Family POLYCITORIDAE

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Family POLYCITORIDAE

Compiler and date details

P. Kott, Queensland Museum, Brisbane, Queensland, Australia

Introduction

The family Polycitoridae Michaelsen, 1904 contains colonial species in which zooids usually are embedded in relatively firm test which often contains embedded sand and other foreign particles. Partially embedded zooids occur only in Archidistoma Garstang, 1891, a genus not recorded from Australia. With the exception of Brevicollus tuberatus Kott, 1990 (an unusual indigenous species), the zooids are long and narrow, the abdomen being many times the length of the relatively short thorax.

In many species of Eudistoma Caullery, 1909, Polycitor Renier, 1804, Eucoelium Savigny, 1816 (=Polycitorella Michaelsen, 1924, a junior synonym) and Cystodytes Drasche, 1884, the zooids, arranged in circles with the separately opening 6-lobed atrial apertures in the centre of the circle, form rudimentary cloacal systems. True cloacal systems are developed only in the monotypic genus Salix Kott, 2005 (replacement name for Exostoma Kott, 1990; Kott 2005). In the genera Cystodytes and Eucoelium the zooids synthesise calcareous spicules—either unique plate-like ones or stellate spicules resembling those of Didemnidae.

Throughout the family, zooids are muscular with an internal layer of circular fibres and external longitudinal fibres on the thorax, the longitudinal ones continuing in a wide band along each side of the abdomen. When disturbed, the zooids contract and withdraw deep into the base or centre of the colony, unlike zooids of Holozoidae and Clavelinidae which appear to draw the abdomen up behind the thorax when disturbed. Kott (1990) has suggested that this reflects the importance of protecting the site of replication from predators—in Polycitoridae replication is from horizontal division of the long, narrow abdomen, while in Clavelinidae and Holozoidae the buds develop from the isolated terminal ampullae or the posterior stolon, both in the stalk or base of the colony.

Fertilisation may occur in the upper part of the oviduct, or in the atrial cavity where embryos are found at various stages of development. Larvae of Polycitoridae are of various sizes, but usually differ from Holozoidae and Clavelinidae in the arrangement of adhesive organs in a median vertical row at the anterior end of the trunk, generally having lost the triradial arrangement which is thought to be more primitive.

Although both have a long abdominal gut loop, Polycitoridae differ from Pycnoclavellidae in having lobed (rather than smooth-rimmed) apertures and larval adhesive organs with an axillary cone in an epidermal cup (rather than the inverted tubes of Pycnoclavellidae). The eggs do not appear to be fertilised at the base of the oviduct as they are in Pycnoclavellidae.

The family is well represented in Australian waters, especially by species of the genus Eudistoma. The monotypic Salix, the only genus with true cloacal systems, is common in tropical waters off the northern coast of the continent. Brevicollus Kott, 1990, an unusual indigenous genus with an equally unusual larva, is assigned to this family on the basis of its two 6-lobed, separately opening apertures, by its larval form and the presence of embryos at different stages of development in the atrial cavity.

Michaelsen (1930) proposed Archidistoma, Eudistoma, Paessleria Michaelsen, 1907 and Hyperiodistoma Michaelsen, 1930 as subgenera of the genus Sigillina Savigny, 1816. The family was revised by Kott (1990) who recognised Eudistoma, Archidistoma and Polycitor as genera of the Polycitoridae, while Sigillina was shown to have closer affinities with Holozoidae. The name Eudistoma has been given precedence over Paessleria (Kott 1995; ICZN Opinion 1865, 1997). Hyperiodistoma is a junior synonym of Sigillina (Holozoidae).