Thouarella antarctica ( Valenciennes, 1846 )

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 : 22-26

1. Thouarella antarctica ( Valenciennes, 1846)

Figs 4 , 5 .

Primnoa antarctica Valenciennes, 1846 : pl. 12, figs 2, 2a (only images); Milne-Edwards 1857: 140; Gray 1857: 286; 1859:

483; Kölliker 1865: 135; Bayer 1956: F220 (list). Thouarella antarctica Gray 1870: 69 ; Wright & Studer 1889: 65–66, pl. 21 fig. 1; Thomson & Henderson 1906: 38 (list);

Gravier 1914: 48–56, pl. 7 figs 31–34, pl.10 figs 52–55 (samples not seen); Molander 1929: 75 (samples not seen) Not Thouarella antarctica Hickson 1907: 9–10 , pl. 2, figs 19, 24 (= unknown) Thouarella (Parathouarella) antarctica Kükenthal 1915: 150 (key); 1919: 433–435; 1924: 299 Not Thouarella (Euthouarella) antarctica Broch 1965: 24–26 , pl. 2, figs 5–7 (= T. pendulina ) Not Thouarella (Thouarella) antarctica Cairns & Bayer 2009: 27 (listed), 33–34, fig. 6g –l (= T. chilensis )

Material examined: Holotype, MNHN, Oct.0000–208, 1836–1839 Venus expedition, Malouine Islands , Falkland Islands, depth unknown.

Other material: ZMH, R/ V W. Herwig, sta. 270, south of Falkland Islands, SW Atlantic , 53˚00’S, 60˚00’W, 375 m, 9 Feb 1971 ; ZMH, R/ V W. Herwig, sta. 256, Burdwood Bank, SW Atlantic, 53˚56’S, 63˚40’W, 400 m, 6 Feb 1971 ; ZMH, R/ V W. Herwig, sta. 311, off Patagonia, SW Atlantic, 46˚54’S, 60˚28’W, 480 m, 18 Feb 1971 ; ZMH, R/ V W. Herwig, sta. 361, west of Falkland Islands, SW Atlantic , 51˚55’S, 61˚50’W, 200 m, 12 Jul 1966 ; NHM 89.5.27.43, H.M.S. Challenger, sta. 148A, off Crozet Island , 46˚53’S, 51˚52’E, 1005 m, 3 Jan 1874 ; USNM 97965, R/V Hero , cruise 715, sta. 870, 54˚34’S, 64˚W, 84 m, 24 Oct 1971.

Thouarella antarctica is the type species of Thouarella . Unfortunately, the holotype, held in MNHN, is in very poor condition. F.M. Bayer studied the holotype and identified one NMNH catalogued specimen as T. antarctica ( USNM 97966). Having studied the holotype we disagree with Cairns and Bayer (2009) that USNM 97966 is T. antarctica ; this is in fact T. chilensis as it has more scales in the abaxial row than is usual for T. antarctica , wider and thicker operculars, and marginal scales that are not smooth edged. Some additional specimens of T. antarctica , listed above, were found at SMF (belonging to ZMH). Description of sclerites and images herein are from three holotype polyps; no SEM images of whole polyps were taken as the specimen is too fragile and thus only an opercular view without a full complement of marginal scales was possible ( Fig. 4e,f ). Colony and polyp descriptions are from the specimens listed above.

Description

Colonies are sparsely branched. Branchlets depart the main stem in up to 5 directions from all sides of branches in a typical bottlebrush arrangement. The branchlets are generally simple, however some secondary branching occurs. The axis is yellow, tough, horny, and can be brittle towards the apex. The holdfast is calcareous.

Polyps are isolated, 1.6–2.3 mm high (average 2 mm), and modestly flared distally; they project from all sides of the branchlet at 60–80˚, and are tightly placed at 11–23 polyps per cm (such that, in lateral view, most polyps overlap), and are more clustered at branchlet tip (25–30). Some polyps occur on the main stem. There are 5–7 scales in each abaxial row ( Fig. 4c ), 5–6 scales in lateral rows and 8 longitudinal rows reducing to 5 at the polyp base.

There are 2 circles of 4 operculars: one upper, one lower ( Fig. 4e ), and they can join to form a full cone, although sometimes there are gaps. Operculars are arrowhead-shaped ( Fig. 5 a-f), 415–820 µm high (average 650 µm), 200–480 µm wide (330 average µm), with a H:W of 1.7–2.1 (average 2) and have a longitudinally concave outer surface with granules radially arranged from a proximal centre, fading towards the distal edges. The inner opercular surface has a large, simple keel with several longitudinal striations, and the proximal half has densely arranged, small tubercles in the centre and serrated striations towards the distal edge. Lateral and distal edges are finely serrate; the proximal edge is coarsely lobate.

Marginal scales are triangular ( Fig. 5g –i ), broader than operculars, 645–870 µm high (average 760 µm), 390–750 µm wide (average of 600 µm), and with a lower average H:W than the operculars (1.3, ranging from 1–1.7). The inner surface of marginals bears a complex multi-keel with lateral extensions ( Fig. 5j ), which can project beyond the scale edges, and thus be visible from the outer surface; tubercles cover the inner surface proximally and are found across the keel base. The outer surface is covered with granules radially arranged from the central proximal area. The distal area of the keel and the lateral edge of marginals are finely serrated; proximal edges are coarsely lobate and covered with tubercles.

Submarginals are circular to widely elliptical in shape ( Fig. 5k–n ), 430–650 µm high (average 760 µm), 370–750 µm wide (average 600 µm), with a H:W of 1–1.7 (average 1.3), and tend to have a pointed distal edge, and a small median distal keel ( Fig. 5m ) or teeth on the inner surface. The scales directly beneath the submarginals are also sometimes modified in the same manner.

Body-walls scales ( Fig. 5o–v ) form a variety of shapes from circular to elliptical. They are 380–460 µm high, 440–530 µm wide (average 420 and 500 µm respectively), usually wider than high, having an average H:W of 0.85 (range from 0.7–1), with small and sometimes strong ridges on the distal edge of the inner surface ( Fig. 5o,t ). Sometimes the abaxial body-wall scale at the polyp base is very wide ( Fig. 5o ). The inner body-wall scale surfaces are covered with dense tubercules and the outer surfaces sparsely covered with granules sometimes arranged radially from the central area.

The coenenchymal scales ( Fig. 5w ) are small, 160–330 µm high, 190–380 µm wide (average of 220 and 300 µm respectively), with a H:W of 0.4–1 (average 0.75), irregularly circular to oval in shape with a regular distribution of peaked granules on the outer surface, and a dense arrangement of tubercules on the inner surface. All polyp sclerites usually have an irregularly coarse, lobate proximal edge and a finely serrate distal edge.

Distribution

This species is known from 500 km north of the Falkland Islands to the southern tip of South America, from 200–480 m depth. An unseen sample has been described from Crozet Island (1005 m depth, Wright & Studer 1889), so the bathymetric range may extend deeper and the geographic range further east.

Remarks

The holotype is from the Falkland Islands and consists of one branch broken off mid-stem ( Fig. 4b ). The original description of Thouarella antarctica (as Primnoa antarctica ) by Valenciennes has a beautiful illustration of the colony and polyps (see Fig. 1 ). Through a microscope, the operculars do appear elongated and the illustration reflects this, although it is exaggerated; the image is otherwise a good likeness. Descriptions thereafter rely heavily upon this drawing as well as Milne-Edwards’ (1857) short description. Kölliker (1865) did give further details of body-wall scale dimensions (still under the name of P. antarctica ,) but Gray’s (1870, p. 45) redescription as T. antarctica only gave a general description: “Coral simple, with long, simple filiform branches, spreading on all side of the stem. Bark formed in large imbricated scales. Polyp-cells smooth, bell-shaped, scattered on upperside of branches, covered with four or five series of imbricate scales.”

Wright and Studer’s (1889) description of Thouarella , alongside several new species, gives the first moderately detailed definition of the genus and more information about T. antarctica . Kükenthal (1915, 1919) correctly described T. antarctica as having marginal scales with lateral projections off the keel, and this remains one of the defining characteristics.

In 1924 Kükenthal listed T. antarctica as having nine or ten scales in the abaxial row but we believe this is a mistake, as he listed 4–6 abaxial scales in both his 1915 and 1919 keys.

A bottlebrush colony form, distally flared polyps, and complex multi-keeled marginal scales make T. antarctica just as representative of Thouarella as any species found within Group 1. Thus, T. antarctica is considered to be its type species.

Comparisons

As mentioned in the historical summary, the ‘ Köllikeri’ ( Kükenthal 1912) and ‘ Antarctica’ groups ( Versluys 1906; Kükenthal 1912) have very similar species. This includes T. koellikeri Wright & Studer, 1889 , T. variabilis , T. versluysi Kükenthal, 1907 , T. striata Kükenthal, 1907 , T. clavata Kükenthal, 1908 , T. brucei Thomson and Richie, 1906 , and T. hicksoni, Thomson, 1911 —the ‘ Köllikeri’ group including species with isolated polyps and foliate, elongated marginals. In 1919, Kükenthal rearranged the entire genus, moving T. antarctica into the ‘ Köllikeri group’, as it has relatively tall marginals, leaving T. crenelata , T. chilensis and T. affinis in the ‘ Antarctica’ group.

The polyps of Thouarella crenelata have marginal scales with distinctively serrate distal borders and a high number of scales in the longitudinal abaxial row and are thus quite different from the remaining ‘ Antarctica’ / ‘Köllikeri’ group species. Thouarella koellikeri , T. viridis Zapata-Guardiola and López-González, 2010 , T. antarctica , and T. chilensis are all very similar, having isolated polyps larger than 1.5 mm and low triangular marginals. All of these species have very small differences in the number of body-wall scales in the abaxial row of the polyps and the complexity of the marginal keel. Although all species have a similar polyp size they vary in polyp density; the number of polyps per cm splits this group into two as T. antarctica and T. chilensis have very clustered polyps, more than 15 per cm (up to double this in fact), whereas the remaining two species have lower polyp densities. Thouarella antarctica and T. chilensis are considered distinct from each other as the latter has polyps with operculars and marginals that have a heavily striated outer surface and a full opercular cone, all absent in T. antarctica . Thouarella chilensis also tends to have more scales in the abaxial row than T. antarctica . Both T. koellikeri and T. viridis have tall opercular cones and the latter has protruding toothed submarginals.

Thouarella koellikeri generally has longer polyps than T. antarctica due to the greater number of body-wall scales in the abaxial row (7–10 as against 5–7), and the operculars of the latter have simple keels whereas in the former there are several small striations adjacent to the keel. Also, the branchlets of the former leave the stem in mainly two directions, whereas T. antarctica has a true bottlebrush form.

The scales of the opercular cone of the polyps of T. viridis are more closely fitted than those of T. antarctica ; and the former has shorter marginals that are less elongated, with wide lateral extensions of the keel that are rarely visible when viewed from the abaxial side of the polyp, something that is normal in T. antarctica . Also, the operculars of T. viridis have a larger, more pronounced keel than those of T. antarctica , the keel of the latter operculars being complex and spread laterally.

Several species have a similar number of longitudinal abaxial body-wall scales as T. antarctica and many were in the original ‘ Köllikeri ’ group and are thus compared here:

Thouarella variabilis differs from T. antarctica in having longer marginals, polyps that are more flared, and smaller, narrower, lanceolate operculars.

The polyps of Thouarella brevispinosa are generally longer than those of T. antarctica , and have more scales in the abaxial row. The marginals of polyps of T. brevispinosa are larger and have a longer, simple keel, lacking the lateral extensions common in T. antarctica .

The polyps of Thouarella brucei and T. antarctica are similarly shaped, but the marginal keels of the former are simpler with the inner and outer opercular scale surfaces tending to be smoother, the abaxial row with fewer scales, and colonies that can appear bilateral rather than bottlebrush, as in T. antarctica .

Thouarella striata has distinctive striations on the marginal scale inner surface that are absent in T. antarctica . In addition, the marginal scales of T. striata have a less complex keel (although not markedly), where the keel is not visible from an abaxial side view, and there are generally fewer scales in the abaxial row.

Thouarella clavata and T. bipinnata have polyps of a similar size as T. antarctica (the polyps of T. bipinnata are only slightly larger), however, polyps of T. bipinnata are clavate ( T. antarctica has more splayed polyps) and its colony is uniplanar (whereas T. antarctica is bottlebrush). Thouarella clavata is bottlebrush (although branchlets sometimes bend creating a bilateral appearance) and has a similar number of abaxial scales as polyps of T. antarctica . However, the polyps of T. bipinnata are less densely arranged and branchlets are longer, narrower, and less rigid than those of T. antarctica .

Thouarella minuta polyps are under 1 mm tall making them considerably smaller than those of T. antarctica .

Figures

FIGURE 1. First illustration of Thouarella antarctica (from Valenciennes 1846): a) full colony; b) close up of polyps.

FIGURE 4. Thouarella antarctica. SMF, R/V W.Hertwig 1971, sta. 256: a) 21 cm colony; d) close up of branchlet. Holotype, MNHN, Oct.0000-208: b) holotype colony; c) abaxial polyp view; e) stereo opercular view of polyp, some marginals missing; f) opercular view of polyp with all marginals.

FIGURE 5. Thouarella antarctica, holotype, MNHN, Oct.0000-208: a,c) outer and b,d–f) inner surface of opercular scales; g,h) outer and i,j) inner surface of marginal scales; k,l) inner and m,n) outer surface of submarginal scales; o–q,s,t) inner and r,u,v) outer surface of body-wall scales; w) coenenchymal scales—inner surface top row first and second left, bottom row second left, remainder are of the outer surface.