Cephalodiscus


Cephalodiscus is a genus of hemichordates in the family Cephalodiscidae of the order Cephalodiscida.

Description

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

19 living species of Cephalodiscus have been described:Cephalodiscus agglutinans Harmer & Ridewood, 1914Cephalodiscus atlanticus Bayer, 1962Cephalodiscus australiensis Johnston & Muirhead, 1951Cephalodiscus calciformis Emig, 1977Cephalodiscus densus Andersson 1907 Cephalodiscus dodecalophus McIntosh 1882Cephalodiscus evansi RidewoodCephalodiscus fumosus John, 1932Cephalodiscus gilchristi Ridewood, 1908Cephalodiscus gracilis Harmer 1905Cephalodiscus graptolitoides Dilly 1993Cephalodiscus hodgsoni Ridewood, 1907 Cephalodiscus indicus Schepotieff 1909Cephalodiscus kempi John, 1932Cephalodiscus levinsoni Harmer 1905Cephalodiscus nigrescens Lankester 1905Cephalodiscus planitectus Miyamoto, Nishikawa and Namikawa, 2020Cephalodiscus sibogae Harmer 1905Cephalodiscus solidus Andersson, 1907
Extinct species include:
Proposed subgenera are Idiothecia, Demiothecia, Orthoecus, and Acoelothecia.

Historical discovery

Cephalodiscus are endemic to the Antarctic and Southern Ocean, 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 of 1901-1903, only C. dodecalphus had been identified. In 1882, William M'Intosh 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, the Falklands, Sri Lanka, and Australia. 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 off the southern coast of Honshu, Japan.