Phallusia polytrema (Herdman, 1906)

KOTT, PATRICIA, 2003, New syntheses and new species in the Australian Ascidiacea, Journal of Natural History 37 (13), pp. 1611-1653 : 1631-1632

Phallusia polytrema (Herdman, 1906)

Ascidia polytrema Herdman, 1906: 306 .

Ascidia pandora Kott, 1985: 48 .

Plurascidia marquesana Monniot and Monniot, 2000: 714 .

Distribution. The species is known from the Gulf of Manaar (Herdman, 1906), Heron I. (Great Barrier Reef: Kott, 1985) and the Marquesas Is (Monniot and Monniot, 2000).

Description. (From Ascidia pandora QM GH2045 .) The species is a long-oval shape, with a terminal branchial aperture on a short siphon and a sessile atrial aperture half-way down the dorsal surface. The test is glassy, transparent and thin with sand absent only from an area around each siphon. The body wall is thin and transparent, and lies against the inner lining of the test. Body muscles consist of narrow bands of circular muscles around each aperture and a broad band of parallel transverse bands across the dorsal surface, interrupted by the atrial aperture about half-way along. The transverse muscles in the vicinity of the atrial aperture exchange fibres with the circular muscles around the opening. Branchial tentacles at the base of the branchial siphon are long and narrow. The dorsal ganglion and gland are just antero-ventral to the left end of the anterior transverse muscle. A trail of minute urn-shaped ciliated openings of the neural duct extends anteriorly to terminate in a similar urn-shaped opening in the base of a short, narrow peritubercular V bounded by a high membranous extension of the prepharyngeal groove, the left membrane crossing over the right and continuing in the dorsal lamina which increases in width from the base of the peritubercular V. The dorsal lamina extends about two-thirds of the way down the long, straight branchial sac to the oesophageal opening. The dorsal part of the posterior third of the branchial sac has a deep pleat on the left side just anterior to the retropharyngeal groove. It flattens out as it extends obliquely antero-ventrally behind the gut loop. The otherwise flat branchial sac is characteristic of Ascidiidae , with many rows of stigmata crossed by internal longitudinal vessels with papillae at their junctions with the transverse vessels. Three or four stigmata crossed by parastigmatic vessels are in each mesh. The gut is embedded in the body wall and the gonads are tightly enclosed in the loop. The stomach is long, elliptical with four or five shallow longitudinal folds. The descending limb of the gut loop is swollen into a large almost globular chamber. Sometimes the rectum (see Monniot and Monniot, 2000) is also swollen, although in other specimens it is a narrow tube. The ovary is a narrow much-branched tube and the testis follicles are diffuse and distributed over the gut wall.

Remarks. This unusual species with a thin sandy test resembles Ascidia (Phallusia) caguayensis Millar and Goodbody, 1974 in most characters other than the test (which in the latter species is soft and translucent) and the number of stigmata per mesh (which is eight or nine in A. caguayensis ).

Ascidia (Phallusia) recifense Millar, 1977 has similar accessory openings on the neural duct, but a minute gut loop (like Microgastra Kott, 1985 ). Since gonads were not seen they may be embedded in the test and these species may be in the genus Microgastra —the form of the gut loop being like Plurellidae rather than Ascidiidae . Monniot and Monniot (2000) appear to have overlooked the characteristic gonads embedded in the test in their redefinitions of Plurella , Microgastra and Plurellidae , and also overlooked the similarities between their material from the Marquesas Is and other conspecific and related species referred to above, distinguished from Microgastra (Plurellidae) by their gonads, contained in the gut loop embedded in the body wall (as in Ascididae) rather than in the test.

The specimen of Ascidia pandora Kott, 1985 is mutilated and obscured by sand which is tangled in the branchial sac. Careful re-examination of this specimen has demonstrated features overlooked (the posterior pleat and the accessory neural openings) and misinterpreted (a U-shaped neural opening in the peritubercular area). The specimen is identical to Plurascidia marquesana Monniot and Monniot, 2000 and Ascidia polytrema Herdman, 1906 .

Nishikawa (1991) examined the type specimen of A. polytrema , and found gonads in the gut loop (as in Ascidia ) rather than embedded in the test as in Microgastra granosa , which Kott (1985) had thought to be the senior synonym of A. polytrema . Specimens of A. polytrema from the type locality (kindly sent by Mrs V Meenakshi, Department of Zoology, PPC Mahalaxmi College for Women, Tuticorin, India) have been re-examined, confirming Nishikawa’s observations, and the synonymy (above).

Based on the accessory neural openings into the atrial cavity, Ascidia polytrema Herdman, 1906 and Ascidia (Phallusia) recifense Millar, 1977 most likely belong to Phallusia as Millar (1977) and Nishikawa (1991) propose. The species also has some similarity to Ascidia scaevola (Sluiter, 1904) which has a small compact gut loop, groups of transverse muscle bands, and a large pleat in the branchial sac, but it lacks accessory neural openings. It should be noted that the family Plurellidae , to which the junior synonym of the present species was originally assigned, contains the genera Microgastra (solitary species) and Plurella (colonial species). The zooids of Plurella , as is usual in colonial species (where replication interrupts growth) are small, are entirely embedded in tough common test and demonstrate a colonial organization in their parallel (rather than random) arrangement. There is no evidence that these zooids are other than a true colony. The lack of evidence of active replication in the relatively few examined specimens (see Monniot and Monniot, 2000) should not be taken as evidence that they do not replicate. In fact, the budding process for most known Polyandrocarpa spp. is not documented, yet it is clear from the structure of the colony and the size of the zooids that they are replicates, as is the case with Plurella spp.