Thouarella affinis Wright and Studer, 1889

TAYLOR, M. L., CAIRNS, S. D., AGNEW, D. J. & ROGERS, A. D., 2013, <strong> A revision of the genus <em> Thouarella </ em> Gray, 1870 (Octocorallia: Primnoidae), including an illustrated dichotomous key, a new species description, and comments on <em> Plumarella </ em> Gray, 1870 and <em> Dasystenella </ em>, Versluys, 1906 </ strong>, Zootaxa 3602 (1), pp. 1-105 : 33-37

4. Thouarella affinis Wright and Studer, 1889

Figs 10 , 11

Thouarella affinis Wright & Studer, 1889: 66–68 , pl.11, fig. 3; Thomson & Henderson 1906: 38 (list); Kükenthal 1912: 302 (listed)

Thouarella (Epithouarella) affinis Kükenthal 1915: 151 (key); 1919: 435–436; 1924: 300 (key)

Thouarella (Thouarella) affinis Cairns & Bayer 2009: 27 (listed)

Material examined: Holotype, NHM 1889.5.27.44, 65 mm fragment, H.M.S. Challenger, sta. 135D, off Inaccessible Island , Tristan de Cunha, 37˚25’S, 12˚22’30”W, 91–128 m, 15 Jul 1874.

Other material: ZMH, R/ V W. Herwig, sta. 232, east of Isla de los Estados, Tierra del Fuego, Argentina, SW Atlantic, 54˚46’S, 62˚30’W, 800 m , 1971.

Description

As only a small fragment was available, descriptions of axis and colony morphology were taken from the type description.

The colony is bottlebrush, ( Fig. 10a ) but can appear bilateral. The axis is yellow, stiff, and brittle, although the branch apex is more flexible. The stem twists in a spiral from base to a quarter-length from the apex whereupon the next twist commences.

The branching of this species is dense, with only 1.5–2 mm between branchlets; becoming denser towards the apex. The branchlets are mostly simple, with some forking (dividing usually in basal region of branchlet), upwardly inclined 60–90˚, and up to 50 mm long. The branchlets are arranged in spirals from 3 sides of the main stem with 4 branchlets occurring within one spiral, but as the stem twists the spiral is difficult to follow.

The polyps are isolated, 1.1–2.1 mm high, at a density of 7–9 per cm (with denser placement at branchlet apex, which generally has a polyp at the tip). The polyps can be modestly flared but generally have a wide, rounded head extending from a slender polyp body making them clavate ( Fig. 10b,c; H :W of 1.3–2.1, average 1.7). The polyps are arranged in short spirals of 3–4 and angled distally at 45–60˚. Each polyp has 8 longitudinal rows with 6–7 scales in each abaxial row and 5 adaxially.

The tall rounded operculum rises above the marginal scales. The operculars are isosceles-triangle shaped ( Fig. 11a,b ), tongue-shaped ( Fig. 11c–f ), or lanceolate ( Fig. 11g ), 540–780 µm high (average 610 µm) and 220–350 µm wide (average 280 µm), with an average H:W of 2.2 (range from 1.8–2.5). The outer scale surface is longitudinally concave with granules across the proximal area and occasional low striations spreading radially from centre towards distal edge ( Fig. 11d–f ). The inner surface has a low, complex multi-keel ( Fig. 11a,b ), or a dense area of low striations ( Fig. 11c ); the proximal half of the scale is tuberculate.

The marginals are somewhat diamond-shaped ( Fig. 11h–j ), 460–610 µm high (average 525 µm) and 460–640 µm wide, with an average H:W of 1 (ranges from 0.8–1.3). The inner surface has a smooth band along the distal edge which is broken by a small, simple keel ( Fig. 11h ) with 2 or 3 adjacent striations whilst the remainder is tuberculate. The adaxial marginals frequently have no defined keel with just 4 or 5 short, sharp striations perpendicular to the distal edge ( Fig. 11i ). The outer surface is mostly smooth with some granules that often form radial striations ( Fig. 11j ).

The body-wall scales occur in a range of shapes from circular and irregular to elliptical ( Fig. 11k–q ). They are generally broader than high, 350–490 µm high (average 430 µm), 380–780 µm wide (average 540 µm), with an average H:W of 0.8 (ranges from 0.6–1.1), and curved slightly away from the polyp body. Tubercles cover the inner scale surface. Granules occur sparsely on the outer surface and sometimes tubercles occur along the proximal edge. The distal edge of all sclerites of this species is finely serrate whilst the proximal edge is irregularly lobate.

There is a single layer of roughly circular ( Fig. 11 r-u) or sometimes elongated coenenchymal scales ( Fig. 11v, w ) that are 190–280 µm high (average 225 µm), 230–430 µm wide (average 300 µm), with an average H:W of 0.8 (range from 0.4–1.1). There are large prominent granules on the outer surface of the scales and fine tubercles on the inner surface.

Distribution

This species is known only from the type location Tristan de Cunha, and also Inacessible Island , mid-South Atlantic. The depth of observed occurrence is 91–800 m.

Remarks

The holotype material examined is damaged so the number of polyps per cm may be an underestimate.

Comparisons

Wright and Studer (1889) considered T. affinis to be very similar to T. antarctica and the polyps of these species do have a similar number of scales in the abaxial rows (6–7 T. affinis , 5–7 T. antarctica ). However, the latter has a more complexly structured marginal keel with large lateral projections compared to the modest keel found on marginals of the former.

The polyps of T. viridis are a similar shape and size to T. affinis and they also have similar shaped marginals, although those of T. viridis are slightly taller. However, the operculars of the polyps of T. viridis are pointed, while those of T. affinis are mostly tongue-shaped (with some triangular-shaped operculars with squared proximal edges). In addition, the body-wall scales of the former have more pronounced striations perpendicular to the distal edge (that are visible in lateral polyp view) than the latter.

The polyps of T. affinis are more rounded than those of T. brucei . The polyps of the latter also have fewer scales in the abaxial row and marginals that are narrower with a higher H:W ratio.

Polyps of T. koellikeri tend to be longer than those of T. affinis , with more scales in the abaxial row. Also, the former has pointed triangular operculars whereas those of the latter are mostly tongue-shaped.

Figures

FIGURE 10. Thouarella affinis, holotype, NHM 1889.5.27.44: a) 13 cm long colony, with polychaete along stem; b) lateral polyp view; c) close up of polyps; d) stereo opercular view.

FIGURE 11. Thouarella affinis, holotype, NHM 1889.5.27.44: a–c,g) inner and d–f) outer surface of opercular scales; h,i) inner and j) outer surface of marginal scales; k–n) outer and o–q) inner surface of body-wall scales; r,w) outer and s–v) inner surface of coenenchymal scales. Some SEM images by ZGR.