Animal Diversity Web

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)

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. (Cuvier, 1805; Davidson, 1888; Davidson, 2012; Duméril, 1806; Huxley, 1869; Nielsen, 2002)

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 . (Carlson, 1995; Holmer, et al., 1995; Popov, et al., 1993; Rowell, 1982; Sperling, et al., 2011; Williams, et al., 1996)

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. (Bourlat, et al., 2008; Cohen and Gawthrop, 1997; Cohen and Weydmann, 2005; Cohen, 2000; Dunn, et al., 2008; Giribet, et al., 2000; Halanych, et al., 1995; Hausdorf, et al., 2010; Helmkampf, et al., 2008; Nielsen, 2001; Nielsen, 2002; Paps, et al., 2009; Sperling, et al., 2011)