Obelia dichotoma (Linnaeus, 1758)

Galea, Horia R., 2010, Notes on a small collection of thecate hydroids (Cnidaria: Hydrozoa) from Tristan da Cunha, south Atlantic, Zootaxa 2336, pp. 1-18 : 13

Obelia dichotoma (Linnaeus, 1758)

( fig. 5 G)

Sertularia dichotoma Linnaeus, 1758: 812 .

Obelia dichotoma – Cornelius, 1975: 265, figs 3–4 . – Cornelius, 1995b: 296, fig. 69.

Material examined. Stn. T10, 15.ii.2007: one small, sterile stem, on plumose bryozoan. Distribution. Nearly cosmopolitan in coastal waters (Cornelius 1995b). This is the first record of this species for Tristan da Cunha.

Figures

FIGURE 3. A and B: Filellum sp. (material from station Stn. I 10) – portion of colony (A); hydrotheca (B). C: Halecium sp. – four erect stems. D: Halecium cf. beanii (Johnston, 1838) – fragment of monosiphonic part of stem with origin of a side branch. E and F: Halecium cf. delicatulum (Coughtrey, 1876), morphotype 1 – portion of stem (E); hydrotheca (F). G to P: Halecium? delicatulum (Coughtrey, 1876), morphotype 2 (part) – whole colony on piece of alga (G); cross section through the stolon (H); basal part of stem (I); portion of colony (J); various branching patterns (K – M); fragment of stem showing internodes (N); various shapes of primary hydrophores (O); gonotheca arising from primary hydrophore (P). Scale bars: 50 µm (H); 100 µm (F); 200 µm (B, F, O); 300 µm (A, E, I, P); 400 µm (D, K, L, M); 500 µm (N); 2 mm (J); 1 cm (G).

FIGURE 4. A to E: Halecium? delicatulum (Coughtrey, 1876), morphotype 2 (continued) – gonotheca replacing a hydrotheca (A) or arising from stolon (B); mature female gonotheca with eggs (C); various shapes of female gonotheca (D); cross section through gonotheca (E). F and G: Amphisbetia minima (Thompson, 1879) – basal portion of stem (F); gonotheca (G). H to M: Sertularella sp. – basal part of stem (H); fragment of stem showing its straight shape (I) and mode of branching (J); two internodes (K); shape of hydrotheca (L); frontal views of two hydrothecal apertures showing 3 and 5 internal projections of perisarc (M). N and O: Sertularella gayi (Lamouroux, 1821) – fragment of monosiphonic part of stem with basal parts of three side branches (N); various shapes of the hydrotheca (O); P and R: Aglaophenia parvula Bale, 1882 – hydrotheca and detail of its margin (P); female corbula (R). S: Antennella quadriaurita Ritchie, 1909 – detail of stem in lateral (left) and frontal (right) views. Scale bars: 100 µm (M, P); 200 µm (F, S); 300 µm (A, B, E, K, L, O); 400 µm (J); 500 µm (C, D, G); 750 µm (H); 1 mm (I, R); 1.5 mm (N).

FIGURE 5. A to C: Plumularia setacea (Linnaeus, 1758) – stem internode and basal part of cladium (A); female gonotheca (B); stolonal nematothecae (C). D: Silicularia rosea Meyen, 1834 – hydrotheca (middle), female (left) and male (right) gonothecae. E: Clytia gracilis (M. Sars, 1850) – hydrotheca atop its pedicel. F: Clytia paulensis (Vanhöffen, 1910) – hydrotheca atop its pedicel. G: Obelia dichotoma (Linnaeus, 1758) – fragment of stem with hydrotheca. H: Obelia geniculata (Linnaeus, 1758) – stem internode with hydrotheca. Scale bars: 100 µm (C); 200 µm (G); 300 µm (A, B, F, H); 400 µm (E); 1 mm (D).