Brachiopoda

Diversity

The phylum Brachiopoda , also known as lamp shells, is a group of bilaterally symmetrical, coelomate organisms that superficially resemble bivalve molluscs. Approximately 450 species of living brachiopods are currently known, and have traditionally been divided into two classes: Inarticulata (orders Lingulida and Acrotretida ) and Articulata (orders Rhynchonellida , Terebratulida and Thecideidina ). Brachiopods range in size from 1 mm to 9 cm in length, and all known species are solitary, benthic, marine animals with a two part shell (valve); the valves of Inarticulata species are attached only by muscles, while the valves of Articulata species have a tooth-and-socket hinge. In the past 20 years, new classification systems based on more rigorous phylogenetic analyses have been proposed to replace traditional brachiopod classification and have been adopted to different degrees by scientists.

All brachiopods filter feed on planktonic organisms and possess a distinctive feeding structure called a lophophore. This structure is composed of a pair of tentacle-bearing arms that have a circular, U-shaped, or highly coiled arrangement, depending on the species, and generates the feeding currents that these organisms use to capture prey. These organisms generally broadcast spawn, although females of a few species take sperm into their mantle cavity, where fertilization occurs and eggs may be brooded. A few species are hermaphroditic. Brachiopods possess a distinct, free-living larval stage called a lobate larva, which have different morphologies and developmental trajectories in Articulata and Inarticulata species.

Although the number of living brachiopod species is relatively low compared to many other phyla, brachiopods have one of the most prolific fossil records of any organismal group, dating back to the early Cambrian Period. Over 12,000 species, most of which are now extinct, have been identified from fossils. Most abundant and diverse during the Devonian Era, the majority of brachiopods were wiped out during the Permian-Triassic mass extinction.

Geographic Range

Brachiopods are found throughout the world's marine environments.

• Biogeographic Regions
• nearctic nearctic
  • native native
• palearctic palearctic
  • native native
• oriental oriental
  • native native
• ethiopian ethiopian
  • native native
• neotropical neotropical
  • native native
• australian australian
  • native native
• antarctica antarctica
  • native native
• oceanic islands oceanic islands
  • native native
• arctic ocean arctic ocean
  • native native
• indian ocean
  • native native
• atlantic ocean atlantic ocean
  • native native
• pacific ocean pacific ocean
  • native native
• mediterranean sea
  • native native

• Other Geographic Terms
• holarctic holarctic
• cosmopolitan cosmopolitan

Habitat

Brachiopods usually attach to substrate (rock outcroppings, crevices, caves, etc.) using their fleshy pedicles, though some species burrow into sediments in shallow waters. They are found at all depths, most commonly on the continental shelf, and often in very cold waters.

• Habitat Regions
• temperate temperate
• tropical tropical
• polar polar
• saltwater or marine saltwater or marine

• Aquatic Biomes
• benthic benthic
• reef reef
• coastal coastal
• abyssal abyssal

Systematic and Taxonomic History

Brachiopods have been described and depicted in scientific works dating to the late 16th century. The name Brachiopoda ( Brachiopodes ) appears to have first been applied to these organisms by the French naturalist Georges Cuvier, who considered them a family of molluscs. André Marie Constant Duméril and his colleague at the Muséum National d'Histoire Naturelle appear to have been the first to use this name in an actual taxonomic classification in the following year (still considering them an order of molluscs), and is generally credited as the taxonomic authority for this name. Brachiopods continued to be considered related to either molluscs or annelids for the following 60 years, with the English biologist T.H. Huxley rejecting the molluscan hypothesis in 1869 and organized them into the two classes, Articulata and Inarticulata , which are used in traditional brachiopod classification. By the early to mid 20th century, biologists realized, based on numerous autapomorphic characters, that these organisms were sufficiently different from all other living groups of animals to justify their recognition as a distinct phylum.

The monophyly of phylum Brachiopoda and its constituent classes has historically been contentious. Modern morphological phylogenetic analyses support the monophyly of the phylum. The monophyly of the two traditional classes, Articulata and Inarticulata has received much weaker, or no support. This has resulted in the proposal of at least two additional classification systems for brachiopods. The first of these divides the phylum based on shell composition, with those species having shells composed of calcite placed in class Calciata (order Craniida and traditional Articulata species), while those having chitinous shells (orders Lingulida and Discinida ) are placed into class Lingulata . The second additional classification system places Craniida in its own subphylum, Craniformea , while the two remaining classes from the previous classification are elevated to subphyla Rhynchonelliformea and Linguliformea .

Historically, morphological examinations of animal phylogeny have considered brachiopods and other organisms having lophophores to be deuterostomes (organisms in which the first embryological opening (blastopore) becomes the anus, as opposed to the mouth in protostomes ). Molecular phylogenetic analyses, however, have placed lophophorates with other protostomes in the superphylum Lophotrochozoa . The analysis of lophotrochozan phylogeny remains an active area of research, with little consensus as to the relationships of its constituent taxa. Within Lophotrochozoa , brachiopods have been considered the sister group to phylum Phoronida , in the clade Brachiozoa , likewise, phoronids are sometimes considered part of Lophotrochozoa , necessitating the creation of the subphylum Phoroniformea within the phylum Brachiopoda . In either form, phylogenetic studies have often recovered this grouping as the sister group to molluscs. Other recent phylogenetic analyses, however, have strongly supported a sister relationship between brachiopods and phylum Nemertea (ribbon worms), or Brachyozoa and Nemertea , which was previously not suspected due to differences between these groups in embryonic cleavage and larval forms. These analyses have recovered this grouping ( Brachyozoa and Nemertea ) as either the sister group to phylum Annelida (segmented worms) or Mollusca within Lophotrochozoa .

Physical Description

Brachiopods may range in size from 1 mm to over 9 cm (measured along the largest shell dimension), but most commonly measure from 4 to 6 cm. They resemble bivalve mollusks but differ in important ways, most obviously in that their valves are divided into ventral and dorsal rather than left and right halves, and are unequal (the ventral (pedicle) valve is larger than the dorsal (brachial) valve). Their multi-layered valves are secreted by the mantle lobes, which arise from the body wall. Mantle lobes may have spines on their edges for protection. The outermost layer of the valve is called the periostracum and it may have spines or other growths to help keep the animal in place. Valves may be punctate, with perforations (typically housing tissue extensions of the mantle) or impunctate. The mantle lobes line the fluid-filled mantle cavity and contain mantle canals (extensions of the coelom). This is surrounded with a peritoneum, forming the outer boundaries of the coelom; a layer of connective tissue made up of longitudinal muscles; and an epidermis. The epidermis is highly ciliated on the lophophore and has columnar and cuboidal cells. Another outgrowth of the body wall, the pedicle, is located at the posterior of the ventral valve. In members of Articulata , the pedicle is operated by muscle bands extending from the body wall; the pedicles of Inarticulata species contain connective tissue, muscles, and a coelemic lumen. Papillae on the pedicles of some species help them adhere to the substrate.

All brachiopod species possess a lophophore, a structure which is formed by folds of the body wall around the mouth, with many ciliated tentacles that are used to catch food. A homologous structure is found in several other phyla. In brachiopods, the lophophore presents as a pair of arms with tentacles, extending anteriorly into the mantle cavity. It is contained within the valves and therefore is not moveable, being held in place either by coelomic pressure ( Inarticulata species) or skeletal elements ( Articulata species). The tentacles have lateral and frontal cilial tracts and create feeding currents using cilial motion. Gas exchange occurs across the mantle and tentacles. The circulatory system of brachiopods consists of a contractile heart (in the dorsal mesentery above the gut) and associated channels into the mesentery. Blood appears to be separate from coelomic fluid, although both have similar components (various coelomocytes, some with hemerythrin).

• Other Physical Features
• ectothermic ectothermic
• heterothermic heterothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• sexes alike

Development

In many species, females brood fertilized eggs in a brooding area (most often the arms of the lophophore, within the nephridia, in regions of the mantle cavity, or in depressions of the valves) until they have reached the larval stage. Cleavage is holoblastic, radial, and nearly equal, leading to a coeloblastula. Brachiopods may undergo mixed or fully indirect development but all go through a free-swimming larval stage. Larvae are known as lobate larvae. Larvae of Inarticulata species look much like the adults, but are able to protrude their lophophores from the mantle lobes and use them for feeding and locomotion. They may remain planktonic for months and sink upon beginning valve secretion. Larvae of Articulata species have anterior, mantle and pedicle lobes. After a short (1 to 2 day) larval stage, during which they feed on yolk, they settle, attaching themselves to the substrate using their pedicles, and undergo metamorphosis. The mantle lobes, which begin to secrete the valves, come forward to cover the anterior lobe, which becomes the body and lophophore.

• Development - Life Cycle
• metamorphosis metamorphosis

Reproduction

Brachiopods have transient gonads that develop from the peritoneum of the metacoel. Gametes are released through the nephridia. In most cases, fertilization is external; in a few species of brachiopods, females pick up sperm from the water and fertilization is internal.

• Mating System
• polygynandrous (promiscuous) polygynandrous (promiscuous)

Depending on environment and species, brachiopods may have a breeding season (often spring or summer for Inarticulata species or fall and winter for Articulata species) or may breed year-round. Brachiopods are most typically dioecious (only a few species including some members of genus Argtrotheca are known to be hermaphroditic) and reproduction is sexual.

• Key Reproductive Features
• iteroparous iteroparous
• seasonal breeding seasonal breeding
• year-round breeding year-round breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• simultaneous hermaphrodite
• sexual sexual
• fertilization fertilization
  • external external
  • internal internal
• oviparous oviparous

Brachiopods do not exhibit any parental investment beyond the production of gametes and, in females of some species, brooding of fertilized eggs.

• Parental Investment
• no parental involvement
• pre-fertilization
  • provisioning
• pre-hatching/birth
  • protecting
    • female

Lifespan/Longevity

Brachiopod species exhibit a reasonably wide range of lifespans, typically living from 3 to 30 years.

Behavior

Although their larvae are planktonic, if only for a few days, adults are sessile and typically attach to substrate by their pedicles. There are some solitary species that do not attach to substrate and remain free-living.

• Key Behaviors
• diurnal diurnal
• nocturnal nocturnal
• crepuscular crepuscular
• sessile sessile
• motile motile
• solitary solitary

Communication and Perception

Nerves extend from a nerve ring and dorsal and ventral ganglia to the lophophore, mantle, and associated muscles. The mantle edges and setae are supplied with tactile receptors. Some species may also be chemoreceptive via their tentacles or mantle edges. In one species of genus Lingula , a pair of statocysts is present; as a burrowing species, these structures may aid in orienting the body in the substrate.

• Communication Channels
• tactile tactile
• chemical chemical

• Perception Channels
• tactile tactile
• chemical chemical

Food Habits

In order to feed, brachiopods must separate their valves. Articulata species use a pair of diductor muscles to open the valves and adductor muscles (both striated and smooth) to close them. Inarticulata species retract their bodies to increase coelomic pressure, forcing the valves open, and use adductor muscles to close them. Brachiopod tentacles have lateral and frontal cilial tracts and a feeding current is created by cilial motion; they are filter feeders of phytoplankton and other particulates. Food is directed toward a brachial axis (lophophoral ridge), where it passes along a brachial food groove to the mouth. These organisms have an open circulatory system; it has been suggested that this system is primarily for nutrient distribution and that the coelomic fluid is the medium for oxygen transport.

• Primary Diet
• planktivore planktivore

• Foraging Behavior
• filter-feeding filter-feeding

Predation

Brachiopod shells are an obvious predator deterrent; however, most species have relatively thin shells and the fossil record suggests that predators may be able to bore through them, if rarely. It appears that the flesh of brachiopods is unpalatable and they therefore are not generally subject to predation, particularly in the presence of bivalves such as mussels, which appear to be much more palatable. An alternate theory is that the amount of energy that must be expended in order to consume a brachiopod is greater than the benefit. Shorebirds appear to feed on Glottidia palmeri in the Gulf of California, based on shell repair scars found in these populations, which are correlated with the migratory patterns of these species.

Ecosystem Roles

The fossil record shows that brachiopods have been hosts to a variety of parasites including polychaetes and gastropods . Present-day brachiopods have been found infested with polychaetes as well. Typically, parasites bore into, but not through, the host's shell. There is evidence that brachiopods create calciferous blisters to prevent parasites from entering the space between the valves.

Economic Importance for Humans: Positive

Beyond the potential for scientific research, there are no positive effects of brachiopods on humans.

• Positive Impacts
• research and education

Economic Importance for Humans: Negative

There are no known negative impacts of brachiopods on humans.

Conservation Status

As a broadly cosmopolitan phylum, of which most members favor deep, temperate or polar waters, brachiopods are not considered in danger of becoming threatened or extinct.

Additional Links

Encyclopedia of Life

Contributors

Jeremy Wright (author), University of Michigan-Ann Arbor, Leila Siciliano Martina (editor), Texas State University.

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