Class PTEROBRANCHIA

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Class PTEROBRANCHIA

Compiler and date details

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

Introduction

The zooids of the extant members of the Class Pterobranchia Lankester, 1877, are small (about 0.5–7.0 mm long) and live in benthic sedentary colonies, produced by budding from a single sexually produced individual. The asexually produced colony or pseudo-colony has a protective coenoecium. This may take the form of a repent branching chambered and completely enclosed tube, open ended, erect tubes rising from each chamber and housing a single zooid, or a compact (Schepotieff 1907a, 1907b) cluster of individual tubes and zooids, or a sponge-like mass of anastomosing interconnecting tubes and tunnels shared by all the zooids and buds in the colony. The colony or pseudo-colony is usually attached to a solid surface. The coenoecia are secreted externally and are composed of a flexible, transparent to translucent, colourless to deep brownish red collagenous material secreted and moulded entirely by the proboscis and may incorporate shells of Foraminifera, sand grains, shell fragments or sponge spicules.

The genera and species are identified primarily by the form of the coenoecia in which they live (Lankester 1884; Allman 1869; Ridewood 1920; Norman 1921; Johnston & Muirhead 1951; Markham 1971). All are adapted to a tubiculous mode of life so the body is sac-like, the digestive tract U-shaped. The proboscis (buccal shield or cephalic shield) is flattened and discoid and ventrally directed covering the mouth; the collar bears one or more pairs of ciliated tentaculate arms. Ventrally, the trunk region is extended into a long contractile stalk which may or may not be attached to a repent stolon (Van der Horst 1939; Dawydoff 1948; Hyman 1959; Stebbing & Dilly 1972; Nishikawa 1986).

All pterobranchs are microphagous suspension feeders. The sexes are separate and recognisable when mature. Eggs and early larval stages remain within the parent coenoecium, and dispersal is by a short planktonic phase. The larvae are lecithotrophic and each one is capable of establishing a new colony (Van der Horst & Helmcke 1956; Burdon-Jones 1957; Stebbing 1970; Dilly 1985; Lester 1985, 1988a, 1988b; Halanych 1993).

Asexual reproduction by budding of new zooids is the dominant phase in the life cycle of all pterobranchs. Both sexual and asexual processes can take place simultaneously. There is some evidence of seasonality in both methods of reproduction, and several generations of zooids may occupy the same coenoecium. For further details on budding, settlement behaviour and establishment of new colonies, substrata preferences, associated fauna and geographic distribution, see Schepotieff (1907a, 1907b); Burdon-Jones (1954); Burdon-Jones & Tambs-Lyche (1960); Stebbing (1970); Dilly (1973); Dilly & Ryland (1985); Lester (1985, 1988a, 1988b).

The class contains two orders: Rhabdopleurida and Cephalodiscida, both with fossil and extant members.