Family POLYCLINIDAE

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Family POLYCLINIDAE

Compiler and date details

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

Introduction

Polyclinidae Milne-Edwards, 1841 is one of the most diverse families of the Aplousobranchia. Six genera are represented in Australian waters. All have embedded zooids with 6-lobed or (occasionally) 8-lobed branchial apertures and smooth-rimmed atrial apertures, usually with a tongue from the upper rim of the opening or from the body wall anterior to the opening. Zooids may be arranged with their atrial openings into sometimes extensive branching cloacal canals or in circular systems around simple cloacal cavities. Colonies vary from small cushions to large irregular and sometimes lobed masses, or stalked heads, and many species have sand or other particles embedded in the test. Gonads are in a thread- or sac-like posterior abdomen with the testis follicles respectively serially arranged or bunched. The ovary is small and is anterior to the testis follicles. Fertilisation appears to occur in the atrial cavity or the distal end of the oviduct, where embryos are found at different stages of development. The gut loop is relatively short and the stomach is about halfway down the descending limb.

The stomach wall has parallel longitudinal folds in Aplidium Savigny, 1816 and is either smooth or has mulberry-like pockets in its wall in Polyclinum Savigny, 1816, Aplidiopsis Lahille, 1890, Synoicum Phipps, 1774, Morchellium Giard, 1872 and Sidneioides Kesteven, 1909. Aplidium often has long, thread-like posterior abdomina. The posterior abdomina of Synoicum and Morchellium are seldom so long. In Polyclinum, Aplidiopsis and Sidneioides the abdomen is separated from a more or less sac-like posterior abdomen by a constriction. The last three genera have larvae with vesicles separating from posteriorly projecting bilateral strands of larval ectoderm, one strand each side of the dorsal midline, and one each side of the postero-ventral corner of the larvae trunk, as well as anterior epidermal ampullae and adhesive organs. Aplidium usually has epidermal vesicles separating from the epidermis at the anterior end of the larval trunk around and amongst the adhesive organs. Synoicum has a stomach wall and larval epidermal vesicles like Polyclinum, Aplidiopsis and Sidneioides. The phylogenetic position of Morchellium is uncertain. It may be allied to Aplidium, the stomach being more barrel-shaped than the dorsally shortened stomach of Polyclinum. Larvae are not known for the Australian species of Morchellium, but the English Channel species, M. argus (see Berrill 1950), has anterior vesicles similar to those of Aplidium.

Polyclinum has branchial papillae, possible vestiges of the inner longitudinal branchial vessels found also in Protopolyclinidae, suggesting a protopolyclinid ancestry for Polyclinum and its related genera Aplidiopsis, Sidneioides and probably Synoicum. However, the folded stomach wall, and thread-like posterior abdomina of Ritterellidae suggest a direct relationship with Aplidium. Polyclinidae may therefore be polyphyletic, for although most of the genera are related to one another and appear to have an ancestor in the Protopolyclinidae, Aplidium and possibly Morchellium may have evolved from Ritterellidae after its isolation from the Protopolyclinidae.

Aplidium is one of the most speciose of the genera in tropical, temperate and polar waters in most parts of the world including Australia where 47 species are recorded. Synoicum (22 species) and Polyclinum (12 species) are also well represented. Two species each of Aplidiopsis and Morchellium are known from Australia, as well as one of the two known species of Sidneioides (see Kott 1992). As in other families, tropical species appear to have a wider range than temperate ones.