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BRACHIOPODA

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(Dum?ril, 1806)

Sections
  • Background
  • Development
  • Zoogeography
  • Background

    Commonly known as ‘lamp shells’, the adults of this exclusively marine phylum superficially resemble bivalve molluscs. Along with phoronids and bryozoans they are grouped together as lophophorates, as during at least part of their development, they all have a specialized feeding structure called a lophophore, an extension of the body wall into a tentaculate structure that surrounds the mouth. Although they can be abundant, they are not a generally well-known group. The current number of species is only around 300 worldwide and 30 in European waters, but at the beginning of the Cambrian era (600 million years ago) they dominated the benthos and around 30,000 species are described from the fossil record. They are sporadically distributed around the world, as remnant members of a once widespread animal group tend to be.
    Brachiopods have an external, bivalve, clam-like shell, but apart from considerable anatomical difference from bivalve molluscs, the shell symmetry is also different, having dissimilar dorsal and ventral valves. Bivalve molluscs, in most cases, have a plane of bilateral symmetry between the shells, whereas brachiopods have a plane of symmetry through the shells, perpendicular to the hinge. The shell is usually composed of calcite or a chitinophosphate material, rather than aragonite (a calcite polymorph), the typical constituent of mollusc shells.
     
    Adult brachiopods are solitary, sessile, slow moving, suspension feeders and live on the seabed. They attach to rocks or other firm substrates with a fleshy stalk or pedicle, or more rarely by cementation of the ventral valve. A few species live in burrows in sediments, but they are not found in areas of soft, fine sediment where there is a high sedimentation rate, or their lophophores would become clogged. They can however survive in strong currents and are most abundant in shallow waters, with densities of up to 400 specimens/m² being recorded. They have evolved to use very little energy or oxygen, so are well suited to marginal environments such as polar, deep-water (to 6000 m), low oxygen and brackish habitats.
     
    Traditionally, brachiopods have been separated into two classes, based on the opening/closing apparatus of the shells. Class Articulata have a pair of teeth on the ventral valve which fit into paired sockets on the dorsal valve and act as a hinge. Class Inarticulata rely solely upon internal muscles to open and close the valves, so have different musculature. It has been argued that hinge articulation is not a good character to separate groups, so their taxonomy is currently under debate. Their free-swimming larvae can occur in plankton samples.
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    Development

    The free-swimming larvae of brachiopods rarely occur in plankton samples, probably because the pelagic phase is very short in many species. Pennington & Stricker (2002) separate four larval types, representative of four main groups: Lingulacea, Discinacea, Craniacea (traditionally accepted as superfamilies within Class Inarticulata) and Articulata.
    Lingulacea: In larval Lingulacea (Figure 1A) there are dorsal and ventral shell valves with mantle tissue internally, a complete gut, a developing pedicle and a lophophore. Eggs are released into the plankton and before the larvae hatch a half-moon shape embryonic shell or protegulum has formed, which expands while in the plankton, until an ovoid larval shell results. The ciliated lophophore develops cirri either side of a median tentacle, a pair at a time, in increasing numbers with growth. Cirrus pair number is used in staging larvae. Larvae may spend around 3-6 weeks in the plankton and there is no clear metamorphosis when they settle. Size: ~0.3-1.5 mm diameter.
    Discinacea: Discinacea also releasing eggs and sperm into the plankton, but the larvae do not superficially resemble lingulacean larvae. They differ in the early developmental stages in that they do not have a protegulum stage. When the larva emerges from the egg a shell is not developed (Figure 1B) up until the stage when two pairs of cirri are present on the lophophore, either side of the median tentacle. Larvae are characterised at this stage by two bundles of 3-5 long setae. By the time 3-4 cirrus pairs appear, the shell has developed. The shell continues to grow, but the lophophore remains at the four pairs of cirrus stage (Figure 1C). The early larval setae are lost, but other setae develop, notably the curved principle larval setae, and a pedicle bud develops.  Again there is no striking metamorphosis. Size: ~0.3-1.5 mm diameter.
     
    Figure 1A-E. Brachipod larvae.
     
     
    In brachiopod larvae with a bivalve shell it is very thin, usually transparent and pale amber in colour. As they usually occur in low numbers, they are easily overlooked in samples and on superficial examination could be confused with large mollusc bivalve larvae. They swim by extending the lophophore cirri and median tentacle and are propelled by ciliary action. In preserved material the cilia, apart from the most robust ones, are generally destroyed, but the cirri are usually very obvious and characteristic.
     
    Craniacea and Articulata: Craniacea and Articulata larvae share a similar appearance, resembling button mushrooms in shape. They do not develop a shell or feed until settlement, at which time they have a metamorphosis.  Craniacea are free spawners and the larvae are fully developed around three days after fertilization Larvae develop an apical lobe and a mantle lobe with three pairs of setal bundles, but no pedicle lobe is formed (Figure 1D). A pedicle is not required as adults attach to the substrate by cementing the shell valve. Settlement taking place around four days after fertilization, so they are in the water column for a very short period. Size: ~ 0.1-0.4 mm.
     
    Articulata includes both free spawners and brooding species. The characteristic larva consists of three sections that are not true segments (Figure 1E), an anterior apical lobe, a central mantle lobe and a posterior pedicle lobe. The apical lobe is ciliated and in some species there is a prominent band of locomotory cilia around the circumference and a tuft of immotile cilia anteriorly. Sometimes there is a row of tiny circular structures lining the posterior margin of the lobe, the vesicular bodies. The mantle lobe bears four bundles of cilia. In later larvae the pedicle lobe develops a concavity called the attachment disc. Larvae are probably in the plankton for less than a week. Size: ~ 0.1-0.4 mm.
     
    Further information: Reed 1991;Ryland 1965, 1970; Todd et al.,1991; Zimmer & Woollacott 1977; Hayward & Ryland 1998, 1999; Temkin & Zimmer 2002; Larink & Westheide 2006. 
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    Zoogeography

    Particularly coastal waters, but can be widely distributed.

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    [BRACHIOPODA on the taxonomic tree]

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