Fig. 38
Sertularia tenella Alder, 1856: 357, pl. 13, figs. 3–6.
Sertularella tenella.— Segerstedt, 1889: 17, 26.— Jäderholm, 1909: 102, pl. 11, fig. 15.— Kramp, 1935b: 178, fig. 73C.— Rees & Rowe, 1969: 19.— Jägerskiöld, 1971: 64.— Cornelius, 1979: 292; 1995b: 81, fig. 19A.
Type locality. UK: England, Northumberland (Cornelius & Garfath 1980: 285).
Museum material. Kosterhavet, 58°53.039’N, 11°05.602’E, 160– 30 m, 06.ix.2010, biological dredge, R / V Nereus, three colonies, with one on a shell fragment, one on Abietinaria abietina, and one on a gastropod egg case, up to 1.2 cm high, without gonophores, ROMIZ B3881.
Remarks. Hydroids of Sertularella tenella Alder, 1856 somewhat resemble those of S. rugosa (Linnaeus, 1758) in being relatively small and in having annulated hydrothecae. Cornelius (1995b: 82) listed six characters useful in distinguishing S. tenella from its congener: (1) the aperture is perpendicular to the hydrothecal axis rather than being outward-inclined, (2) hydrothecae are widely separated vertically rather than being bunched together, (3) a notch is absent just below the rim on the abcauline wall of the hydrotheca, (4) hydrothecae are adnate for about ¼ rather than ¼–½ of their length, (5) the number of hydrothecal annulations is 5–6 rather than 3–4, and (6) its typical substrate is usually other hydroids rather than flustrid bryozoans.
Records indicate that S. tenella is common along the coast of western Sweden from the mid-Kattegat northwards (see Checklist). By contrast, it is reportedly infrequent in Danish waters (Kramp 1935b) and in the Oslofjord (Christiansen 1972).
Records of this typically boreal species from the Caribbean region (e.g., Fraser 1944) are almost certainly erroneous.
Reported distribution. West coast of Sweden.—From the border with Norway to mid-Kattegat (Jägerskiöld 1971).
FIGURE 39. Sertularia tenera: part of branch with two pairs of subopposite hydrothecae, ROMIZ B3927. Scale equals 0.5 mm.
Elsewhere.—In the North Atlantic from the Barents Sea and Svalbard to the Atlantic coast of France (Cornelius 1995b; Ronowicz 2007), and from Hudson Bay and Greenland to Cape Cod (Fraser 1944; Calder 1970). In the North Pacific from the Bering Sea southeastwards to California and southwestwards to Japan (Fraser 1937; Yamada 1959; Naumov 1960).