Stylaster bithalamus Broch, 1936

Cairns, Stephen D. & Zibrowius, Helmut, 2013, Stylasteridae (Cnidaria, Hydrozoa, Filifera) from South Africa, Zootaxa 3691 (1), pp. 1-57 : 26-28

Stylaster bithalamus Broch, 1936

Figs. 2 A–B, 12A–I, 27

Allopora oculina: Studer, 1878: 636 .—Hickson, 1900: 94.—Boschma, 1966b: 112.

Stylaster (Allopora) bithalamus Broch, 1936: 13 , 18, 74–76, pl. 12, fig. 34, text-fig. 25.

Allopora bithalamus, Boschma, 1957: 18 ; 1960b: 435–445, pls. 1–3, text-figs. 1–4 (redescription).—not Eguchi, 1964: 7 (= S. eguchii ).—Boschma, 1966b: 110–112, figs. 1e–h (comparison to A. eguchii ).—Vervoort & Zibrowius, 1981: 38.

Stylaster bithalamus: Broch, 1936: 75 .—Cairns, 1983b: 429 (listed).

Types and Type Locality. Holotype, ZMC. Type Locality: 33°57’S, 18°15’E (Table Bay, South Africa), 93 m.

Material Examined. MN SM185, 1 male colony, SAM, and SEM stub 1702 (USNM); UCTES SCD 316, 1 male colony, RMNH 15821; UCTES SCD289, 1, SAM; UCTES SCD316, 1, SAM; UCTES AFR801, RMNH 15827, and SEM stub 1703 (USNM 76534); Valdivia 93, 1, ZMB 7040; Valdivia 104, 7 colonies, ZMB 7042; specimens mentioned by Boschma (1960b), Naturalis Biodiversity Centre; Allopora oculina reported by Studer (1878) from Gazelle station 16, ZMB 1654.

Description. Colonies are of moderate size, robust, uniplanar, and sparsely branched, the largest colony (UCTES AFR801 described by Boschma 1960b) 6 cm in height and 8 x 12 mm in basal branch diameter. Branching is dichotomous and somewhat irregular, the distal branches being fairly slender (2–3 mm in diameter), not blunt, but usually terminating in an apical cyclosystem. Commensal polynoid tubes are present in all specimens examined ( Figs. 2 B, 12A), including the type, all eight specimens reported by Boschma (1961), and the specimens reported herein (see Remarks). The polynoid gall tubes are fairly large, up to 6 x 3 mm in cross section, and dominate the posterior face of larger coralla. The coenosteal texture is reticulate-granular ( Fig. 12 C), the strips ranging from 52–67 µm in width, covered with irregularly-shaped granules. Nematopores were not observed. The colonies are uniformly white.

Cyclosystems are homogeneously arranged on the anterior and lateral surfaces of most branches; the polynoid gall tube occupying the posterior face does not bear cyclosystems. Distal branches tend to have a sympodial arrangement of cyclosystems ( Fig. 12 A), like those Stylaster species in Group B. Cyclosystems are circular to slightly elliptical in shape, 0.85–1.5 mm in diameter, and flush to slightly raised above the coenosteum. Cyclosystems are never arranged in rows. Based on 25 cyclosystems, the range of dactylopores per cyclosystem is 7–16; the average is 11.0 (ơ = 0.85); and the mode is 10. Even though based on a low number of cyclosystems these numbers are consistent with those reported by Broch (1936) based on much larger numbers, range, 9–16; average, 11.5; and mode, 11, and Boschma (1961), range,1–19; average, 10.8; and mode, 10. Diastemas are common but not very wide.

The gastropores are circular (about 0.45 mm in diameter), and the gastropore tube may be as deep as 1.6 mm. As explained in detail by Boschma (1961), the gastrostyles are variable in shape, Boschma describing four distinct but intergrading shapes. In general, the style has a short cylindrical to discoidal base that supports the mid-section of the style, which is the widest part of the structure. Above this middle section, which can be torus-shaped, the elongate apical spine either gradually or abruptly diverges from the lower section, ending in a pointed tip ( Fig. 12 D). The entire gastrostyle may be as tall as 0.55 mm and occupy the lower third to half of the gastropore tube. At the level of the lower gastrostyle apical spine is a solid constriction of the tube (analogous to a ring palisade, see Fig. 12 D), called a sphincter by Broch (1936), and thus suggesting a comparison to the double-chambered gastropore tubes of Conopora . This belt-like structure is similar to that described for S. griseus . The dactylotomes are about 0.1 mm wide, and the pseudosepta are thick (0.15–0.27 mm in width) with convex upper surfaces. Each dactylopore contains several dactyloglossae ( Figs. 12 E–G), which occlude most of the tube.

Female ampullae are primarily internal, hardly seen in apical view except as low bulges about 1.2 mm in diameter. They communicate to the upper chamber of an adjacent gastropore tube via an efferent pore. Male ampullae are partially internal, seen in apical view as small (0.3–0.5 mm in diameter) mounds ( Fig. 12 I), with apical efferent pores.

Comparisons. Stylaster bithalamus is compared to the morphologically similar S. griseus in the account of that species. Among the other South African Stylaster species in Group A ( Table 1 ), S. bithalamus differs in having dactyloglossae, internal female ampullae with efferent pores that open into gastropore chambers, and nonlanceolate gastrostyles.

Remarks. Although Boschma (1961) stated that polynoid gall tubes were often associated with this species, but that they were not associated with his specimen from UCTES AFR801, examination of a specimen from UCTES AFR801 (RMNH 15827) did show evidence of polynoid gall tubes.

Distribution. Continental shelf of southern South Africa from Saint Helena Bay (most of Western and Eastern Cape Provinces)(Fig. 27), 11– 155 m.

Figures

FIGURE 2. Colonies of various species: A – B, Stylaster bithalamus, MN SM 185, male, anterior and posterior views of same colony, respectively. C, Stylaster griseus, holotype. D – F, Stylaster amphiheloides, syntypes, BM 1842.12.2.79. G – H, Stylaster amphiheloides, MN SM 228, posterior and anterior views of same colony. I, Stylaster lonchitis, Liltved specimen. J, Stylaster kenti, holotype.

FIGURE 12. Stylaster bithalamus (A – B, D, F – I, MN SM 185, SAM; C, E, female from UCTES AFR 801, USNM 76534): A, stereo view of branch segment bearing lateral cyclosystems, polynoid gall tube to right. B, cyclosystems. C, reticulate-granular coenosteal texture. D, stereo view of gastropore tube, gastrostyle, and sphincter. E, stereo view of a cyclosystem showing one small dactyloglossa. F – G, large and small dactyloglossae, respectively. H – I,? male ampullae.