Family PSEUDODISTOMIDAE

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Introduction

The family Pseudodistomidae Harant, 1931 has, like Pycnoclavellidae and Euherdmaniidae, 6-lobed apertures opening separately to the exterior, gonads in a long, narrow posterior abdomen, and a moderately long vascular appendix. Like Sigillina Savigny, 1816 (Holozoidae), the family consistently has three rows of stigmata, and a gut loop of moderate length with the stomach (divided into four chambers) half to three-quarters of the distance down the descending limb. Fertilisation occurs at the base of the oviduct and embryos develop as they pass up the oviduct into a brood pouch projecting, and sometimes constricted off, from the zooid at the top of the abdomen (rather than in the thorax). Zooids are completely embedded, and the colonies are often (but not always) stalked. The test frequently is hardened with embedded sand.

Genera of this family are distinguished from Sigillina by the very long posterior abdomina containing the gonads. As in other families of Aplousobranchia, longitudinal thoracic muscles extend posteriorly in a band along each side of the abdomen and posterior abdomen, but usually not onto the vascular appendix as they do in Sigillina. The larvae are large, the trunk from 0.6 to more than 1.0 mm long. Unlike Sigillina, the large antero-median adhesive organs have unusual hollow eversible papillary cones (scyphate with an axial vesicle: Cloney 1990) and large lateral epidermal ampullae.

When Kott (1992) defined the family Pseudodistomidae, she overlooked the subfamily Pseudodistominae Harant, 1931. Although Kott's definition excludes other genera (Ritterella and Placentela) which Harant included, both family level taxa are based on the same nominal genus, and Harant is the author of the family, albeit redefined by Kott (1992).

The family contains two genera, the diverse Pseudodistoma and the monotypic Anadistoma Kott, 1992. The latter genus, known only from one Australian record, is distinguished by its inner coat of transverse muscles on the thorax, which suggests an affinity with Eudistoma Caullery, 1909 (Polycitoridae) rather than with Sigillina (Holozoidae). It is possible, therefore, that the family, as presently constituted, is polyphyletic.

The genus Pseudodistoma is well represented in Australian waters, especially in temperate waters, although two species are known from the tropics.

General References

Caullery, M. 1909. Recherches sur la famille des Distomidae. Bulletin Scientifique de la France et de la Belgique 42: 1-59

Cloney, R.A. 1990. Urochordata-Ascidiacea Pt 14. pp. 392-451 in Aida, K.G. (ed.). Reproductive Biology of Invertebrates. Vol. 4, Pt B: Fertilisation, Development and Parental Care. New Delhi : Oxford & IBH.

Harant, H. 1931. Contributions à l'histoire naturelle des ascidies et de leurs parasites. Annales de l'Institut Océanographie, Monaco & Paris 8(4): 229-389

Kott, P. 1992. The Australian Ascidiacea Pt 3, Aplousobranchia (2). Memoirs of the Queensland Museum 32(2): 377-620

Michaelsen, W. 1924. Ascidiae Krikobranchiae von Neuseeland, den Chatham und den Auckland Inseln. Videnskabelige Meddelelser fra Dansk Naturhistorisk Forening i Kjøbenhavn 77: 263-434

Savigny, J.C. 1816. Recherches anatomiques sur les ascidies composées et sur les ascidies simples—Système de la classe des Ascidies pp. 1–239. In Mémoires sur les Animaux sans Vertèbres, Pt 2. Paris : G. Dufour.

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