''Cephalodiscus'' is a genus of

hemichordate Hemichordata ( ) is a phylum which consists of triploblastic, eucoelomate, and bilaterally symmetrical marine deuterostome animals, generally considered the sister group of the echinoderms. They appear in the Lower or Middle Cambrian and incl ...

s in the

monotypic In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unisp ...

family

Cephalodiscidae ''Cephalodiscus'' is a genus of hemichordates in the monotypic family Cephalodiscidae of the order Cephalodiscida. Description All known species live in a secreted coenecium attached to a rock substrate. Unlike ''Rhabdopleura'', ''Cephalodisc ...

of the order

Cephalodiscida Cephalodiscida is one of two orders in the class Pterobranchia, which are small, worm-shaped animals. Members belong to the hemichordata, hemichordates. Species in this order are Sessility (zoology), sessile, living in clear water and secretion, ...

Description

All known species live in a secreted coenecium attached to a rock substrate. Unlike ''

Rhabdopleura ''Rhabdopleura'' is a genus of colonial sessile hemichordates belonging to the Pterobranchia class. They are exclusively marine, benthic organisms whose species occur within all major oceans and range in habitat from intertidal to c. 900 m.Gord ...

'', ''Cephalodiscus'' species do not form large colonies and are only pseudocolonial, but they do share a common area with individual buds for each zooid. ''Cephalodiscus'' zooids are also more mobile than their ''Rhabdopleura'' counterparts, and are able to move around within tubaria. ''Cephalodiscus'' zooids can be produced via asexual budding. There are a few pairs of tentacled arms, whereas ''Rhabdopleura'' has only one pair of arms.

Species

Cephalodiscus nigrescens ''Cephalodiscus nigrescens'' is a sessile hemichordate belonging to the order Cephalodiscida. Parasites The parasitic protozoa Protozoa (: protozoan or protozoon; alternative plural: protozoans) are a polyphyletic group of single-celled e ...

'' Lankester 1905 * '' Cephalodiscus planitectus'' Miyamoto, Nishikawa and Namikawa, 2020 * '' Cephalodiscus sibogae'' Harmer 1905 * '' Cephalodiscus solidus'' Andersson, 1907 Extinct species include: * †'' Cephalodiscus lutetianus'' Abrard, Dollfus & Soyer 1950 * †'' Cephalodiscus nusplingensis'' Schweigert & Dietl 2013 Proposed subgenera are ''idiothecia'', ''demiothecia'', ''orthoecus'', and ''acoelothecia''.

Historical discovery

''Cephalodiscius'' are

endemic Endemism is the state of a species being found only in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also foun ...

to the

Antarctic The Antarctic (, ; commonly ) is the polar regions of Earth, polar region of Earth that surrounds the South Pole, lying within the Antarctic Circle. It is antipodes, diametrically opposite of the Arctic region around the North Pole. The Antar ...

and

Southern Ocean The Southern Ocean, also known as the Antarctic Ocean, comprises the southernmost waters of the world ocean, generally taken to be south of 60th parallel south, 60° S latitude and encircling Antarctica. With a size of , it is the seco ...

, whose relative inaccessibility has historically limited human study of the genus. The ''Erebus'' and ''Terror'' may have unwittingly encountered ''C. nigrescens'' specimens, and the ''Challenger'' ''C. densus''; but until the

Swedish Antarctic Expedition The Swedish Antarctic Expedition of 1901–1903 was a scientific expedition led by Otto Nordenskjöld and Carl Anton Larsen. It was the first Swedish endeavour to Antarctica in the Heroic Age of Antarctic Exploration. Background Otto Nordensk ...

of 1901-1903, only ''C. dodecalphus'' had been identified. In 1882, M'Intosh (later spelled McIntosh) had identified ''Dodecalphus'' from dredged Magellanic-Straits material, work published 5 years later, but the discovery left cephalodiscid phylogeny unclear. M'Intosh proposed placement amongst the polyzoa, whilst Harmer suggested the modern placement amongst hemichordates. The Swedish expedition provided a plethora of new species, and subsequent researchers began to recognize cephalodiscid species in the relatively temperate waters off

South Africa South Africa, officially the Republic of South Africa (RSA), is the Southern Africa, southernmost country in Africa. Its Provinces of South Africa, nine provinces are bounded to the south by of coastline that stretches along the Atlantic O ...

, the

Falklands The Falkland Islands (; ), commonly referred to as The Falklands, is an archipelago in the South Atlantic Ocean on the Patagonian Shelf. The principal islands are about east of South America's southern Patagonian coast and from Cape Du ...

Sri Lanka Sri Lanka, officially the Democratic Socialist Republic of Sri Lanka, also known historically as Ceylon, is an island country in South Asia. It lies in the Indian Ocean, southwest of the Bay of Bengal, separated from the Indian subcontinent, ...

, and

Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...

. At the same time, researchers also determined that ''C. rarus'' and ''andersonii'' were in fact ''C. densus'' specimens. '' Cephalodiscus planitectus'' is the most recently discovered species. It was described in 2020 from specimens found in

Sagami Bay lies south of Kanagawa Prefecture in Honshu, central Japan, contained within the scope of the Miura Peninsula, in Kanagawa, to the east, the Izu Peninsula, in Shizuoka Prefecture, to the west, and the Shōnan coastline to the north, while the i ...

off the southern coast of

Honshu , historically known as , is the largest of the four main islands of Japan. It lies between the Pacific Ocean (east) and the Sea of Japan (west). It is the list of islands by area, seventh-largest island in the world, and the list of islands by ...

, Japan.

References

External links

* * Hemichordate genera Pterobranchia {{hemichordate-stub