Pterobranchia is a class of small worm-shaped animals. They belong to the Hemichordata, and live in secreted tubes on the ocean floor. Pterobranchia feed by filtering plankton 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 projecti ...

attached to tentacles. There are about 25 known living pterobranch species 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. ''Cephalod ...

'', 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 specime ...

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

Cambrian Period The Cambrian Period ( ; sometimes symbolized C with bar, 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 ...

. The class Pterobranchia was established by Ray Lankester in 1877. It contained, at that time, the single genus '' 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 l ...

, 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 d ...

, had affinities with the Enteropneusta. Electron microscope 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 term, ...

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 ...

graptolites, 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 worms, their body is divided into three parts: an anterior proboscis, 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 zooids 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, d ...

projects upwards, lying dorsal to the collar. ''

'' and ''

'' have a single pair of gill slits in the pharynx, although '' Rhabdopleura'' has none. Development of pterobranchs have been studied only in '' Rhabdopleura'' from Plymouth (''

Rhabdopleura compacta ''Rhabdopleura compacta'' is a sessile hemichordate. It is a suspension feeder that secretes tubes on the ocean floor. Unlike all other ''Rhabdopleura'' species, the zooids have black elongated arms with no tentacles. Distribution ''Rhabdopleura ...

'') and from Bermuda (''

Rhabdopleura normani ''Rhabdopleura normani'' is a small, marine species of worm-shaped animal known as a pterobranch. It is a sessile suspension feeder, lives in clear water, and secretes tubes on the ocean floor. Description This species grows in colonies. Each ...

''). 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 reproductio ...

, with the fertilised egg hatching to produce a free-swimming ciliated larva. 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 ''Yuknessia'' is an early pterobranch, known from the Burgess shale, the Chengjiang and the Wheeler shale. Long, unbranched fronds emerge from a central holdfast-like body covered in small conical plates. 23 specimens of ''Yuknessia'' are known ...

'' and '' Galeaplumosus'', are known from mid-Cambrian Lagerstätten. Earlier

small carbonaceous fossils Small carbonaceous fossils (SCFs) are sub-millimetric organic remains of organisms preserved in sedimentary strata. This category of fossils has traditionally included robust or thick-walled entities such as plant spores, acritarchs and chitinoz ...

are known from the Buen Formation.

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 c ...

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 Th ...

Fowler 1892 stat. nov. ** Order

Fowler 1892 * Subclass Graptolithina Bronn 1849 ** Order ?†

Camaroidea Cysticamaridae 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 ...

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 Filter feeders are a sub-group of suspension feedin ...

Bulman 1970 ** Order ?†

Dithecoidea Dithecodendridae 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 Filter feeders are a sub-group of suspension feedi ...

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 Filter feeders are a sub-group of suspension feeding ...

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 Nicholson 1872 *** Order † Graptoloidea Maletz, Carlucci and Mitchell 2009

Genomics

Genetic code

The two pterobranch taxa ''

'' 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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