Brachiopod

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. Brachiopods now live mainly in cold water and low light. Fish and crustaceans seem to find brachiopod flesh distasteful and seldom attack them. Among brachiopods, only the lingulids (Lingula sp.^[4]) have been fished commercially, on a very small scale. One brachiopod species (Coptothyrus adamsi) may be a measure of environmental conditions around an oil terminal being built in Russia on the shore of the Sea of Japan. The word "brachiopod" is formed from the Ancient Greek words brachion ("arm") and podos ("foot").^[5] They are often known as "lamp shells", since the curved shells of the class Terebratulida resemble pottery oil-lamps.^[2]

Brachiopods are the state fossil of the U.S. state of Kentucky.^[6]

Anatomy

Shell structure and function

Modern brachiopods range from 1 to 100 millimetres (0.039 to 3.937 in) long, and most species are about 10 to 30 millimetres (0.39 to 1.18 in).

The brachial valve is usually smaller and bears brachia ("arms") 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. (R. C. Moore, 1952) 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 ("lower") valve actually lies above the dorsal ("upper") 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. (R. C. Moore, 1952)

Articulate ("jointed") 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. (R. C. Moore, 1952)

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 (main body cavity) 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.^[10]

The inarticulate species Lingula anatina, showing the long pedicle, flattened shells and prominent chaetae around the front edge of the shells

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 (a form of calcium carbonate) under that, and innermost a mixture of proteins and calcite.^[10] 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.^[11][12]

Lingulids and discinids, which have pedicles, have a matrix of glycosaminoglycans (long, unbranched polysaccharides), in which other materials are embedded: chitin in the periostracum;^[10] apatite containing calcium phosphate in the primary biomineralized layer;^[13] and a complex mixture in the innermost layer, containing collagen and other proteins, chitinophosphate and apatite.^[10][14] Craniids, which have no pedicle and cement themselves directly to hard surfaces, have a periostracum of chitin and mineralized layers of calcite.^[10][15] 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.^[16]

Mantle

Brachiopods, as with

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.^[10] Experiments show that a brachiopod's oxygen consumption drops if petroleum jelly is smeared on the shell, clogging the diverticula.^[17]

Lophophore

Like

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 an opening in the hinge.^[10] However, some genera have no pedicle, such as the inarticulate Crania and the articulate Lacazella; they cement the rear of the "pedicle" (ventral) valve to a surface so that the front is slightly inclined up away from the surface.^[2][10] In these brachiopods, the ventral valve lacks a pedicle opening.^[24] In a few articulate genera such as Neothyris and Anakinetica, the pedicles wither as the adults grow and 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.^[2]

Pedicles of inarticulate species are extensions of the main coelom, which houses the internal organs. A layer of longitudinal muscles lines the

The pedicle of articulate brachiopods has no coelom, and its homology is unclear. It is constructed from a different part of the larval body, and has a compact 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 the 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.^[10]

Some early brachiopods—for example strophomenates, kutorginates and obolellates—do not attach using their pedicle, but with an entirely different structure known as the "pedicle sheath", which has no relationship to the pedicle.^[25][26] This structure arises from the umbo of the pedicle valve, at the centre of the earliest (metamorphic) shell at the location of the protegulum. It is sometimes associated with a fringing plate, the colleplax.^[26]

Biology

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 lingulids the entrance and exit channels are formed by groups of chaetae that function as funnels.

The majority of food consumed by brachiopods is digestible, with very little solid waste produced.[29] The cilia of the lophophore can change direction to eject isolated particles 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 onto the lining of the mantle. This has its own cilia, which wash the lumps out through the opening between the valves. If the lophophore is clogged, the adductors snap the valves sharply, which creates a "sneeze" that clears the obstructions.^[17] In some inarticulate brachiopods the digestive tract is U-shaped and ends with an anus that eliminates solids from the front of the body wall.^[27] Other inarticulate brachiopods and all articulate brachiopods have a curved gut that ends blindly, with no anus.^[10] These animals bundle solid waste with mucus and periodically "sneeze" it out, using sharp contractions of the gut muscles.^[17]

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.^[10] The maximum oxygen consumption of brachiopods is low, and their minimum requirement is not measurable.

Brachiopods also have colorless blood, circulated by a muscular heart lying in the dorsal part of the body above the stomach.^[10] 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 fluid and blood must mix to a degree.^[17] The main function of the blood may be to deliver nutrients.^[10]

Nervous system and senses

The "brain" of adult articulates consists of two

Brachiopod fossils show great diversity in the morphology of the shells and lophophore, while the modern genera show less diversity but provide soft-bodied characteristics. Both fossils and extant species have limitations that make it difficult to produce a comprehensive classification of brachiopods based on morphology. The phylum also has experienced significant convergent evolution and reversals (in which a more recent group seems to have lost a characteristic that is seen in an intermediate group, reverting to a characteristic last seen in an older group). Hence some brachiopod taxonomists believe it is premature to define higher levels of classification such as order, and recommend instead a bottom-up approach that identifies genera and then groups these into intermediate groups.^[33]

However, other taxonomists believe that some patterns of characteristics are sufficiently stable to make higher-level classifications worthwhile, although there are different views about what the higher-level classifications should be.^[33] The "traditional" classification was defined in 1869; two further approaches were established in the 1990s:^[14][34]

• In the "traditional" classification, brachiopods are divided into the Articulata and Inarticulata. The Articulata have toothed hinges between the valves, while the hinges of the Inarticulata are held together only by muscles.^[10][14]
• A classification devised in the 1990s, based on the materials of which the shells are based, united the
  Craniida and the "articulate" brachiopods in the Calciata, which have calcite shells. The Lingulida and Discinida, combined in the Lingulata, have shells made of chitin and calcium phosphate.^[14]
• A three-part scheme, also from the 1990s, places the
  Craniiformea. The Lingulida and Discinida are grouped as Linguliformea,^[35] and the Rhynchonellida and Terebratulida as Rhynchonelliformea.^[36][37]

About 330 living species are recognized,[14] grouped into over 100 genera. The great majority of modern brachiopods are rhynchonelliforms (Articulata).^[2]

Modern classification

Genetic analysis performed since the 1990s has extended the understanding of the relationship between different organisms. It is now clear the brachiopods do not belong to the

• Subphylum Linguliformea
  • Class Lingulata
    • Order Lingulida
    • Order †Acrotretida
    • Order †Siphonotretida
  • Class †Paterinata
    • Order †
      Paterinida
• Subphylum
  Craniiformea
  • Class Craniata
    • Order Craniida
    • Order †
      Craniopsida
    • Order †Trimerellida
• Subphylum Rhynchonelliformea
  • Class †Chileata
    • Order †
      Chileida
    • Order †Dictyonellida
  • Class †Obolellata
    • Order †
      Obolellida
    • Order †
      Naukatida
  • Class †Kutorginata
    • Order †
      Kutorginida
  • Class †Strophomenata
    • Order †Billingsellida
    • Order †Strophomenida
    • Order †Productida
    • Order †Orthotetida
  • Class Rhynchonellata
    • Order Rhynchonellida
    • Order Terebratulida
    • Order Thecideida
    • Order †Protorthida
    • Order †Orthida
    • Order †Pentamerida
    • Order †Atrypida
    • Order †Athyridida
    • Order †Spiriferida
    • Order †Spiriferinida

Ecology

Distribution and habitat

Brachiopods are an entirely marine phylum, with no known freshwater species. Most species avoid locations with strong currents or waves, and typical sites include rocky overhangs, crevices and caves, steep slopes of

Among brachiopods only the lingulids have been fished commercially, and only on a very small scale.^[44] It is mostly the fleshy pedicle that is eaten.^{[45][46][47][48]} Brachiopods seldom settle on artificial surfaces, probably because they are vulnerable to pollution. This may make the population of Coptothyrus adamsi useful as a measure of environmental conditions around an oil terminal being built in Russia on the shore of the Sea of Japan.^[1]

Evolutionary history

Fossil record

Over 12,000 fossil species are recognized,

However, fossils from 2007 onwards have supported a new interpretation of the Early-Cambrian tommotiids, and a new hypothesis that brachiopods evolved from tommotiids. The "armor mail" of tommotiids was well-known but not in an assembled form, and it was generally assumed that tommotiids were slug-like animals similar to Halkieria, except that tommotiids' armor was made of organophosphatic compounds while that of Halkieria was made of calcite. However, fossils of a new tommotiid, Eccentrotheca, showed an assembled mail coat that formed a tube, which would indicate a sessile animal rather than a creeping slug-like one. Eccentrotheca's organophosphatic tube resembled that of phoronids,^[54] sessile animals that feed by lophophores and are regarded either very close relatives or a sub-group of brachiopods.^[55] Paterimitra, another mostly assembled fossil found in 2008 and described in 2009, had two symmetrical plates at the bottom, like brachiopod valves but not fully enclosing the animal's body.^[56]

See also

• Taxonomy of the Brachiopoda
• Evolution of brachiopods
• List of brachiopod genera
• List of brachiopod species
• Novocrania anomala
• Margaret Jope

Notes

References

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Further reading

• Candela, Y. (2003). "Late Ordovician brachiopods of the Bardahessiagh Formation of Pomeroy, Ireland". Monograph of the Palaeontographical Society London (618, part of Volume 156): 1–95, pls 1–12.
• Harper, D.A.T. (2006) [1984]. "Brachiopods from the Upper Ardmillan succession (Ordovician) of the Girvan district, Scotland. Parts 1-3". Monograph of the Palaeontographical Society London: 1–187, pls 1–33.
• Moore, R.C.; Lalicker, C.G.; Fischer, A.G. (June 1952). Invertebrate Fossils. Mcgraw-Hill College.
  ISBN 978-0-07-043020-4
  .
• Vinn, O.; Wilson, M.A. & Toom, U. (2014). "Earliest rhynchonelliform brachiopod parasite from the Late Ordovician of northern Estonia (Baltica)". Palaeogeography, Palaeoclimatology, Palaeoecology. 411: 42–45.
  doi:10.1016/j.palaeo.2014.06.028
  . Retrieved 2014-01-09.

External links

Wikimedia Commons has media related to Brachiopoda.

Wikisource has the text of the 1911 Encyclopædia Britannica article "Brachiopoda".

• The Evolution of Brachiopoda – 2016 overview paper by Sandra J. Carlson, Department of Earth and Planetary Sciences, University of Davis, California, at Annual Reviews Archived 2020-04-23 at the Wayback Machine
• BrachNet
• Brachiopoda Database
• Brachiopoda World Database - used from 1995 to 2015
• Information from the Kansas Geological Survey
• Site of R.Filippi
• UC-Berkeley Museum of Paleontology
• Palaeos Brachiopoda (last updated 2002)