Obelia dichotoma ( 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

Obelia dichotoma ( Linnaeus, 1758)

Figs. 51, 52

Sertularia dichotoma Linnaeus, 1758: 812 .

Obelia dichotoma .— Segerstedt, 1889: 12, 25.— Lönnberg, 1898: 52; 1899b: 17.— Jäderholm, 1909: 63, pl. 5, fig. 15.— Rees & Rowe, 1969: 17.— Östman, 1982a: 156; 1982b: 228; 1983: 7.

Laomedea dichotoma .— Jägerskiöld, 1971: 63.— Östman, 1979a: 6; 1999: 19.

Obelia dichotoma form dubia.— Östman, 1983: 7; 1999: 19.

Obelia dichotoma form plicata.— Östman, 1983: 7; 1999: 19.

Type locality. UK: coast of SW England ( Cornelius 1975a: 267) .

Museum material. Kosterhavet, 58°52.567’N, 11°06.313’E, 6–30 m, 07.ix.2010, SCUBA , on algae and Sertularella polyzonias , one colony, up to 2.5 cm high, with gonothecae, coll. B.E. Picton, ROMIZ B3898 .

Remarks. Linnaeus (1758) evidently based his account of Sertularia dichotoma (= Obelia dichotoma ) on the earlier report of Ellis (1755), giving its provenance merely as “Habitat in Oceano.” Cornelius (1975a) restricted the type locality of the species to southwest coast of England, where it was found by Ellis “...in great abundance.”

Identification of material examined here was based on morphology and examination of the cnidome. In addition to A-type b-rhabdoids (microbasic b-mastigophores), exceedingly slender, elongate, and curved I d - type isorhizas ( Fig. 52c ), and curved but somewhat thicker I D - type isorhizas ( Fig. 52b ), as described in this species by Östman (1982b, 1987, 1999), were observed on hydranths of the specimens.

Details of the natural history of O. dichotoma , a well-known and widespread species, are given in Cornelius (1975, 1990, 1995). Although generally supposed to be virtually cosmopolitan, many identifications of the species remain doubtful. It was reported from Denmark by Kramp (1935b, as Laomedea dichotoma ) but not from the Oslofjord, Norway, by Christiansen (1972).

Reported distribution. West coast of Sweden.—From northern Kosterfjord to the Öresund ( Lönnberg 1898; Rees & Rowe 1969).

Elsewhere.—North Atlantic from the Barents Sea to the Mediterranean Sea in Europe, and from the Gulf of St. Lawrence to the Gulf of Mexico in North America ( Broch 1910; Fraser 1944; Bouillon et al. 2004).

Figures

FIGURE 52. Obelia dichotoma, nematocysts, ROMIZ B3898: a, A-type b-rhabdoids (microbasic b-mastigophores); b, ID-type isorhiza; c, Id-type isorhiza.