Agnezia abyssa, 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 : 325-328

Agnezia abyssa sp. n.

(®gures 13, 14)

? Agnesia sp. : Monniot and Monniot, 1982.

Material examined. St. 866, 7694±7934 m, about 400 specimens; st. 867, 8004± 8116 m, 16 specimens. HOLOTYPE KIE 1/1028 (st. 866).

Description. The specimens are 4±9 mm high and 3±6 mm in greatest diameter, many of them are attached to small sponges and form large clusters, resembling somewhat a colony of Perophora (®gure 13B). The shape is irregular, but usually the body narrows posteriorly to a distinct peduncle, which terminates in one or more thin test processes. The test is thin and translucent, the surface is smooth or has a small amount of sand and mud adherent to it. In most specimens apertures are sessile, well removed from each other and situated on the opposite ends of the upper surface, but in some strongly contracted specimens there are pronounced diverging siphons. The siphons, however, appear only on the body itself, and not on the test, which is no longer attached to the body in many specimens. Probably both apertures have six lobes, but these were di cult to count.

The body is contracted into a compact mass much smaller than the less contractile test. The muscles around the apertures are diOEerently developed: the atrial siphon has strongly developed circular muscles, and few thin and short radial muscles, while circular muscles of the branchial siphon are weak and do not extend below the branchial velum. A number of thick muscles radiate from the branchial aperture, some are short, but others continue to the end of the body. Short parallel transverse muscles form a band on each side of the endostyle. The longitudinal muscles are con®ned mostly to the posterior end of the body and between the circular muscles of the atrial siphon and the band of ventral transverse muscles (®gure 14).

Short, thick and numerous tentacles of various sizes are in several circles. A high velum well separated from the tentacles makes a distinct dorsal V. An unusually thick prepharyngeal band runs close to the velum, or even on the velum laterally and ventrally, and makes a pronounced V dorsally, with the tip curving to the left side. The dorsal tubercle was not detected, the neural ganglion is on the left of the dorsal V. Each side of the branchial sac has three double rows of about six wide stigmata with no more than 1.5 coils. The stigmata are enlarged and the tissue between them is greatly reduced. Radial parastigmatic vessels are present on some stigmata. The double rows are separated by two transverse vessels, each with a high triangular dorsal languet. One or two low ¯attened papillae are present on each transverse vessel on the left side, and two papillae on the right. The ventral margin of the branchial sac is attached to the body wall far from the endostyle, along the line between the lateral longitudinal and ventral transverse muscles.

The shape of the intestine is variable and depends on the degree of contraction of the muscular body. In relaxed specimens, including the holotype, the gut loop is straight and parallel to the last row of stigmata, but in some specimens the rectum bends dorsally and makes an angle with the axis of the primary loop. The long narrow oesophagus curves through a right angle to enter into the stomach. The shape of the stomach resembles that of the genus Synoicum , it is small, oval, asymmetrical, smooth-walled and distinctly demarcated from the oesophagus and intestine. The intestine is ®lled with ®ne sediments. The anus in all specimens examined is small and apparently has a smooth margin. The gonad, consisting of a central ovary surrounded by large oval male follicles, is present in the gut loop. The male and female ducts are thick and contain large eggs and spermatozoa. The gonoducts follow the rectum and open near the anus.

Remarks. In the unequal development of circular muscles around the branchial and atrial apertures and the presence of two rows of stigmata between the transverse vessels, the present species resembles A. celtica ( Monniot and Monniot, 1974) , an abyssal species described from the north Atlantic and later reported from the south Atlantic and east Indian Ocean. This species has, however, ten rows of stigmata, and each stigma makes several coils. Another abyssal Atlantic species, A. atlantica ( Monniot and Monniot, 1973) , diOEers greatly in many features, especially in the structure of the branchial sac.

In the limited number of papillae A. abyssa resembles A. tenue ( Monniot and Monniot, 1983) , from the south-western Atlantic, from 18 m. This similarity is convergent and is related to the small size of A. tenue ; in the other features (body muscles, branchial sac, gut loop) the species are diOEerent.

Monniot and Monniot (1982) recorded three specimens of Agnezia sp. from exactly the same locality as the present material (55 ss 07 ¾ S, 26 ss 01 ¾ W, 7600 m). They are similar to the present material externally, but have four rows of spirals, separated by transverse vessels with four papillae. According to Monniot and Monniot (1982: 111), their specimens cannot belong to any known species because of the`peculiar muscle arrangement’. They have strong circular muscles around the apertures and`two lateral symmetrical areas, which come together on the dorsal axis’. It is di cult to understand without a ®gure what these`symmetrical lateral areas’ are, but they may correspond to the longitudinal muscles on the body sides in the present species. In any case it seems unlikely that at such a great depth and in the same place two diOEerent species of the same genus may exist. The Monniots’ specimens were not in a good condition and this may explain the mentioned diOEerences.

This is one of the most deep-water species and it is probably abundant in the South Sandwich Trench.