Pterobranchia is a

class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used differently ...

of small worm-shaped animals. They belong to the Hemichordata, and live in secreted tubes on the

ocean floor The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean. All floors of the ocean are known as 'seabeds'. The structure of the seabed of the global ocean is governed by plate tectonics. Most of ...

. Pterobranchia feed by filtering

plankton Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a cruc ...

out of the water with the help of

cilia The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike proje ...

attached to

tentacle In zoology, a tentacle is a flexible, mobile, and elongated organ present in some species of animals, most of them invertebrates. In animal anatomy, tentacles usually occur in one or more pairs. Anatomically, the tentacles of animals work main ...

s. There are about 25 known living pterobranch

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriat ...

in three genera, which are '' Rhabdopleura'', ''

Cephalodiscus ''Cephalodiscus'' is a genus of hemichordates in the monotypic family Cephalodiscidae of the order Cephalodiscida. Description Unlike ''Rhabdopleura'', ''Cephalodiscus'' species do not form large colonies and are only pseudocolonial. ''Cephalo ...

'', and ''

Atubaria ''Atubaria heterolopha'' is a species of hemichordates in the monotypic genus ''Atubaria'' and in the monotypic family Atubaridae. This taxon belongs to the pterobranchian order Cephalodiscida. It was described by Tadao Sato in 1936 from speci ...

''. On the other hand, there are several hundred extinct genera, some of which date from the

Cambrian Period The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ...

. The class Pterobranchia was established by

Ray Lankester Sir Edwin Ray Lankester (15 May 1847 – 13 August 1929) was a British zoologist.New International Encyclopaedia. An invertebrate zoologist and evolutionary biologist, he held chairs at University College London and Oxford University. He was th ...

in 1877. It contained, at that time, the single

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

'' Rhabdopleura''. ''Rhabdopleura'' was at first regarded as an aberrant

polyzoon Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum of simple, aquatic invertebrate animals, nearly all living in sedentary colonies. Typically about long, they have a special feeding structure called a ...

, but when the ''Challenger'' report on ''

'' was published in 1887, it became clear that ''Cephalodiscus'', the second genus now included in the

order Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of ...

, had affinities with the Enteropneusta.

Electron microscope An electron microscope is a microscope that uses a beam of accelerated electrons as a source of illumination. As the wavelength of an electron can be up to 100,000 times shorter than that of visible light photons, electron microscopes have a hi ...

studies have suggested that pterobranchs belong to the same

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English ter ...

as the

extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

graptolite Graptolites are a group of colonial animals, members of the subclass Graptolithina within the class Pterobranchia. These filter-feeding organisms are known chiefly from fossils found from the Middle Cambrian ( Miaolingian, Wuliuan) through t ...

s, and phylogenetic analysis suggests that the pterobranchs are living members of the graptolite clade.

Biology

Pterobranchs are small worm-like

filter feeder Filter feeders are a sub-group of suspension feeding animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feedin ...

s living on the ocean floor, often in relatively deep waters. Like their relatives, the

acorn worm The acorn worms or Enteropneusta are a hemichordate class of invertebrates consisting of one order of the same name. The closest non-hemichordate relatives of the Enteropneusta are the echinoderms. There are 111 known species of acorn worm in th ...

s, their body is divided into three parts: an anterior

proboscis A proboscis () is an elongated appendage from the head of an animal, either a vertebrate or an invertebrate. In invertebrates, the term usually refers to tubular mouthparts used for feeding and sucking. In vertebrates, a proboscis is an elong ...

, a collar, and a trunk. The proboscis is wide and flattened at the tip, and in most species contains glands that secrete a tube of organic material in which the pterobranch spends its adult life. The animals are mostly colonial, with several

zooid A zooid or zoöid is a single animal that is part of a colonial animal. This lifestyle has been adopted by animals from separate unrelated taxa. Zooids are multicellular; their structure is similar to that of other solitary animals. The zooi ...

s living together in a cluster of tubes. In some species, the individual zooids within the colony are connected by

stolon In biology, stolons (from Latin '' stolō'', genitive ''stolōnis'' – "branch"), also known as runners, are horizontal connections between organisms. They may be part of the organism, or of its skeleton; typically, animal stolons are external s ...

s. The single member in the genus ''

'' is unusual in lacking the tubes typical of other pterobranchs, living as a naked zooid on corals. Recently, ''

'' has been regarded as a questionable species by Tassia et al. (2016) and is no longer considered valid. The collar bears a number of large arms, each of which includes a row of tentacles along one side. The number of arms varies between species, with anything from one to nine pairs. The tentacles are covered in cilia and aid in filtering food from the water. The trunk includes a simple tubular gut, and is curved over so that the

anus The anus (Latin, 'ring' or 'circle') is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, the residual semi-solid waste that remains after food digestion, which, ...

projects upwards, lying dorsal to the collar. ''

'' and ''

'' have a single pair of gill slits in the

pharynx The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its st ...

, although '' Rhabdopleura'' has none. Development of pterobranchs have been studied only in '' Rhabdopleura'' from Plymouth ('' Rhabdopleura compacta'') and from Bermuda ('' Rhabdopleura normani''). Both of these species are

dioecious Dioecy (; ; adj. dioecious , ) is a characteristic of a species, meaning that it has distinct individual organisms (unisexual) that produce male or female gametes, either directly (in animals) or indirectly (in seed plants). Dioecious reproducti ...

, with the fertilised egg hatching to produce a free-swimming ciliated

larva A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. ...

. Despite the close relationship between the two groups, the larva does not resemble that of the acorn worms; they are "planula-like". Eventually, the larva settles onto the substrate and metamorphoses to an adult. Alternatively, they also reproduce asexually by budding to create a new colony.

Evolution

Paleontology

The earliest pterobranchs, including '' Yuknessia'' and '' Galeaplumosus'', are known from mid-Cambrian

Lagerstätte A Lagerstätte (, from '' Lager'' 'storage, lair' '' Stätte'' 'place'; plural ''Lagerstätten'') is a sedimentary deposit that exhibits extraordinary fossils with exceptional preservation—sometimes including preserved soft tissues. These f ...

n. Earlier small carbonaceous fossils are known from the

Buen Formation The Buen Formation is a geologic formation and Lagerstätte in Peary Land, North Greenland. The shale preserves fossils dating back to the Early Cambrian period (Atdabanian in the local timescale, about 520 to 513 Ma).

Taxonomy

Comparison of

18S ribosomal RNA 18S ribosomal RNA (abbreviated 18S rRNA) is a part of the ribosomal RNA. The S in 18S represents Svedberg units. 18S rRNA is an SSU rRNA, a component of the eukaryotic ribosomal small subunit ( 40S). 18S rRNA is the structural RNA for the small ...

sequences indicated that pterobranchs are closely related to enteropneust hemichordates. Class Pterobranchia Lankester 1877 * Subclass

Cephalodiscida Cephalodiscida is one of two orders in the class Pterobranchia, which are small, worm-shaped animals. Members belong to the hemichordates. Species in this order are sessile, living in clear water and secrete tubes on the ocean floor. Taxonomy T ...

Fowler 1892 stat. nov. ** Order

Fowler 1892 * Subclass Graptolithina Bronn 1849 ** Order ?† Camaroidea Kozlowski 1928 sensu Kozlowski 1949 ** Order ?†

Crustoidea Wimanicrustidae is an extinct family of graptolite Graptolites are a group of colonial animals, members of the subclass Graptolithina within the class Pterobranchia. These filter-feeding organisms are known chiefly from fossils found from t ...

Bulman 1970 ** Order ?†

Dithecoidea Dithecodendridae is an extinct family of graptolites. Genera List of genera from Maletz (2014): *†'' Archaeolafoea'' Chapman, 1919 *†'' Bulmanidendrum'' Obut, 1974 *? †''Dalyia ''Dalyia'' is a pterobranch known from the middle Cambria ...

Obut, 1960 ** Order ?†

Tuboidea Cyclograptidae is an extinct family of graptolite Graptolites are a group of colonial animals, members of the subclass Graptolithina within the class Pterobranchia. These filter-feeding organisms are known chiefly from fossils found from th ...

Kozlowski 1938 sensu Kozlowsk 1949 ** Order

Rhabdopleurida Rhabdopleurida is one of three orders in the class Pterobranchia, which are small, worm-shaped animals, are the only surviving graptolites. Members belong to the hemichordates. Species in this order are sessile, colonial, connected with a stolon ...

Fowler 1892 sensu Beklemishev 1951 ** Clade † Eugraptolithina Mitchell et al., 2013 *** Order †

Dendroidea Graptolites are a group of colonial animals, members of the subclass Graptolithina within the class Pterobranchia. These filter-feeding organisms are known chiefly from fossils found from the Middle Cambrian ( Miaolingian, Wuliuan) through t ...

Nicholson 1872 *** Order †

Graptoloidea Graptolites are a group of colonial animals, members of the subclass Graptolithina within the class Pterobranchia. These filter-feeding organisms are known chiefly from fossils found from the Middle Cambrian ( Miaolingian, Wuliuan) through t ...

Maletz, Carlucci and Mitchell 2009

Genomics

Genetic code

The two pterobranch taxa '' Rhabdopleura compacta'' and ''

'' use alternative genetic codes in their mitochondrial genome.

References

External links

*
Cameron, Christopher B. (2016). A comprehensive list of extant hemichordate species with links to images

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