surface. The brachial and pedicle valves are often call the dorsal ("upper") and ventral ("lower"), but some paleontologists regard "dorsal" and "ventral" as incorrect terms, since they believe that the "ventral" valve was formed by folding of the upper surface under the body. Irrespective of this debate, the valves of brachiopods are differently arranged of those of bivalve molluscs, which lie on the left and right sides of the body. In most brachiopod species both valves are convex, the surfaces often bear growth lines or other ornaments, and the pedicle valve is larger than the brachial. However, the linguids, which burrow into the seabed, have valves that are smoother, flatter and of similar size and shape.

Brachiopod valves have a hinge, in which the rearmost end of the brachial valve rocks on an internal projection of the pedicle valve. The major classification of brachiopods is determined by the form of the hinges. The internal projections of articulate ("jointed") brachiopods have teeth which fit into sockets on the brachial valve, an arrangement that locks the valves together. Inarticulate brachiopods have no matching teeth and sockets, and their valves are held together only by muscles.

All brachiopods have adductor muscles, that are set on the inside of the pedicle valve and 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 lay 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 (main body cavity) and make it bulge outwards, pushing the valves apart. Both classes open the valves to about 10°. The more complex set of muscles employed by inarticulate brachiopods can also operate the valves as scissors, a mechanism that linguids use to burrow.

Each valve consists of three layers, a outer periostracum made of organic compounds and two biomineralized layers. Articulated brachiopods have a periostracum made of proteins, a "primary layer" of calcite (a form of calcium carbonate under that, and finally a mixture of proteins and calcite. Inarticulate's shells have a similar sequence of layers, but their composition is different from that articulated brachiopods and also varies between the classes of inarticulate brachiopods. Linguids and discinids, which have pedicles, have a matrix of glycosaminoglycans (long, unbranched polysaccharides), in which other material 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 hard surfaces, have a periostracum of chitin and mineralized layers of calcite.

Mantle

Like molluscs, brachiopods have a mantle, an epithelium that 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 (main body cavity) 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 upper side 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 (hollow extensions) 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. The body excluding lophophore and mantle cavity account for about 25% of the internal space among articulate species with diverticula, which are majority, but about 60% among articulate species that have no diverticula accounts. This suggests that the lophophore is a severe constraint on the body mass of brachiopods, and that diverticula provide extra living space.

Lophophore

Like bryozoans and phoronids, brachiopods have a lophophore, a crown of tentacles whose cilia (fine hairs) create a water current that enables them to filter food particles out of the water. However a bryozoan or phoronid lophophore is a ring of tentacles mounted on a single, retracted stalk, while the basic form of the brachiopod lophophore is U-shaped, forming the brachia ("arms") from which the phylum gets its name. Brachiopod lophophores are non-retractable and occupy the frontmost two-thirds of the internal space, where the valves gape when opened. To provide enough filtering capacity in this restricted space, lophophores of larger brachiopods are folded in moderately to very complex shapes – loops and coils are common, and some species' lophophores resemble a hand with the fingers splayed. In all species the lophophore is supported by cartilage and by a hydrostatic skeleton (in other words by the pressure of its internal fluid), and the fluid extends into the tentacles. Some articulate brachiopods also have a pair of brachidia, calcareous struts suspended on the inside of the brachial valve and shaped similarly to the lophophore that it supports.

The tentacles bear cilia (fine mobile hairs) on their edges and along the center. The beating of the outer cilia drives a water current from the tips of the tentacles to their bases, where it exits. Food particles that collide with the tentacles are trapped by mucus, and the cillia down the middle drive this mixture to their bases. A brachial groove runs rounds the bases of the tentacles, and its own cilia pass food along the groove towards the mouth. The method used by brachiopod is known as "upstream collecting", as food particles are captured as they enter the field of cilia that creates the feeding current. This method is used by the related phoronids and bryozoans, and also by pterobranchs. Entoprocts use a similar-looking crown of tentacles, but theirs are solid and the flow runs from bases to tips, forming a "downstream collecting" system that catches food particles as they are about to exit.

Attachment to substrate

Most modern species attach to hard surfaces by means of a cylindrical pedicle ("stalk"), an extension of the body wall. This has a chitinous cuticle (non-cellular "skin") and protrudes through a opening in the hinge. However, some genera such as the inarticulate Crania and the articulate Lacazella have no pedicle, and cement the rear of the "pedicle" valve to a surface so that the front is slightly inclined up away from the surface. In a few articulate genera such as Neothyris and Anakinetica, the pedicles wither as the adults grow, and they finally lie loosely on the surface. In these genera the shells are thickened and shaped so that the opening of the gaping valves is kept free of the sediment.

Pedicles of inarticulate species are extensions of the main coelom, which houses the internal organs. A layer of longitudinal muscles lines the epidermis of the pedicle. Members of the order Lingulida have long pedicles, which they use to burrow into soft surfaces, to raise the shells to the opening of the burrow to feed,and to retract them when disturbed. A lingulid moves its body up and down the top two-thirds of the burrow, while the remaining third is occupied only by the pedicle, with a bulb on the end that builds a "concrete" anchor. However, the pedicles of the order Discinida are short and attach to hard surfaces.

An articulate pedicle has no coelom, is constructed from a different part of the larval body, and has a core composed of connective tissue. Muscles at the rear of the body can straighten, bend or even rotate the pedicle. The far end of the pedicle generally has rootlike extensions or short papillae ("bumps"), which attach to hard surfaces. However, articulate brachiopods of genus Chlidonophora use a branched pedicle to anchor in sediment. The pedicle emerges from the pedicle valve, either through a notch in the hinge or, in species where the pedicle valve is longer than the brachial, from a hole where the pedicle valve doubles back to touch the brachial valve. Some species stand with the front end upwards, while others lie horizontal with the pedicle valve uppermost.

Feeding and excretion

The water flow enters the lophophore from the sides of the open valves, and exits at the front of the animal. In linguids the entrance and exit channels are formed by groups of chaetae that function as funnels. In other brachiopods the entry and exit channels are organized by the shape of the lophophore. The lophophore captures food particles, especially phytoplankton (tiny photosynthetic organisms), and deliver them to the mouth via the brachial grooves along the bases of the tentacles. The cilia of the lophophore can change direction to eject isolated partciles of indigestible matter. If the animal encounters larger lumps of undesired matter, the cilia lining the entry channels pause and the tentacles in contact with the lumps move apart to form large gaps and then slowly use their cilia to dump the lumps on to the lining of the mantle. This has its own cilia, which wash the lumps out through the gape between the valves. If the lophophore is clogged, the adductors snapped the valves sharply, which creates a "sneeze" that clears the obstructions.

The mouth is at the base of the lophophore. Food passes through the mouth, muscular pharynx ("throat") and oesophagus ("gullet"), all of which are lined with cilia and cells that secrete mucus and digestive enzymes. The stomach wall has branched ceca ("pouches") where food is digested, mainly within the cells.

Nutrients are transported throughout the coelom, include the mantles lobes, by cilia. The wastes produced by metabolism are broken into ammonia, which is eliminated by diffusion through the mantle and lophophore. Brachiopods have metanephridia, used by many phyla to excrete ammonia and other dissolved wastes. However, brachiopods have no sign of the podocytes which perform the first phase of excretion in this process, and brachiopod metanephridia appear to be used only to emit sperm and ova.

The majority of brachiopods' food is digestible, with very little solid waste to be removed. Some inarticulate brachiopods have a U-shaped digestive tract ending with an anus that eliminates solids from the front of the body wall. Other inarticulate brachiopods and all articulate brachiopods have a curved gut that ends blindly, with no anus. These animals bundle solid waste with mucus and periodically "sneeze" it out, using sharp contractions of the gut muscles.

Circulation and respiration

The lophophore and mantle are the only surfaces that absorb oxygen and eliminate carbon dioxide. Oxygen seems to be distributed by the fluid of the coelom, which is circulated through the mantle and driven either by contractions of the lining of the coelom or by beating of its cilia. In some species oxygen is partly carried by the respiratory pigment hemerythrin, which is transported in coelomocyte cells.

Brachipods also have colorless blood, circulated by a muscular heart which lies in the dorsal part of the body above the stomach. The blood passes through vessels that extend to the front and back of the body, and branch to organs including the lophophore at the front and the gut, muscles, gonads and nephridia at the rear. The blood circulation seems not to be completely closed, and the coelomic and blood fluids must mix to a degree. The main function of the blood may be to deliver nutrients.

Nervous system and senses

The "brain" of adult articulates consists of two ganglia, one above and the other below the oesophagus. Adult inarticulates have only the lower ganglion. From the ganglia and the commissures where they join, nerves run to the lophophore, the mantle lobes and the muscles that operate the valves. The edge of the mantle has probably the greatest concentration of sensors. Although not directly connected to sensory neurons, the mantle's chaetae probably send tactile signals to receptors in the epidermis of the mantle. Many brachiopods close their valves if shadows appear above them, but the cells responsible for this are unknown. Some brachiopods have statocysts which detect changes in the animals' balance.

Reproduction and lifecycle

Lifespans range from from 3 to over 30 years. Adults of most species are of one sex throughout their lives. The gonads are masses of developing gametes (ova or sperm), and most species have four gonads, two in each valve. Those of articulates lie in the channels of the mantle lobes, while those of inarticulates lie near the gut. Ripe gametes float into the main coelom and then exit into the mantle cavity via the metanephridia, which open on either side of the mouth. Most species release both ova and sperm into the water, but females of some species keep the embryos in brood chambers until the larvae hatch.

The cell division in the embryo is radial (forms stacks of rings directly above each other), holoblastic (cells are separate, although adjoining) and regulative (the type of tissue into which a cell develops is controlled by interactions between adjacent cells, rather than rigidly within in each cell). While some animals develop the mouth and anus by deepening the blastopore, a "dent" in the surface of the early embryo, the blastopore of brachiopods closes up, and their mouth and anus develop from new openings.

The larvae of inarticulates swim as plankton for months, and are miniature adults, with valves, mantle lobes, a pedicle that coils in the mantle cavity, and a miniature lophophore, which is used for both feeding and swimming – except that Craniids have no pedicle. As the shell becomes heavier, the juvenile sinks to the bottom and becomes a sessile adult. The larvae of an articulate species live only on yolk, and remain only among the plankton for only a few days. This type of larva has a ciliated frontmost lobe that becomes the body and lophophore, a rear lobe that becomes the pedicle, and a mantle like a skirt, with the hem towards the rear. On metamorphosing into an adult, the pedicle attaches to a surface, the front lobe develops the lophophore and other organs, and the mantle rolls up over the front lobe and starts to secrete the shell.

Brachiopods' maximum oxygen consumption is low, and their minimal is not measurable. In cold seas, brachiopod growth is seasonal and the animals often lose weight in winter. These variations in growth often form growth lines in the shells. Members of some genera have survived for a year in aquaria without food.

Taxonomy

In addition the "traditional" classification, defined in 1869, two further approaches were established in the 1990s:
  • In the "traditional" classification, the Articulata have toothed hinges between the valves, while the hinges of the Inarticulata are held together only by muscles.
  • In another approach, based on the materials of which the shells are based, the Craniida and the traditional the "articulate" group are united in the Calciata, having calcite shells, while the Lingulida and Discinida, combined in the Lingulata, have shells made of chitin and calcium phosphate.
  • A three-part scheme places the Craniida in a group separate of its own, the Craniformea. The Lingulida and Discinida are grouped as Linguliformea, and the Rhynchonellida and Terebratulida as Rhynchonelliformea.
Classifications of brachiopods
"Traditional" classification Inarticulata Articulata
"Calciata" approach Lingulata Calciata
Three-part approach Linguliformea Craniformea Rhynchonelliformea
Orders Lingulida Discinida Craniida Terebratulida Rhynchonellida
Hinge No teeth Teeth and sockets
Anus On front of body, at end of U-shaped gut None
Pedicle Contains coelom with muscles running through No pedicle No coelom, muscles where joins body
Long, burrows Short, attached to hard surfaces None, cemented to surface Short, attached to hard surfaces
Periostracum Glycosaminoglycans and chitin Chitin Proteins
Primary (middle) mineralized layer of shell Glycosaminoglycans and apatite (calcium phosphate) Calcite (a form of calcium carbonate)
Inner mineralized layer of shell Collagen and other proteins, chitinophosphate and apatite (calcium phosphate) Calcite Proteins and calcite
Chetae around opening of shells Yes No Yes
Coelom fully divided Yes No Yes


About 330 living species are recognized, grouped into over 100 genera. The great majority of modern brachiopods are rhynchonelliforms (Articulata excluding Craniida).

Ecology

Distribution and habitat

Brachiopods live only in the sea. Most species avoid locations with strong currents or waves, and typical sites include rocky overhangs, crevices and caves, steep slopes of continental shelves, and in the bottoms of deep oceans. However, some articulate species attach to kelp, and in exceptionally sheltered sites in intertidal zones. The smallest living brachiopod, Gwynia, is only about long, and lives between gaps in gravel.

Rhynchonelliforms (Articulata excluding Craniida), whose larvae live only their yolks and settle quickly, specialize in specific areas and form dense populations that can reach thousands per meter, and young adults often attach to the shells of more mature ones. One other hand inarticulate brachipods, whose larva swim for up to a month before settling, have wide ranges and members of the genus discinoid Pelagodiscus are found all over the seas.

Interactions with other organisms

Brachiopods' metabolisms are 3 to 10 times slower than those of bivalves. While brachiopods were abundant in warm, shallow seas during the Cretaceous period, they have been out-bred by bivalves, and now live mainly in cold and low-light conditions.

Brachiopod shells occasionally show evidence that they have suffered and sometimes repaired damage caused by predators. Fish and crustaceans seem to find brachiopod flesh distasteful. The fossil record shows that drilling predators like gastropods attacked molluscs and echinoids 10 to 20 times more often than they do brachiopods, and suggests that such predators attacked brachiopods by mistake and/or only when other prey was scarce.

In waters where food is scarce, the snail Capulus ungaricus steals food from bivalves, snails, tube worms, and brachiopods.

Among brachiopods only the lingulids have been fished commercially, on a very small scale.

Evolutionary history

Fossil record

 


 
Timeline of major fossil brachiopod groups
Era  Paleozoic Mesozoic Cen
Period  Cm O S D C P Tr J K Pg N
                       


 
Lingulata

   
 

 
Obolellida

   
 

 
Strophomenida

   
 

 
Orthodida

   
 

 
Pentamerida

   
 

 
Rhynchonellida

   
 

 
Spiriferida

   
 

 
Terebratulida

   
 



Over 12,000 fossil species are recognized, grouped into over 5,000 genera. While the largest modern brachiopods are long, a few fossils measure up to wide. The earliest confirmed brachiopods have been found the early Cambrian, with the hingeless, inarticulate forms appearing first, followed soon after by the hinged, articulate forms. Three unmineralized species have also been found in the Cambrian, and apparently represent two distinct groups that evolved from mineralized ancestors. The inarticulate Lingula is often called a "living fossil", as very similar genera have been all the way back to the Ordovician. On the other hand, articulate brachiopods have produced major diversifications, and severe mass extinctions – but the articulate Rhynchonellida and Terebratulida, the most diverse present-day groups, appeared at the start of the Ordovician and Carboniferous respectively.

At their peak in the Paleozoic the brachiopods were among the most abundant filter-feeders and reef-builders, and occupied other ecological niches, including swimming in the jet-propulsion style of scallops. However, after the Permian–Triassic extinction event, informally known as the "Great Dying", brachiopods recovered only a third of their former diversity. They were still quite diverse and adundant in the Jurassic and Cretaceous, but have slowly declined.

Scientists have found that brachiopod fossils have been useful indicators of climate changes during the Paleozoic era. When global temperatures were low, as in much of the Ordovician, the large range of temperatures between equator and the polar created different collections of fossils at different latitudes. On the other hand, warmer periods, such much of the Silurian, created smaller ranges of temperatures, and the low to midde latitudes had only a few brachiopod species that lived in all the warmer seas.

Brachiopods are extremely common fossils throughout the Paleozoic. The major shift came with the Permian extinction. Before this extinction event, brachiopods were more numerous and diverse than bivalve mollusks. Afterwards, in the Mesozoic, their diversity and numbers were drastically reduced and they were largely replaced by bivalve mollusks. Mollusks continue to dominate today, and the remaining orders of brachiopods survive largely in fringe environments.

The origin of the brachiopods is unclear; two hypotheses suggest how a bivalved lifestyle could have emerged.

The most abundant modern brachiopods are the Class Terebratulida. The perceived resemblance of terebratulid shells to ancient oil lamps gave the brachiopods their common name "lamp shell". The phylum most closely related to Brachiopoda is probably the small phylum Phoronida (known as "horseshoe worms"). Along with the Bryozoa and possibly the Entoprocta, these phyla constitute the informal superphylum Lophophorata.

The brachiopods evolved in the lower Cambrian, and became particularly numerous in shallow water habitats during the Ordovician & Silurian, in some cases forming whole banks in much the same way as bivalves (such as mussels) do today. In some places, large sections of limestone strata and reef deposits are composed largely of their shells.

Throughout their long geological history, the brachiopods have gone through several major proliferations and diversifications, and have also suffered from major extinctions.

It has been suggested that the slow decline of the brachiopods over the last 100 million years or so is a direct result of (1) the rise in diversity of filter feeding bivalves, which have ousted the brachiopods from their former habitats; (2) the increasing disturbance of sediments by roving deposit feeders (including many burrowing bivalves); and/or (3) the increased intensity and variety of shell-crushing predation. However, a famous paper by Stephen Jay Gould suggested that the rise in bivalves which accompanied the downfall of the brachiopods was nothing more than coincidence - the two lineages were like "ships that pass in the night". The greatest successes for the bivalves have been in habitats that have never been adopted by the brachiopods, such as burrowing.

The abundance, diversity, and rapid evolution of brachiopods during the Paleozoic make them useful as index fossils when correlating strata across large areas.

Classification

In older classification schemes, Phylum Brachiopoda was divided into two classes: Articulata and Inarticulata. Since most orders of brachiopods have been extinct since the end of the Paleozoic Era, classifications have always relied extensively on the morphology (that is, the shape) of fossils. In the last 40 years further analysis of the fossil record and of living brachiopods, including genetic study, has led to changes in taxonomy.

The taxonomy is still unstable, however, so different authors have made different groupings. This article follows Williams, Carlson & Brunton (2000), who subdivide Brachiopoda into three subphyla, eight classes, and 26 orders. These categories are believed to be approximately monophyletic. Brachiopod diversity declined significantly at the end of the Paleozoic. Only five orders in three classes include forms that survive today, a total of between 300 and 500 extant species. In their zenith during the mid-Silurian Period, 16 orders of brachiopods coexisted.

Extant taxa in green, extinct taxa in grey
after Williams, Carlson, and Brunton, 2000
     


   


 
Brachiopod Taxonomy

Subphyla Class Order Extinct
Linguliformea Lingulata
Lingulida no
Siphonotretida Ordovician
Acrotretida Devonian
Paterinata Paterinida Ordovician
Craniformea Craniforma
Craniida no
Craniopsida Carboniferous
Trimerellida Silurian
Rhynchonelliformea
Chileata Chileida Cambrian
Dictyonellidina Permian
Obolellata Obolellida Cambrian
Kutorginata Kutorginida Cambrian
Strophomenata
Orthotetidina Permian
Triplesiidina Silurian
Billingselloidea Ordovician
Clitambonitidina Ordovician
Strophomenida Carboniferous
Productida Permian
Rhynchonellata
Protorthida Cambrian
Orthida Carboniferous
Pentamerida Devonian
Rhynchonellida no
Atrypida Devonian
Spiriferida Jurassic
Thecideida no
Athyridida Cretaceous
Terebratulida no


Comparison with bivalves

While brachiopods superficially resemble bivalve molluscs, the similarities are purely a product of convergent evolution; brachiopods and bivalves belong to different phyla and thus differ markedly. Bivalves usually have a plane of symmetry between the valves of the shell, which are mirror images of each other; most brachiopods have a plane of bilateral symmetry through the valves and perpendicular to the hinge. The two brachiopod valves differ in shape and size from one another. Bivalves use adductor muscles to hold their two valves closed, and they open them by means of an external or internal ligament once the adductor muscles are relaxed. Brachiopods use internal diductor muscles to pull their two valves apart; they close the two with adductor muscles.

Furthermore, brachiopod shells are made of different minerals. While bivalves construct their shells from aragonite, Linguliformea use apatite, and the Rhynchonelliformea and Craniiformea produce calcite shells.

Gallery

Image:Onniella.jpg|Brachiopod fossils are often found in dense assemblages, such as these specimens of the Ordovician species Dalmanella meeki.Image:Brachiopoda-morphology.png|Brachiopod morphologyImage:Brachiopod Neospirifer.jpg|A Carboniferous brachiopod Neospirifer condor, from Bolivia. The specimen is 7 cm across.Image:Rhynchotremadentatum.jpg|Rhynchotrema dentatum, a rhynchonellid brachiopod from the Cincinnatian (Upper Ordovician) of southeastern Indiana.Image:HederellaOH3.jpg|A Devonian spiriferid brachiopod from Ohio that served as a host substrate for a colony of hederellids. The specimen is 5 cm wide.Image:Syringothyris01.JPG|Syringothyris sp.; a spiriferid brachiopod from the Logan Formation (Lower Carboniferous) of Wooster, Ohiomarker (internal molds).Image:PetrocraniaOrdovician.jpg|Petrocrania brachiopods attached to a strophomenid brachiopod; Upper Ordovician of southeastern Indiana.Image:LingulaanatinaAA.JPG|Lingula anatina from Stradbroke Island, Australia.

References

  • Part H of
  • MLF (Moore, Lalicker and Fischer); Invertebrate Fossils, McGraw-Hill Book, 1952


External links



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