Subfamily Cephalodiscinae

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Subfamily Cephalodiscinae

Compiler and date details

C. Burdon-Jones, Queensland Museum, Brisbane, Queensland, Australia

Introduction

The Cephalodiscidae Harmer, 1905 are the larger of the extant families in the order Cephalodiscida, and comprise two genera: Cephalodiscus M'Intosh, with four subgenera and 17 species, and Atubaria Sato which is monospecific. In Cephalodiscus the division into subgenera and species is determined mainly on the structure of the coenoecium.

Since the genus Cephalodiscus was established by M'Intosh (1882), 22 species have been described, five have been shown to be synonyms of other species, and the remaining 17 species have been grouped into the following subgenera, based on the structure of the coenoecium: Cephalodiscus sensu stricto, Demiothecia Ridewood, Idiothecia Ridewood, Orthoecus Andersson, and Acoelothecia John (see Ridewood 1906, 1920; Andersson 1907; John 1931; Van der Horst 1939). In accordance with the International Code of Zoological Nomenclature, Johnston & Muirhead (1951) recommended the suppression of the subgenus Demiothecia in favour of Cephalodiscus, a procedure which seems to have been overlooked by some authors (see Hyman 1959; Benito 1982).

With the exception of Atubaria (Sato 1936), all the subgenera and the greatest diversity of species have been recorded from the Antarctic and Subantarctic regions. Six species in three subgenera, Orthoecus, Idiothecia and Cephalodiscus, have been recorded from waters adjacent to the Australian Antarctic Territory (Ridewood 1918a, 1918b; Johnston & Muirhead 1951). One species in Orthoecus occurs off Albany, Western Australia, and one in Cephalodiscus has been found on the Great Barrier Reef of Queensland and will probably prove to be widely distributed throughout the reefs and adjacent islands around the continent. The reports on various expeditions to the Antarctic and Subantarctic indicate that four of the species from those regions have circum-polar distributions. Their presence, frequently in abundance, at so many stations in the Antarctic, makes it one of the richest collecting grounds known for the genus.

Cephalodiscus species frequently occur together, or alone, and in association with calcareous sponges, tunicates, gorgonians, corals, bryozoans and hydroids, on hard substrata in tropical and warm temperate, polar and subpolar, regions. Their bathymetric range extends from the lower intertidal in the tropics to 1550 m and more in the Antarctic.

Atubaria differs from Cephalodiscus in not having a coenoecium, and in having more clearly delineated regions of the alimentary canal, a rectal caecum, the anus in a different position, and a pair of long arms with glandular, non-tentaculate ends. The similarity between the juveniles and the male zooids of Cephalodiscus sibogae is striking. Halanych (1996) thinks they are just another form of Cephalodiscus. When discovered, the female and juvenile zooids were clinging to a hydroid, but the probability that they were foraging beyond the confines of a coenoecium located nearby or forced out by a predator, cannot be discounted. Furthermore, although these anatomical differences merit the establishment of a new genus, they do not justify rank at family level—a decision that has been supported by most contemporary workers, except Benito (1982), who places Atubaria in the Atubaridae. For taxonomic keys to subgenera and species, see works by Ridewood (1920) and Markham (1971), for descriptions of new species, see Emig (1977) and Dilly (1993).