Pterobranchia is a class of small worm-shaped animals. They belong to the Hemichordata, and live in

secreted 440px Secretion is the movement of material from one point to another, such as a secreted chemical substance from a cell or gland. In contrast, excretion is the removal of certain substances or waste products from a cell or organism. The classical ...

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 crucia ...

out of the water with the help of cilia 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 ma ...

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 appropriate s ...

in three genera, which are ''

Rhabdopleura ''Rhabdopleura'' is a genus of colonial sessile hemichordates belonging to the Pterobranchia class. As one of the oldest living genera with a fossil record dating back to the Middle Cambrian, it is also considered to be the only living genus ...

'', '' Cephalodiscus'', and '' Atubaria''. On the other hand, there are several hundred extinct genera, some of which date from the Cambrian Period. 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'' was at first regarded as an aberrant polyzoon, but when the ''Challenger'' report on '' Cephalodiscus'' was published in 1887, it became clear that ''Cephalodiscus'', the second genus now included in the order, had affinities with the

Enteropneusta 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 the ...

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 as the extinct

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 feeders 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 zoo ...

s living together in a cluster of tubes. In some species, the individual zooids within the colony are connected by stolons. The single member in the genus '' Atubaria'' is unusual in lacking the tubes typical of other pterobranchs, living as a naked zooid on corals. Recently, '' Atubaria'' 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 projects upwards, lying dorsal to the collar. '' Cephalodiscus'' and '' Atubaria'' 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 ''

'' has none. Development of pterobranchs have been studied only in ''

'' from Plymouth ('' Rhabdopleura compacta'') and from Bermuda ('' Rhabdopleura normani''). Both of these species are dioecious, 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 Fowler 1892 stat. nov. ** Order Cephalodiscida Fowler 1892 * Subclass Graptolithina Bronn 1849 ** Order ?† Camaroidea Kozlowski 1928 sensu Kozlowski 1949 ** Order ?† Crustoidea Bulman 1970 ** Order ?† Dithecoidea Obut, 1960 ** Order ?† Tuboidea Kozlowski 1938 sensu Kozlowsk 1949 ** Order Rhabdopleurida 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 '' Cephalodiscus'' use alternative genetic codes in their

mitochondrial genome Mitochondrial DNA (mtDNA or mDNA) is the DNA located in mitochondria, cellular organelles within eukaryotic cells that convert chemical energy from food into a form that cells can use, such as adenosine triphosphate (ATP). Mitochondrial ...

References

External links

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

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