Brachiopodalamp-shells(Also: brachiopods)

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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. ("Brachiopoda", 2013; Brusca and Brusca, 2003; Carlson, 1995; Cohen and Gawthrop, 1997; Holmer, et al., 1995; Popov, et al., 1993; Ramel, 2012; Rowell, 1982; Sperling, et al., 2011; Williams, et al., 1996; Zhang, 2011)

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. ("Brachiopoda", 2013; Brusca and Brusca, 2003; Campbell, 2012; Kaulfuss, et al., 2013; Pennington and Stricker, 2002; Ramel, 2012)

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. (Balthasar and Butterfield, 2008; Brusca and Brusca, 2003; Gould and Calloway, 1980; Skovsted, et al., 2007; Ushatinskaya, 2008)

Geographic Range

Brachiopods are found throughout the world's marine environments. (Brusca and Brusca, 2003; Ramel, 2012)

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. (Brusca and Brusca, 2003; Ramel, 2012; Waggoner, 1995)

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. (Brusca and Brusca, 2003; Holt, 2013; Ramel, 2012)

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). (Brusca and Brusca, 2003; Holt, 2013; Ramel, 2012)

  • 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. (Brusca and Brusca, 2003; Cohen, 2007; Pennington and Stricker, 2002; Ramel, 2012)

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. (Brusca and Brusca, 2003; Pennington and Stricker, 2002; Ramel, 2012)

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. (Brusca and Brusca, 2003; Kaulfuss, et al., 2013; Pennington and Stricker, 2002; Ramel, 2012)

Brachiopods do not exhibit any parental investment beyond the production of gametes and, in females of some species, brooding of fertilized eggs. (Brusca and Brusca, 2003; Pennington and Stricker, 2002; Ramel, 2012)

  • 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. (Cohen, 2007)

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. (Brusca and Brusca, 2003; Campbell, 2012)

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. (Brusca and Brusca, 2003)

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. (Brusca and Brusca, 2003; Campbell, 2012)

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. (Campbell, 2012; Kowalewski and Flessa, 2000; Margulis and Chapman, 2009; McClintock, et al., 1993; Thayer, 1985)

  • Known Predators
    • willets (Catoptrophorous semipalmatus)

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. (Hoffmeister, et al., 2003; Kiel, 2008; Rodrigues, 2007; Rodrigues, et al., 2005)

Commensal/Parasitic Species
  • Serpula species (Class Polychaeta, Phylum Annelida)
  • Spionidae species (Class Polychaeta, Phylum Annelida)
  • Polydora species (Class Polychaeta, Phylum Annelida)

Economic Importance for Humans: Positive

Beyond the potential for scientific research, there are no positive effects of brachiopods on humans. (Brusca and Brusca, 2003)

  • Positive Impacts
  • research and education

Economic Importance for Humans: Negative

There are no known negative impacts of brachiopods on humans. (Brusca and Brusca, 2003)

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. (Brusca and Brusca, 2003; IUCN, 2013)

  • IUCN Red List [Link]
    Not Evaluated

Contributors

Jeremy Wright (author), University of Michigan-Ann Arbor, Leila Siciliano Martina (editor), Animal Diversity Web Staff.

Glossary

Antarctica

lives on Antarctica, the southernmost continent which sits astride the southern pole.

Arctic Ocean

the body of water between Europe, Asia, and North America which occurs mostly north of the Arctic circle.

Atlantic Ocean

the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.

World Map

Australian

Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.

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Ethiopian

living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.

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Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

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Neotropical

living in the southern part of the New World. In other words, Central and South America.

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Pacific Ocean

body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.

World Map

Palearctic

living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.

World Map

abyssal

on or near the ocean floor in the deep ocean. Abyssal regions are characterized by complete lack of light, extremely high water pressure, low nutrient availability, and continuous cold (3 degrees C).

benthic

Referring to an animal that lives on or near the bottom of a body of water. Also an aquatic biome consisting of the ocean bottom below the pelagic and coastal zones. Bottom habitats in the very deepest oceans (below 9000 m) are sometimes referred to as the abyssal zone. see also oceanic vent.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

chemical

uses smells or other chemicals to communicate

coastal

the nearshore aquatic habitats near a coast, or shoreline.

cosmopolitan

having a worldwide distribution. Found on all continents (except maybe Antarctica) and in all biogeographic provinces; or in all the major oceans (Atlantic, Indian, and Pacific.

crepuscular

active at dawn and dusk

diurnal
  1. active during the day, 2. lasting for one day.
ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

external fertilization

fertilization takes place outside the female's body

fertilization

union of egg and spermatozoan

filter-feeding

a method of feeding where small food particles are filtered from the surrounding water by various mechanisms. Used mainly by aquatic invertebrates, especially plankton, but also by baleen whales.

heterothermic

having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

holarctic

a distribution that more or less circles the Arctic, so occurring in both the Nearctic and Palearctic biogeographic regions.

World Map

Found in northern North America and northern Europe or Asia.

internal fertilization

fertilization takes place within the female's body

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

metamorphosis

A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

nocturnal

active during the night

oceanic islands

islands that are not part of continental shelf areas, they are not, and have never been, connected to a continental land mass, most typically these are volcanic islands.

oriental

found in the oriental region of the world. In other words, India and southeast Asia.

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oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

planktivore

an animal that mainly eats plankton

polar

the regions of the earth that surround the north and south poles, from the north pole to 60 degrees north and from the south pole to 60 degrees south.

polygynandrous

the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

reef

structure produced by the calcium carbonate skeletons of coral polyps (Class Anthozoa). Coral reefs are found in warm, shallow oceans with low nutrient availability. They form the basis for rich communities of other invertebrates, plants, fish, and protists. The polyps live only on the reef surface. Because they depend on symbiotic photosynthetic algae, zooxanthellae, they cannot live where light does not penetrate.

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

seasonal breeding

breeding is confined to a particular season

sessile

non-motile; permanently attached at the base.

Attached to substratum and moving little or not at all. Synapomorphy of the Anthozoa

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

solitary

lives alone

tactile

uses touch to communicate

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

year-round breeding

breeding takes place throughout the year

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