Sigillina exigua Kott 2006, sp. nov

Sigillina exigua sp. nov. (Figures 2A–C, 8D) Distribution Type locality. South Australia (Kangaroo I., ‘‘The Arches’’, Western River, on rock wall, 10–12 m, coll. K. Gowlett Holmes, 10 December 1999, syntypes SAM E3300). Description Colonies are small flat-topped cushions, about 2 cm diameter and to 5 mm high attached to calcareous debris. Zooids open on the upper surface. The test is soft and transparent. Zooids are short, about 3 mm long, the thorax, abdomen and posterior abdomen being each about equal in length. A short vascular stolon is at the posterior end of the body. Both six-lobed apertures are on the anterior end of the thorax. Three long rows of stigmata are in the branchial sac. Gonads are in the abdomen. A large almost spherical brood pouch constricted off from the postero-dorsal corner of the thorax contains a single embryo. The larval trunk is spherical and 1.1 mm diameter. An otolith and ocellus and three rows of stigmata are in the oozooid. Two large flat-topped adhesive organs, each in a thin-walled epidermal cup and with a short thick stalk, are depressed into the trunk in the anteromedian line. Three large median ectodermal ampullae alternate with the adhesive organs and curve around their stalks. The tail winds completely around the trunk. Remarks The cushion-like colonies and the zooids are particularly small for this genus. The species most closely resembles S. signifera, having flat-topped colonies and two large adhesive organs. However, both zooids and larvae of the present species are smaller (S. signifera larvae having a trunk 0.5 mm long). The present species from the southern Atlantic may be a sister species isolated from S. signifera which has a wide range and is common in tropical waters. The separately opening zooids, large larval trunks, wide, flat-topped, thick stalked larval adhesive organs, the three long rows of stigmata and the postero-dorsal thoracic brood pouch are characteristic of both this genus and Pseudodistoma, which is distinguished by its long posterior abdomen containing the gonads and the lack of the posterior abdominal vascular stolon of Sigillina. Kott (1992a) overlooked the similarity of Sigillina and Pseudodistoma and, although she later (Kott 1998, 2005b) emphasized the remarkable, large larval adhesive organs in Pseudodistoma, she again overlooked their likeness to the adhesive organs of Sigillina. It is possible the relationship of Sigillina to the Holozoidae based on the pattern of replication from the large vascular stolon (see Caullery 1909) could be less important than it previously was thought to be; and that the presence of the gonads in a posterior abdomen with conspicuous bands of longitudinal muscles along each side (that distinguishes Pseudodistomidae from Sigillina) may be less compelling than the characters (listed above) that are shared by these genera and suggest a closer phylogenetic relationship than previously was proposed.
Published as part of Kott, Patricia, 2006, Observations on non-didemnid ascidians from Australian waters (1), pp. 169-234 in Journal of Natural History 40 (3 - 4) on pages 184-185, DOI: 10.1080/00222930600621601, http://zenodo.org/record/5232431
{"references":["Kott P. 1992 a. The Australian Ascidiacea, Pt 3 Aplousobranchia (2). Memoirs of the Queensland Museum 32: 377 - 620.","Kott P. 1998. Tunicata. In: Wells A, Houston WWK, editors. Zoological catalogue of Australia. Volume 34, Hemichordata, Tunicata, Cephalochordata. Melbourne: CSIRO Publishing. p 51 - 252, 259 - 261 (Appendices I - III), 265 - 292 (Index).","Kott P. 2005 b. Catalogue of Tunicata in Australian waters [CDROM]. Canberra: Australian Biological Resources Study. 301 p.","Caullery M. 1909. Recherches sur la famille des Distomidae. Bulletin Scientifique de la France et de la Belgique 42: 1 - 59."]}