Ritterella chetvergovi, Sanamyan & Sanamyan, 2002

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 : 312-313

Ritterella chetvergovi sp.n.

(®gure 4)

Material examined. St. 916, 4664±5631 m, one colony. Holotype 1/1042.

Description. The colony is 12 mm in greatest diameter near the base, and 8 mm in height. It consists of a wide oval basal part and a short, upright, ¯at-topped zooid-bearing column (®gure 4A). The super®cial layer of the test is fragile, coated by minute sand grains and can be easily removed. Sand grains coat the whole surface apart from small areas above the thorax of the zooids. The inner test is soft, transparent, without embedded sand. It contains small blue pigment granules, persisting after 29 years of storage in alcohol; the granules are also present in zooids, especially in the stomach folds. The colony contains few vertical zooids forming a single circle in the zooid-bearing column. Branchial and atrial siphons of each zooid open separately on the top of the sides of the column. The branchial opening is on the short, protruding siphon directed toward the base of the colony, and a sessile atrial opening is above it. There is a central stem of ®rm test in the zooid-bearing column; many other species with cylindrical lobes and zooids arranged in a single circle have similar supporting stems.

The zooids are thick, about 6 mm long in contracted state (®gure 4B). Most are in rather poor condition and only a few can be examined. A wide, nearly rectangular thorax is slightly shorter than the abdomen. About ten widely separated longitudinal muscles are on each side of the thorax and form a ribbon on the ventral side of the abdomen. There are numerous thin transverse thoracic muscles. The branchial and atrial openings are on short siphons at the top of the thorax. They are well removed from each other and both have six rather indistinct lobes. About 30 long and thick tentacles may reach the bottom of the contracted thorax or protrude outside from the branchial siphon. Such long tentacles are present in other colonial deep-water species without true stigmata (e.g. Pharyngodictyon and Protoholozoa ). The present species, however, has true narrow longitudinal stigmata arranged into ten rows. There are about ten stigmata per half row in the middle of the branchial sac, but apparently no more than ®ve stigmata in most posterior row, although they cannot be counted exactly. Rows of stigmata are separated by nine raised transverse vessels, each with a long dorsal languet. Each row is crossed by a parastigmatic vessel. Parastigmatic vessels are as high as transverse vessels separating rows of stigmata, and at ®rst view the branchial sac appears to have much more numerous rows of stigmata.

The abdomen is a little curved ventrally by contraction of ventral muscle ribbon. The oesophagus is of moderate length. The large symmetrical cylindrical stomach is in the middle of the abdomen and has about 20 raised longitudinal and more or less regular folds. The intestine is ®lled by mud and its subdivision into regions is unclear. A stem of parenchymal tissue, apparently epicardium, continues from the bottom of the branchial sac to the end of the abdomen along its ventral side.

The small sac with gonads is just under and somewhat to the left of the pole of the gut loop. Few large male follicles are in a compact bunch, ovary not present.

Remarks. The species seems to have a true postabdomen and therefore should be assigned to Ritterella . Although the postabdomen is small and inconspicuous, and its presence may be doubted, it resembles that in such species as Aplidium circumvolutum (Sluiter, 1900) in which the posterior abdomen (with bunched testis follicles) gets pulled up behind the thorax when the strong muscle bands contract.

A similar species was assigned to Polycitor by Millar (1970) (see remarks under Pharyngodictyon mirabile ). The present species does not appear to belong to Polycitor , which has a long oesophagus, and gonads usually are clearly within the gut loop.

Ritterella has not been recorded previously from below 460 m and the present record from more than 5000 m is the ®rst from the abyssal waters. The only previously known Antarctic species of Ritterella was R. miri W ca Monniot and Monniot, 1983. It diOEers from the present species by numerous features, including number of tentacles, stomach folds, length of postabdomen.

Etymology. The species is named after Alexander Chetvergov.