Family HEXACROBYLIDAE

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Introduction

The family Hexacrobylidae Seeliger, 1906 is characterised by a large buccal cavity surrounded by six arms (usually branched) containing muscles (continuous with longitudinal bands of the body wall), a reduced pharynx (limited to a short band behind the buccal cavity), ciliated stigmata opening into the atrial cavity from complex branchial pouches in the pharyngeal wall, a long oesophagus (with or without a large digestive diverticulum), a large stomach, gonads of the molgulid type with testis follicles separate from the ovarian tube, and a large vesicle (that appears to be a homologue of the molgulid kidney).

A proposal (Monniot et al. 1975) that the family represents a class of Tunicata separate from the Ascidiacea cannot be sustained (see Kott 1989). The family has close affinities with Molgulidae—closer than between Molgulidae and other taxa of the Ascidiacea. It appears to have evolved, probably from a molgulid ancestor, by profound changes in the pharynx and other parts of the gut. The powerful, branched, muscular, external arms, large stomach and conspicuous gut diverticula appear to be adaptations for a carnivorous habit, probably following isolation in ocean trenches.

This is one of the few deep (abyssal) water families so far recorded from the Australian region. Its occurrence in Australian waters has been reviewed by Kott (1989, 1992).

General References

Kott, P. 1989. The family Hexacrobylidae Seeliger, 1906 (Ascidiacea, Tunicata). Memoirs of the Queensland Museum 27(2): 517-534

Kott, P. 1992. The Australian Ascidiacea, supplement 2. Memoirs of the Queensland Museum 32(2): 621-655

Monniot, C., Monniot, F. & Gaill, F. 1975. Les Sorberacea: une nouvelle classe des tuniciers. Archives de Zoologie Expérimentale et Générale 116: 77-122

Seeliger, O. 1906. Appendicularien und Ascidien, Tunicata. Manteltiere. pp. 1041-1168 in Bronn, H.G. (ed.). Klassen und Ordnungen des Tier-Reichs. Leipzig : C.F. Winter Vol. 3 Suppl.

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