Thouarella brucei Thomson and Ritchie 1906

Thouarella brucei Thomson and Ritchie, 1906

Figures 2–6

Thouarella brucei Thomson and Ritchie, 1906: 852 –854, pl. 1, fig.1, pl. 2, fig. 1.; Kükenthal, 1919: 439; 1924: 301. not Thouarella brucei, Broch, 1965: 27 –28, pl. 4, figs. 11–13.

Material examined. Lectotype (here designated): NMS.Z.1921.143.1298 “Scottish Antarctic Expedition (1902-1904), Burdwood Bank, Gough Island (St. Helena)”, one colony. Paralectotypes (here designated): NMS.Z.2010.038.1 and BM 1912.11.9.2, Scottish Antarctic Expedition (1902-1904), stn. 346, 54º25’S, 57º32’W, Burdwood Bank, 102.4 m depth, 1 December 1903, one colony each; NMS.Z.2010.038.2, same data as in the lectotype; ZMA COEL 3574, Scottish Antarctic Expedition (1902-1904), South Atlantic Ocean, fragment of a colony. Additional material: ZMH C11748, ANT XIX/5, stn PS61/167-01, 53º23.68’S, 42º42.23’W, west of South Georgia Island, 308.1–334.5 m depth, 9 April 2002, five fragments.

Description of the lectotype. The specimen is only a fragment of the parent colony (Figure 2 A), 11 cm in total height and about 8.6 cm in width. Main stem ramified up to third order giving simple branchlets (Figure 2 B) up to 2 cm in length, distributed all around, up to 8 branchlets per centimetre. Axis brown, broken at its proximal portion, without holdfast. Basal axis diameter 2.8 mm.

Individual polyps (Figure 2 B) slightly bent upward and arranged in spirals around branchlets; 10–12 polyps per cm. Polyps also present on main stem. Polyps (Figure 3) clavate, about 1.5–2.1 mm in height and 0.6–0.8 mm in diameter with a low operculum. Polyp body with 8 longitudinal rows of scales overlapping one another, 4–5 scales on each longitudinal abaxial row (Figure 3 A) and 3 scales on each adaxial row (Figure 3 B).

Opercular scales (Figure 4, 5A), 0.38–0.66 mm in height and 0.23–0.50 mm in width, arranged in two cycles of four: inner cycle concave isosceles-shaped with bilobed base and rounded tip with an incipient keel; outer cycle larger, isosceles-triangle-shaped with distal inner surface multi-keeled. Proximal inner surface tuberculate, covering about half of their length. Outer surface radially granular. Basal margin irregular. Free margin finely serrated.

Marginal scales (Figure 5B) eight in number, 0.41–0.66 mm in height and 0.47–0.54 mm in width, roughly rhomboidal in shape with a complex tip; adaxials reduced. Inner proximal surface tuberculate, covering about half of their length, distal surface smooth with medial process multi-keeled. Outer surface granular. Basal margin with small granular processes, free margin finely serrated.

Body scales (Figure 6 A) irregular-fan shaped, 0.28–0.49 mm in height and 0.32–0.50 mm in width. Inner surface almost completely tuberculate, upper body scales with short keel. Outer surface granular. Free margin finely serrated.

Coenenchymal scales (Figure 6 B) roughly round to oval-shaped, 0.14–0.43 mm in maximum length; inner surface tuberculate, outer surface granulate forming ridges, free margin irregular.

Geographic and bathymetric distribution. The species is known from Burdwood Bank and Gough Island (Saint Helena), SubAntarctic, and from west of South Georgia Island, (Figure 1), between 100 and 334.5 m in depth.

Variability. The paralectotypes and the additional material examined have a similar bottlebrush colonial structure to that of the lectotype. The main stem can be unbranched or ramified up to third order. The polyps have a wider range in size, from 1.3 to 2.4 mm in height and from 0.46 to 0.84 mm in diameter. The opercular scales can vary from 0.28 to 0.77 mm in height and from 0.10 to 0.50 mm in width. The marginal scales vary from 0.42 to 0.78 mm in height and from 0.34 to 0.58 mm in width. The body scales vary from 0.24 to 0.54 mm in height and from 0.28 to 0.67 mm in width. The coenenchymal scales vary from 0.06 to 0.46 mm in maximum length. Distribution and form of the sclerites from polyps and coenenchyme are similar to that of the lectotype.

Thouarella brucei Thomson and Ritchie, 1906, lectotype (NMS.Z.1921.143.1298):, opercular scales; B,

marginal scales. ∗ inner surface view.

Remarks. Thomson and Ritchie (1906) described Thouarella brucei from the material collected during the Scottish Antarctic Expedition. The original description refers to specimens from different localities “Burdwood Bank, 56 fathoms, December 1, 1903; Gough Island, 100 fathoms, April 22, 1904; St. Helena ”. However, they did not associate the specimens described with the localities. Due to this lack of information in the original description, on the label of the specimens and also in database of the NMS (Fiona Ware pers. comm.), we cannot determine the exact collection locality of the lectotype and some paralectotypes. However, the presence of this species at Burdwood Bank is assured from the data for the paralectotypes NMS.Z.2010.038.1 and BM 1912.11.9.2.

After this study, the only known material recognised for Thouarella brucei will be restricted to the specimens described in the original description and the additional material from west South Georgia here reported. However, several specimens from Smithsonian’s collection will be attributable to this species in the near future, increasing our knowledge of the geographic and bathymetric distribution of this species (Taylor et al. submitted). Broch (1965: 27) reported an additional colony from Burdwood Bank, but the examination of the specimen has shown it should no longer be considered as Thouarella brucei (see remarks on Digitogorgia brochi sp. nov. in this paper).

It should be noted that Kükenthal (1919: 439) mentioned that Thomson and Ritchie could have confused the opercular scales with the marginal scales and also had doubts about the number of the “opercular” scales reported. Furthermore he emphasised the lack of measurements and pictures of polyps, later classifying T. brucei as species dubiae incertae sedis (Kükenthal, 1924: 301).

One of the characters used to differentiate Thouarella species (Kükenthal, 1924; Cairns and Bayer, 2009) is the number of scales in the longitudinal abaxial row. T. brucei shows 4–5 scales in the abaxial row, being comparable to T. pendulina (Roule, 1908), T. versluysi Kükenthal, 1907 and T. hickson i Thomson, 1911. A useful character to differentiate those species is the number of polyps per centimetre present on the branchlets. While polyps of Thouarella brucei have an arrangement of about 12 polyps per centimetre, polyps of T. pendulina have a very crowded arrangement, up to 70 polyps per centimetre (Roule, 1908). There are only up to 4–5 polyps per centimetre in T. versluysi (Kükenthal, 1907: 202) and even fewer in T. hicksoni (Thomson, 1911: 886).

The examination of the type material has allowed us to give a more accurate diagnosis of the species wherein the polyp scales are shown in more detail than in the original description and some initial ranges of variability observed by Thomson and Ritchie have been increased.