Protoholozoa pedunculata Kott, 1969

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 : 315-320

Protoholozoa pedunculata Kott, 1969

(®gure 6)

Protoholozoa pedunculata Kott, 1969: 35 ; Monniot and Monniot, 1983: 103.

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

Description. The colony much resembles Kott’s (1969) original drawing. The complete peduncle is about 65 mm long, its diameter is only 5 mm below the head and gradually increases toward the proximal end, where a ¯at attachment area is 25 mm in diameter. The head, 40 mm in height and 35 in greatest diameter, is an inverted and somewhat laterally ¯attened cone with a more or less ¯at top. The transparent test is soft but relatively ®rm. White opaque zooids are in vertical or oblique rows, up to six in each row.

The zooids are contracted, most of them are 9±13 mm long, but near the base of the head they are much smaller, 2±3 mm long. Both apertures are on the ¯attened dorsal side of the thorax, on short wide siphons. The branchial aperture is some distance down from the top of the thorax, the atrial is near its posterior end. Siphonal margins are not even, although the lobes are indistinct and in most zooids di cult to count, but at least in one zooid both siphons have six distinct lobes. Thoraces are parallel to the surface of the colony. The abdomina may be curved ventrally and sometimes are nearly at right angles to the thorax and the surface of the colony, but many zooids are straight. Longitudinal muscles extend from the branchial siphon (®ve or six), the intersiphonal region (one to three) and the atrial siphon (several thinner bands) and along the thorax toward its posterior ventral corner where they form a single band along the ventral side of the abdomen. Twentythree branchial tentacles were counted in two zooids, they are unusually long and thick, all of the same size. There are only two transverse branchial vessels; on each side of the thorax they are connected by one short external longitudinal bar. The stomach has internal longitudinal folds obscured in most zooids by strong contraction of the abdomen. Gonad consists of a compact mass of numerous small male follicles, one or two ova in the gut loop.

Remarks. The genus Protoholozoa contains six species, four of which have been recorded only once. Kott (1992b) suggested that many features used to distinguish species, particularly the number of transverse branchial vessels, may be variable. All known specimens of P. pedunculata , including the present one, have two vessels, and this feature seems to be stable.

The species is known from the Scotia Sea, Antarctic Peninsula and south-east Paci®c Basin.

Situla rebainsi Vinogradova, 1975 (®gure 7) Situla rebainsi Vinogradova, 1975: 297 .

Material examined. St. 4093, 3700±3970 m, two specimens.

Description. Both specimens are in good condition and their shape and structure are identical, although one specimen is much larger than the other. The body shape is peculiar: two branchial lips are widely open and almost symmetrical. The body resembles a ¯at oval disk attached to the substratum by a long peduncle (®gure 7A). The disk of the larger specimen is 10 Ö 5 cm, the peduncle is 16 cm long and 1.5± 2 cm thick; and the smaller specimen has a disk 5.5 Ö 2.5 cm and peduncle 7 Ö 0.8 ± 1 cm. The peduncle is attached to the right side of the outer surface of the ventral lip and has a tuft of hair-like outgrowths on its proximal end. The atrial aperture is on a short dome-shaped siphon on the outer surface of the disk and somewhat displaced to the left from the mid-dorsal line. The test is soft, transparent and covered by sparse minute papillae and by ®ne mud particles, especially on the proximal half of the peduncle.

Circular muscle ®bres are thin, more or less regularly spaced on the branchial lips and gathered together between the lips. The most marginal circular muscles meet together at the lip corners and form a fan of short thick radial muscle bands. Such arrangement of muscles resembles those of Octacnemus . Radial muscles are ®ne, sparse and dividing into several thin terminal branches. The atrial siphon has thin radial and circular muscles. No muscles present on peduncle.

The branchial sac occupies the central half of the disk. Branchial tentacles are in the characteristic more or less elliptical single series (®gure 7A), far from the branchial sac dorsally and ventrally and close to it laterally. Ventrally they are on the free upper rim of a high velum (or ventral pocket). The tentacular line has weak inēxions on the level of the upper rim of the perforated zone of the branchial sac and on its most dorsal point. The leaf-like tentacles are 0.5 mm long in the small specimen and up to 1 mm long in the large one. Dorsal and lateral tentacles have small ®nger-like oOEshoots or papillae on their outer surface, but these were not detected on tentacles on the ventral velum. The prepharyngeal groove is an oval line, close to the perforated zone of the branchial sac dorsally and laterally; ventrally it is on the bottom of the ventral pocket and far from the perforated zone. Sensory papillae were not detected on the branchial sac.

The stigmata are crowded and large (0.25±0.4 mm in diameter, and up to 0.5 mm in larger specimen). They form a ring around the visceral mass interrupted ventrally. The perforated zone occupies about one-third of the surface of the branchial sac. The branchial tissue is thin, its structure is not as complicated as in other species of Situla . A well-developed dorsal lamina and three transverse vessels are on each side of the branchial sac. The visceral mass has the usual structure.

Remarks. The present species was previously known only from the single specimen described from the South Sandwich Trench from 5530±5651 m. Vinogradova’ s (1975) original and very particular description allows us, without any doubts, to identify the two present specimens from the Orkney Trench as S. rebainsi , although there are some diOEerences. In the type specimen the body gradually tapers to the short peduncle, which is ®ve times shorter than the body, and the body is not so ¯at and disk-shaped as in our specimens. Ampullae on the branchial tentacles were apparently overlooked by Vinogradova, they become visible only after staining and are absent on the ventral tentacles. Such ampullae are characteristic for S. galeata Monniot and Monniot, 1991 , Megalodicopi a rineharti (Monniot and Monniot, 1989) and were found in some tentacles of Kaikoja multitentaculata ( Vinogradova, 1975) .

Megalodicopia rineharti (Monniot and Monniot, 1989) (®gures 8, 9) Situla rineharti Monniot and Monniot, 1989a: 19 ; 1991: 391.

Material examined. St. 4093, 3700±3970 m, one specimen.

Description. The single specimen (®gure 8) is about 8 cm high and 4 cm wide. The oval and laterally ¯attened body tapers to a distinct short peduncle that is about 1.5 Ö 1.5 cm and has a ¯at attachment area on its end. The glossy transparent test is thin (no more than 0.5 mm), cartilaginous, free from any kind of outgrowth and, unlike other species of Situla , is more or less rigid. The test on two oral lips and around them is wrinkled and somewhat thinner than around the rest of the body. A few sparse foraminiferans, hydrozoans and polychaetae tubes are attached to the test surface. The terminal atrial aperture is small and inconspicuous. The oral aperture, in the middle of the body side, has two equal lips, and is closed in the preserved specimen.

Circular and radial muscles form a dense network with nearly rectangular meshes on the branchial lips (®gure 9). Circular muscles are crowded around the margin of each lip and gathered together at the corners of the branchial siphon (where the lips meet). Fine circular and radial muscles are on the atrial siphon. A circular ribbon of muscle ®bres encircles the body dorsally and laterally at the level of the oral tentacles; dorsal ®bres are much thinner than the lateral. The ribbon is interrupted ventrally, its two free extremities ending abruptly near the end of the peduncle. Strong longitudinal muscles on the ventral side of the body extend from the level of the ventral margin of the perforated area of the branchial sac and form a wide ribbon running along each side of the peduncle.

The branchial sac has the shape of a shallow wide cup with a ¯at bottom. Leaflike oral tentacles, each with a small ®nger-like oOEshoot, are arranged in an oval (®gure 9A) on a low rim. The rim slightly increases in height ventrally, but does not form the deep ventral`pocket’ found in Situla . The prepharyngeal band is close to the ring of tentacles and has a long narrow dorsal indentation. Numerous low inconspicuous papillae are on the dorsal and lateral sides of the branchial sac, between the prepharyngeal band and the stigmata. Papillae were not detected on the ventral side of the branchial sac. The oesophageal opening is on the anterior third of the branchial sac, and the perforated area is limited to its central third, below the oesophagus opening. The stigmata are large, oval and not numerous (®gure 9A, C). The retropharyngeal groove runs from the oesophageal opening to the ventral limit of the perforated area. The endostyle is in the posterior third of the branchial sac.

The gut forms an almost circular loop.

Remarks. The holotype of this species from the Galapagos Islands has a short but distinct peduncle (see Monniot and Monniot, 1989a, ®gure 2F). In the present specimen the peduncle is much longer, and this is the only diOEerence between this specimen and the description of the type.

Monniot and Monniot (1989a: 22) had some doubts to which genus the species should be assigned, to Situla or to Dicopia , and they decided that`the Galapagos species is closer to Situla ’. In another paper ( Monniot and Monniot, 1991) this species was described as Situla , but the ®gure caption refers to `Dicopia rineharti ’. Although the cup-like branchial sac of M. rineharti is intermediate between the ¯at one of Situla and the cone-shaped branchial sac of Dicopia or Megalodicopia , this species has the muscular stalk characteristic of Megalodicopia and lacks the ventral`pocket’ characteristic of Situla .

Megalodicopia rineharti resembles M. hians Oka, 1918 , the type species of Megalodicopia , recently redescribed by Sanamyan (1998), but it diOEers in the size and position of the perforated zone of the branchial sac, the shape of the tentacles and the prepharyngeal groove and in some other characters.