Obelia geniculata ( Linnaeus, 1758 )

Calder, Dale R., 2012, On a collection of hydroids (Cnidaria, Hydrozoa, Hydroidolina) from the west coast of Sweden, with a checklist of species from the region 3171, Zootaxa 3171 (1), pp. 1-77 : 50-51

Obelia geniculata ( Linnaeus, 1758)

Fig. 53

Sertularia geniculata Linnaeus, 1758: 812 ; 1761: 541; 1767: 1312.

not Campanularia geniculata .— Lovén, 1836: 281, pl. 6, figs. 1–10, pl. 7, figs. 11–18 [= Gonothyraea loveni ( Allman, 1859) ].

Obelia geniculata .— Segerstedt, 1889: 12, 25.— Jäderholm, 1909: 62, pl. 6, fig. 1.— Östman, 1982a: 156; 1982b: 228; 1983: 6; 1999: 19.

Obelia (Laomedea) geniculata .— Gislén, 1930: 338.

Laomedea geniculata .— Jägerskiöld, 1971: 63.— Östman, 1979a: 6.

Type locality. UK: England, Kent, Dover ( Cornelius 1975a: 273) .

Museum material. Saltö naturreservat, outer ledge, 58°52’40”N, 11°06’53”E, <1 m, 11.ix.2010, collected manually, on attached red algae, one colony, up to 1.3 cm high, with gonothecae, ROMIZ B3924 .

Remarks. One of the most familiar of all hydroids, Obelia geniculata ( Linnaeus, 1758) has been recorded many times along the coast of western Sweden (see Checklist, p. 66). It is also one of the most common hydroids in Danish waters ( Kramp 1935b, as Laomedea geniculata ). However, in the nearby Oslofjord, southern Norway, it is not common anywhere and is apparently of irregular occurrence ( Christiansen 1972). Obelia geniculata is the easiest of the local species of the genus to identify because of its asymmetrically thickened internodes, short hydrothecal pedicels, and thickened hydrothecae. The species also differs from others of the genus Obelia Péron & Lesueur, 1810 from Europe in acid phosphatase patterns ( Östman 1982a) and cnidome ( Östman 1982b, 1999).

Detailed accounts of the biology of O. geniculata have been given by Cornelius (1975a, 1990, 1995b). Reports indicate that this hydrozoan is virtually “cosmopolitan” in shallow waters, but recent evidence implies that cryptic species may exist ( Govindarajan et al. 2005).

Reported distribution. West coast of Sweden.—From Koster to southern Kattegat ( Jägerskiöld 1971: 63).

Elsewhere.—North Atlantic-Arctic: from the White Sea and Jan Mayen Island to the Mediterranean Sea in Europe, and from Hudson Bay to the Gulf of Mexico in North America ( Linko 1911; Cornelius 1975a).

Figures

FIGURE 53. Obelia geniculata: part of colony with a hydrotheca and a gonotheca, ROMIZ B3924. Scale equals 0.5 mm. FIGURE 54. Obelia longissima: part of colony with three hydrothecae, ROMIZ B3889. Scale equals 0.5 mm.