Hypsistozoa Brewin , 1953

Kott, Patricia, 2007, Taxonomic affinities of three stalked colonial species of the Ascidiacea (Tunicata) from the central coast of New South Wales and indications of a trans-Tasman connection, Journal of Natural History 41 (9 - 12), pp. 633-645 : 636-638

Genus Hypsistozoa Brewin, 1953

Type species: Distaplia fasmeriana Michaelsen, 1924

Only two species are known. Both have stalked colonies with soft transparent test, although the outer layer of the stalk is often horny. As in other genera ( Sycozoa and Distaplia ) of the Holozoidae , zooids are arranged in well-organised, circular to elongate common cloacal systems, large sessile atrial openings have a pronounced anterior lip and expose a large part of the branchial sac directly to the regular circular or longitudinal cavities or canals, branchial sacs have four rows of long, narrow stigmata crossed by parastigmatic vessels, replicates are generated from the vegetative stolon at the posterior end of the zooid in a process involving the epicardial sac, fertilisation is in a brood pouch formed by a U-shaped loop of the oviduct that projects out from the postero-dorsal corner of the thorax and larval adhesive organs, each consisting of a terminal cone of adhesive cells in a shallow epidermal cup on a large bulbous stalk, are triradially arranged at the anterior end of the larval trunk. Kott’s (1990) report of muscle bands extending along the abdomen and the broad posterior abdominal vegetative stolon of the present genus is incorrect, muscles in the present genus as in all Holozoidae being confined to the thorax. However, the present genus is unique in the position of its gonads in the top of the posterior abdominal extension (rather than in the loop of the gut or in a sac attached to the zooid on the right side of the gut as in other genera of the Holozoidae ). In the larvae, the S-shaped larval vegetative stolon, prolific larval budding, larval endodermal tubes and associated ectotrophic membranes are unique characteristics of Hypsistozoa (see Brewin (1956) and references therein).

Brewin (1956) first documented the development of the embryo of the New Zealand Hypsistozoa fasmeriana ( Michaelsen, 1924) , describing its large stolon producing an unusually large number (up to 14) of blastozooids and the pair of straight endodermal tubes from the larval oesophageal endoderm to the upper surface of the larval ectoderm just behind the cerebral vesicle on each side of the dorsal mid-line (connecting the lumen of the larval oesophagus to the interior of the oviduct where it forms the brood pouch). A very thin ectotrophic membrane, circular in outline and lobed around its outer margins, is formed as an external flap from the ectoderm on each side of the external openings of the endodermal tubes. These membranes enclose the sides of the larval trunk outside the larval tunic. The deep rounded scallops around the outside of each ectotrophic membrane interdigitate with one another around the anterior and ventral margins of the larval trunk. The endodermal tubes and the flimsy bilateral ectotrophic membranes are of doubtful homology and appear to be unique in the Ascidiacea . Brewin (1956) proposed that the endodermal tubes provide a route for nutriment for the developing yolk-free larva. The evidence for this proposal is entirely circumstantial, viz. that such a source of nutriment would be needed through the five and a half months of larval development and the fact that the tubes open into the oesophagus. The provenance of the food that would pass down the oviduct into the brood pouch, and thence into the endodermal tubes is problematical, however, incurrent water (from which food has already been filtered by the mucous net before it passes through the pharyngeal wall) is not known to enter the oviduct. Nor is there an apparent role for the ectotrophic membranes that envelop the whole larva while it remains in the brood pouch. There is no evidence supporting Brewin’s suggested placenta-type role of diffusion of nutrients across the ectotrophic membranes as an important source of nutriment during the later stages of embryonic development. The absence of test over the ectotrophic membranes is offered in support of this role for the membranes but again the evidence is circumstantial and neither homologues nor even analogues of these structures have been identified in other organisms. In the absence of yolk, some of the needed energy for the developing embryo may come from the thick layers of follicle cells around the embryo rather than from the exogenous source that Brewin implied.

On the basis of the position of their gonads, Kott (1990) assigned Australian specimens, formerly thought to be Distaplia distomoides ( Herdman, 1899) , to Hypsistozoa ( Brewin, 1953) . The newly recorded specimens of the Australian species resemble those of the New Zealand species and the single embryos in each long-stalked brood pouch confirm Kott’s (1990) assignation of the Australian species and establish that the larvae Brewin (1956 and 1959) described are characteristic of the genus Hypsistozoa .