Family CEPHALODISCIDAE

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Family CEPHALODISCIDAE

Compiler and date details

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

Introduction

The living members of the order Cephalodiscida Fowler, 1892 form sedentary colonies or aggregations of small zooids which are 1–12 mm long, with 1–9 pairs of tentaculate arms, and a single pair of branchial pores. They are devoid of any organic linkage, and live in individual coenoeces or share a common protective housing which they construct with secretions produced and moulded by their proboscides.

Each zooid has a long muscular stalk with an adhesive terminal portion which enables the animal to anchor itself on a marginal filament (see Fig. 3a) while feeding at an ostium or on the surface of the coenoecium (Lester 1985). The distal part of the stalk is capable of budding new zooids. Zooids, with numerous buds attached, move freely throughout the coenoecium, and over its outer surface. Detached buds are presumed to remain within the parent colony (Fig. 3b), and contribute to the growth and repair of the coenoecium. Usually zooids and their buds are the same sex, but Ridewood (1907) recorded a male zooid with a female bud in Cephalodiscus (Cephalodiscus) hodgsoni. Colonies are often dominantly one sex.

Sexes are separate, but hermaphrodite zooids have been recorded. Gonads are paired, ova are large and heavily yolked. According to Masterman (1900), Andersson (1903, 1907), Harmer (1905), Schepotieff (1909) and Gilchrist (1915), development into a ciliated planula larva takes place within the coenoecium. John (1931) described post-planula stages, but dispersal, settlement and the formation of a new colony by budding have not been fully described.

The structure of the coenoecia varies considerably between species and forms the basis for the grouping of the various species into several subgenera (Andersson 1907; Ridewood 1907; John 1931). In the Cephalodiscus sub-genus, Cephalodiscus, the colony is branching, each ostium leading into a cavity which is continuous throughout the colony, and is occupied by the zooids and their buds. In the subgenus Orthoecus Andersson, the colony consists of a cluster or cake-like mass of individual tubular coenoecia each attached to its neighbour for part or almost its entire length by common coenoecial material. The coenoecia may or may not include sand grains, sponge spicules, echinoderm spines, sponge spicules and shell fragments. The coenoecia may be closely set and parallel or irregularly bent and straggling (see Fig. 4a). Colonies vary in size and shape (Ridewood 1907, 1918; John 1931; Hyman 1959), from a diminutive, brittle disc-like structure c. 10 mm in diameter and a few millimetres high, to a bulkier mass, some 30 cm diameter and 10 cm high. Tropical species of the subgenus Orthoecus are very much smaller than those species found in the Antarctic.

By contrast, the colonies of the subgenus Idiothecia Ridewood, have a branching structure, each branch studded with short projecting ostia, leading to individual tubular coenoecia, with blind ends connecting up at the centre. In the irregularly branched and more elaborately structured housing of the nominate subgenus Cephalodiscus (Fig. 4b), each ostium leads into a common coenoecial cavity, continuous throughout the colony, and occupied in common by all the zooids and their buds. Such colonies may attain a height of 25–30 cm and be 15–20 cm in diameter, and can form a fairly dense carpet on the sea-bed.

In the sub-genus Acoelothecia John, the colony has two forms—one small and tuft-like, the other larger and branching. Internally, they both consist of a meshwork of bars and spines without any true coenoecial cavities. The zooids and buds occupy the irregular and continuous spaces between the sponge-like meshwork of the housing.

The order dates from the Upper Cambrian and contains two families—Eocephalodiscidae Kozlowski, 1949, all of which are extinct (Kozlowski 1949; Bulman 1970), and the Cephalodiscidae Harmer, 1905 which includes both living and fossil forms (Chapman et al. 1995).

A third family, the monospecific Atubaridae Benito, 1982, has been included in some classifications (e.g. see Benito 1982) but omitted from others (Dawydoff 1948; Van der Horst & Helmcke 1956; Hyman 1959; Bulman 1970; Markham 1971; Nishikawa 1986). In the Catalogue, the genus Atubaria Sato has been accorded equal rank with Cephalodiscus M'Intosh in the family Cephalodiscidae.