Pharyngodictyon mirabile, Herdman, 1886

Sanamyan, K. E. & Sanamyan, N. P., 2002, Deep-water ascidians from the south-western Atlantic (RV Dmitry Mendeleev, cruise 43 and Academic Kurchatov, cruise 11), Journal of Natural History 36 (3), pp. 305-359 : 310-311

Pharyngodictyo n mirabile Herdman, 1886

(®gure 3)

Pharyngodictyon mirabile Herdman, 1886: 152 ; Monniot and Monniot, 1985b: 14.? Pharyngodictyo n mirabile: Kott, 1969: 45 .

? Polycitor fungiformis Millar, 1970: 106 .

Material examined. St. 916, 4664±5631 m, one colony.

Description. Mushroom-shape d colony has a thick and short, obliquely growing stalk expanded supporting a terminal ¯at disk about 14 mm in diameter and expanding to a hemispherical base 16± 12 mm diameter and 13 mm high. The colony is attached to the substratum by the whole ¯at lower surface. The test is grey, hard and contains dispersed minute sand grains. The zooids open over the entire upper surface of the disk, although most of them are con®ned to its margin. They are small, 2±4 mm long, strongly contracted and damaged, and their detailed structure cannot be determined. The thorax forms a right angle with the abdomen. The branchial siphon is terminal, well developed and has six indistinct lobes; the atrial aperture is close to the posterior end of the thorax but its shape cannot be determined. About six thick longitudinal muscles arising on the branchial siphon on each side of the thorax. Twenty tentacles were counted in one zooid only; they are long, laterally ¯attened at the base, arranged in two circles, and may project from the branchial siphon. The prepharyngeal band is as high as the ®rst transverse vessel. The branchial sac has ®ve transverse vessels with triangular dorsal languets displaced to the left from mid-dorsal line. Transverse vessels are connected by thin longitudinal strips, but their number and arrangement is obscured. The stomach has approximately ten longitudinal folds. Most zooids are no longer connected with their postabdomens, which are situated in the stalk of the colony and ®lled by parenchymatous tissue. Gonads are not present.

Remarks. Although the zooids are in poor condition, the specimen can be identi®ed more or less certainly with the type species of the genus, P. mirabile , on the basis of the colony shape, and the structure of the thorax with ®ve transverse branchial vessels. This is the fourth record of this species, the previous three were from the south-west Indian Ocean (type locality), from the south-east Atlantic, Cape Basin ( Monniot and Monniot, 1985b) and South Orkney Islands ( Kott, 1969). Millar (1982) re-examined the holotype and found ®ve transverse branchial vessels. According to him, P. mirabile: Kott, 1969 , is another species, while Kott (personal communication) believes that it was assigned correctly:`Perhaps I mistakenly counted an extra transverse vessel. The shape of colony, albeit arborescent, is so like yours and Herdman’ s, and so is the orientation of the zooidsÐto accommodate the posterior position of the atrial siphon’.

Polycitor fungiformi s Millar, 1970, an abyssal species recorded from the Argentine Basin, has some features similar to the present species. It had no gonad and it is unclear why it was assigned to Polycitor . Millar (1970: 107) reported ®ve rows of stigmata,`the stigmata are elongated in a dorsoventral direction’. His ®gure 6D suggested that there are no true stigmata; the branchial sac seems to be similar to those of Pharyngodictyon , it probabl y has only transverse branchial vessels connected by one longitudinal bar.