Pterygascidia mirabilis

Description. In life, photographed specimens that appear to be of this species (although they were not collected) are upright, pale blue, translucent and they appear to be turgid (pers. com. Karen Sanamyan) Individuals are solitary, up to 15 cm long, the upper half a long, vertical, soft, oval head and the lower half a thick cylindrical stalk attached basally to a hard substrate (rock or pebble) by a small tuft of roots. The whole body, including the stalk is encased in thin, transparent test. The upper half of the individual contains the main body of the organism, consisting of a large branchial sac with the gut embedded in the pallial body wall to the left of the dorsal mid line. The gut forms a gentle vertical J-shaped arc extending from the oesophageal opening at the posterior end of the branchial sac to within about one third of the body length of the atrial aperture. The branchial aperture is terminal and is on a conspicuous terminal branchial siphon with its dorsal wall produced forward into a recurved hood-like structure that directs the opening toward the substrate. A large bilobed flap of test protrudes between the branchial and atrial openings and two smaller lobes project from behind the atrial siphon. A narrow projection from the body wall at the posterior end of the head extends down to the base of the stalk. In these specimens the stalk is collapsed, flattened and wrinkled and blood vessels were not detected in it. It is not clear how the turgor of the stalk is maintained in the living organism and the only available explanation is that it may be a positive pressure in the haemocoele of the stolonial extension. Circular muscles are around each siphon and these have a few short longitudinal branches projecting back onto each side of the body. The only other body muscles detected are the short, thick, parallel, transverse muscle bands arranged in rows along each side of the mid-ventral and mid-dorsal lines. These rows begin just behind the branchial siphon on each side and extend the full length of the body on the right side of the middorsal line and the left side of the mid-ventral line. They were not detected in the posterior half of the body either on the right side of the ventral mid-line or the left side of the dorsal mid-line. The row on the anterior half of the body to the left of the mid-dorsal line is continuous with a row of longer transverse bands extending down the posterior half of the middle of the left side of the body just ventral to the gut. The ends of these short muscle bands sometimes are frayed into a short brush of separate fibres; or they come to a point or the fibres are exactly the same length and are tightly bunched together. The muscles are not symmetrical, one end often being different from the other A ring of fine, pointed branchial tentacles are at the base of the branchial siphon. The dorsal tubercle, in a roomy peritubercular area, has a reverse C-shaped opening with slightly irregular in-turned horns. A dorsal ganglion is just dorsal to the tubercle. The dorsal lamina consisting of numerous finely tapered languets extends the whole length of the branchial sac to the oesophageal opening at the postero-dorsal corner of the body. The branchial sac has many rows of short, oval and very even perforations. Fine parallel internal longitudinal vessels supported on small papillae extend longitudinally over the internal surface of the branchial wall at right angles to the rows of perforations, and create meshes containing 2 – 3 perforations occasionally crossed by parastigmatic vessels. Ciliated epithelium was not detected lining these perforations. Despite the highly contracted condition of the muscles in these specimens, these pharyngeal perforations appear not to be distorted. The short oesophagus curves anteriorly to open into the elliptical, obliquely orientated stomach. The internal lining of the stomach is raised into about thirty fine, transparent, crowded, sometimes crinkled, parallel folds. The cylindrical intestine, opening from the tapered, distal end of the stomach, extends anteriorly in a gentle arc, only slightly concave dorsally. The anal opening, about a third of the body length behind the atrial opening, has a margin broken into shallow lobes. The tubular ovary, in the body wall, outside and almost completely covered by the gut, extends from the stomach to about halfway up the intestine. A crowded network of very fine testis tubules covers the walls of the stomach and extends to about two-thirds of the distance up the intestine. The vas deferens continues along the dorsal midline opening, with the oviduct, at the base of the atrial siphon. The network of testis tubules can be seen from the inside of the stomach through the thin stomach wall. They tend to obscure the crowded fine parallel slightly crinkled folds that line the internal wall of the stomach.

Remarks. Millar (1963) examined the type specimens of Ciallusia longa Van Name, 1918 although he did not recognise its possible relationship with Pterygascidia Sluiter, 1904. Tokioka 1967 first noted the similarity of P. mirabilis Sluiter, 1904 and Ciallusia longa Van Name, 1918. He observed the row of pointed languets along the dorsal mid-line in both species and established that Sluiter’s report of a plain dorsal lamina in P. m i r a b i l i s was incorrect. Nevertheless, although Tokioka (1971 a) thought the genera synonymous (a view now well established) he thought their type species were distinct species of the genus Pterygascidia, separated from each other by small differences in the arrangement of muscle bundles, orientation of the stomach, the presence of stomach folds, differences in the size of the external flaps of test around the apertures and (in P. mirabilis) an oblique row of short muscles about one third of the distance down each side of the body (that appear to be the isolated terminal ends of longitudinal siphon muscles separated by sudden contraction). Tokioka (1967) counted about 60 internal longitudinal branchial vessels on each side in C. longa but does not record the number of rows of perforations, which invariably are numerous and usually are not precisely recorded. Tokioka (1971 a) also thought the pharyngeal wall of C. longa to be perforated by ciliated stigmata (although these have not been demonstrated in any of the material). A digestive gland observed only by Millar (1963) may be obscured by the sheath of testis follicles around the wall of the intestine (see Monniot F and C. 2003). In his account of P. mirabilis, Tokioka’s (1971 b) reports that in one of the specimens the distal end of the vas deferens at the base of the atrial siphon is divided into separate short branches. This has not been observed in other specimens and it may be an abnormality. Most of these differences between specimens appear to be variations associated with the size, state of contraction of the body wall and / or the condition of the specimens. A few (such as the absence of stomach folds and a smooth-edged dorsal lamina) result from errors in interpreting the material. It appears that all recorded specimens are conspecific and that the species are synonymous.

Distribution. Previously recorded: Philippines (Van Name 1918, Tokioka 1971 a, Monniot F. and C. 2003); Timor (Sluiter 1904); Indonesia (Millar 1988). New record: CSIRO SS 05 / 07 (Stn 129 - 034, 121.0584 E 15.7928 S, 119 m, 1 July 2007, Beam Trawl QM G 328489 — 12 specimens). The species is known from waters 12 m to 216 m deep.