Styela andeepensis Maggioni & Tatián, 2022

Maggioni, Tamara, Rimondino, Clara, Taverna, Anabela, Reyna, Paola, Lagger, Cristian, Alurralde, Gastón, Calcagno, Emilia & Tatián, Marcos, 2022, Abyssal ascidians (Chordata, Tunicata) from the Weddell Sea, Antarctica, including a new Styela species and stomach content identifications, Zootaxa 5093 (3), pp. 296-314 : 302-303

Styela andeepensis Maggioni & Tatián sp. nov.

Figure 4

Material examined: 63°34’S, 50°41’W, station 13 , 2610 m, 14 March 2005, one specimen epibiotic of Culeolus suhmi . Holotype: MZUCVI0195

Etymology: In reference to the oceanographic campaign ANDEEP III in which the specimen was collected.

Description. The specimen measures 0.37 cm high and 0.4 cm wide, with both apertures pointing towards the oral aperture of its basibiont. These are located at a short distance from each other. The tunic is thick and opaque, with a yellowish-brown tone ( Fig. 4A ). There are 22 simple oral tentacles of three different sizes, disposed in a circle on a thickening of the mantle. The pre-pharyngeal band is simple and undulated. It encloses a rounded dorsal tubercle in a small indentation. The dorsal lamina has a smooth border with slight undulations. It is continuous and tall, with its distal border curving on itself. The musculature consists of very thin fibers evenly distributed on the mantle, which is very thick. The branchial sac has thick and compressed longitudinal and transverse vessels. The latter alternate with thinner transverse vessels. The branchial longitudinal vessels are disposed as follows:

Left side: DL - 4 (7) 3 (6) 4 (10) 6 (6) 2 - E

Right side: DL - 7 (9) 5 (11) 6 (10) 4 (8) 6 - E

The gut forms a very closed loop. There is a short esophagus that connects with a cylindrical stomach, which occupies almost ⅓ of the distance between the endostyle and the dorsal lamina. The stomach has 10 internal longitudinal folds. The intestine forms a swelling at its most proximal end. The border of the anus is lobed. There are two gonads on each side of the body. The tubular ovaries are long and thin, of an undulated, almost snaking shape. The most distant part of the posterior ovary on the left side is covered by the intestine. Small masses of testicular follicles surround the distal ends of each ovary, all disposed in close contact, although not attached, to them. In total, there are 13 endocarps on the right side of the body and nine on the left side. On the left side, the antero-dorsal ovary is completely covered by a row of six endocarps attached dorsally, while the postero-ventral ovary has a small row of two endocarps on its ventral side. In addition, there is a big endocarp at a mid-distance between both left gonads. On the right side, the antero-dorsal ovary is partially covered by a row of four endocarps dorsally. There is also one small endocarp on its ventral side, close to the testicular follicles. The postero-ventral ovary has a row of four endocarps on its ventral side. In addition, there are three small endocarps located dorsal, ventral and next to the testicular follicles of this gonad. Finally, there is a big endocarp at a mid-distance between the right gonads ( Fig. 4B ).

Stomach contents. No items were found in this specimen.

Remarks. Styela andeepensis sp. nov. is very close to Styela glans Herdman, 1881 , whose geographic range encompasses the South Western Atlantic and the Southern oceans (including the Weddell Sea) at a bathymetric range from 35 m to 1680 m ( Herdman 1881; Monniot C. & Monniot F. 1976; Monniot C. 1978; Monniot C. & Monniot F. 1980; Monniot C. & Monniot F. 1983; Monniot C. & Monniot F. 1994). The main differences between both species include: the structure of the branchial sac, with S. andeepensis sp. nov. having true folds and S. glans having only an accumulation of longitudinal vessels; the size and shape of the ovaries, being shorter and wider in S. glans ; the number and disposition of endocarps, being more numerous in S. glans ; the presence of parastigmatic vessels in S. glans , being completely absent in S. andeepensis sp. nov.; and the presence of atrial tentacles, being numerous in S. glans but completely absent in S. andeepensis sp. nov.

Styela andeepensis sp. nov. is also very close to Styela magalhaensis Michaelsen, 1898 , a species cited for the SE Pacific, SW Atlantic and Southern oceans ( Michaelsen 1900; Ärnbäck-Christie-Linde 1929; Millar 1960; Millar 1970; Diehl 1977; Monniot C. & Monniot F. 1983; Monniot C. & Andrade 1983) at a maximum depth of 495 m ( Diehl, 1977). However, they differ in the following characters: the ovaries have similar lengths, but are wider and have a straight shape in S. magalhaensis (not the characteristic snaking shape found in S. andeepensis sp. nov.); the testicular follicles in S. magalhanesis form compact clusters disposed very close to the lower end of the ovaries, with almost no distance between them, while in S. andeepensis sp. nov. they form smaller clusters disposed close but at some distance from the distal end of the ovaries; atrial tentacles are present in S. magalhaensis , while they are completely absent in S. andeepensis sp. nov.; endocarps are more numerous in S. magalhaensis ; the gut-loop is wider and the stomach is longer in S. magalhaensis ; and the pre-pharyngeal band makes a pronounced V around the dorsal tubercle in S. magalhaensis .

Styela brevigaster Millar (1988) is found in the SE Pacific at bathyal depths (550–750 m). Millar (1988) acknowledged its close similarity to S. magalhaensis , from which it was differentiated by the dimension and shape of the stomach. The specimens observed by Millar (more than 100) presented shorter and wider stomachs in comparison to the long and cylindrical stomachs observed by Michaelsen (1900) in S. magalhaensis . Styela andeepensis sp. nov. has a long, cylindrical stomach, more similar to that of S. magalhaensis .

Styela atlantica ( Van Name, 1912) and Styela sigma Hartmeyer, 1906 are two almost identical species. Both resemble very closely S. andeepensis sp. nov. when morphological characters are compared. However, the number of endocarps are fewer than those reported for S. sigma (approximately 83 on one side; Nishikawa T. 1991, figure 32A) and S. atlantica (approximately 30 on the right side; Monniot F. 2016, figure 26A,B). In addition, there are no tentancles near the atrial aperture as observed for S. atlantica (Monniot F. 2016) nor an “atrial velum welldeveloped, sprinkled with numerous fine atrial tentacles over its inner surface” as described for S. sigma (Nishikawa T. 1991) . The species S. atlantica is found in the northern hemisphere, in Atlantic waters down to 300 m (Monniot F. 2016); S. sigma is found in the northern hemisphere, in Pacific waters down to 800 m (Sanamyan K. 2000).

Other Styela species with two gonads on each side of the body, which are also very close to S. andepeensis sp. nov. are Styela canopus ( Savigny, 1816) and Styela partita ( Stimpson, 1852) . However, these species have testis follicles distributed along both sides of the ovaries (not forming apical clusters as in S. andeepensis sp. nov.). Both were collected from shallow waters in the central Atlantic ( Van Name 1945; Monniot C. 1983; Rocha et al. 2012).

Figures

FIGURE 4. Styela andeepensis sp. nov.. A: individual attached to the tunic of Culeolus suhmi; B: scheme showing internal anatomy (without branchial sac).