Aplidium radiatum ( Sluiter, 1906 )

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 : 308-310

Aplidium radiatum ( Sluiter, 1906)

(®gures 1B, 2)

Psammaplidium radiatum Sluiter, 1906: 25 .

Sidnyum radiatum: Monniot, 1978: 10 (synonymy and type revision); Monniot and Monniot, 1983: 30.

Macroclinum pererratum Sluiter, 1912: 458 .

Sidnyum pererratum: Monniot, 1978: 14 (synonymy and type revision); Monniot and Monniot, 1983: 31.

Not Synoicum pererratum: Millar, 1982: 15 .

Material examined. St. 4096, 255± 285 m, 40 colonies.

Description. The smaller colonies, 1±1.5 cm in diameter, are nearly spherical or hemispherical, the larger, up to 3 cm in greatest diameter, are oval in outline and relatively ¯at, no more than 1 cm in height (®gure 1B). The brittle test is densely impregnated with sand. Vertical zooids open on the upper surface of the colony. They are not visible through the test but their position sometimes is marked by a low elevation of the test. Rather irregular radial rows of zooids are distinguishable on some colonies.

The zooids are usually 4±5 mm long, but are smaller in the contracted state. Thorax, abdomen and postabdomen are of about equal length in most zooids (1.2 ±1.5 mm), but relaxed thorax may be much larger (up to 3 mm long). The branchial siphon has eight triangular pointed lobes. A large and wide atrial languet extends out from the small atrial aperture and divides into three long lobes. Thin longitudinal muscles are widely spaced on the thorax, and form two bands continuing along ventral side of abdomen and postabdomen. The branchial sac has 12 rows, with 16±18 stigmata per half row, but stigmata were counted only in one wellexpanded zooid. The stomach is in the middle of the abdomen and has ®ve or six rather regular but sometimes indistinct ¯attened folds. A group of large male follicles is in the anterior half of the postabdomen.

An unusual elongated horizontal body sometimes curves around the ventral surface at the top of the oesophageal neck (®gure 2B). It has its own thin membraneous wall and is ®lled by granules of unknown nature. We have examined similar bodies only once previously, in poorly preserved undescribed Aplidium from the north-west Paci®c, but in those specimens the bodies were present in all zooids.

Remarks. All zooids examined have eight lobes of the branchial siphon and are identical to those redescribed by Monniot (1978) from the type material of A. radiatum . Zooids from type colonies of A. pererratum have fewer stigmata per row (14) but otherwise they are identical with those of A. radiatum . Colonies of these two species were treated as very similar by Monniot and Monniot (1983), although they described hemispherical cushions in A. radiatum and ¯attened oval masses in A. pererratum . This is not a valid diOEerence, both types and intermediate forms are present in our material from st. 4096. These species are obviously conspeci®c and we do not support the opinion of Monniot and Monniot (1982) that they should be treated as distinct because the larva of A. pererratum is not known.

We examined many colonies collected on the New Zealand Plateau, from close to the location of? Synoicum pererratum: Millar, 1982 and conforming exactly to his description and ®gure ( Millar, 1982, ®gure 4a, b). The zooids have six branchial lobes and therefore belong to another species.

The species with eight-lobed branchial siphon were usually included in Sidnyum Savigny, 1816 , but it was synonymized with Aplidium by Monniot and Monniot (1987) (see also Kott, 1992a). Nakauchi (1986) discussed the problem of the genus Sidnyum . It was pointed out that the type species of this genus, S. turbinatum Savigny, 1816 , according to the original description of the branchial siphon has six lobes, so the only character that diOEerentiates both genera is invalid. Most probably, however, the six-lobed instead of eight-lobed siphon was indicated in the original description of S. turbinatum by mistake, and several species with the eight-lobed branchial siphon were synonymized with the S. turbinatum ( Circinalium concrescens Giard, 1872 ; Parascidium crispum Huitfeldt-Kaas, 1896 ).