Halecium articulosum Clark, 1875

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 : 30-31

Halecium articulosum Clark, 1875

Fig. 29

Halecium articulosum Clark, 1875: 63 , pl. 10, fig. 6.— Jäderholm, 1909: 58, pl. 5, fig. 7.

Halecium sessile .— Segerstedt, 1889: 16, 26.— Cornelius, 1975b: 406 [not Halecium sessile Norman, 1867 ].

Halecium articulosus .— Jägerskiöld, 1971: 62 [incorrect subsequent spelling].

Type locality. USA: New England coast, several locations between Eastport , Maine , and Long Island Sound ( Clark 1875: 63) .

Museum material. Kosterhavet, 58°52.424’N, 11°06.178’E, 35– 11 m, 06.ix.2010, biological dredge, R / V Nereus , one colony, 10.5 cm high, without gonophores, ROMIZ B3887 .

Remarks. Halecium articulosum Clark, 1875 resembles H. halecinum ( Linnaeus, 1758) in general colony form. It differs in having branches given off in several planes instead of one, and its branches tend to be more curved. Female gonothecae are kidney-shaped to mitten-shaped with a lateral aperture, instead of being oblong with a terminal aperture. Hydrothecae are strictly sessile. Secondary hydrothecae, when present, also appear sessile because they arise directly from the diaphragm of the hydrothecae beneath them rather than being pedicellate.

Confusion exists in the literature about the identity and synonymy of this species. Cornelius (1975b) examined type material of H. articulosum from Casco Bay, Maine, USA, and concluded that it was conspecific with H. sessile Norman, 1867 , originally described from Scotland. Later, he regarded them as questionably conspecific ( Cornelius 1995a). Still later, when describing their hydranths ( Cornelius 1998), he treated them as distinct species. Kramp (1935b) distinguished H. articulosum from H. sessile in having (1) adjacent hydrothecae at an angle of about 90° rather than being in the same plane, (2) internodes about twice as long as wide, instead of greater, (3) stems that are thick and strongly branched instead of thin and sparsely branched. Hydroids illustrated as H. sessile in Cornelius (1995a) correspond with Kramp’s (1935b: 144, 149, fig. 62C) account and illustration of H. articulosum . Earlier, Jäderholm (1909) had concluded that Swedish hydroids of the kind examined here were identical with H. articulosum and not H. sessile .

The identity of Halecium filiforme Alder, 1862 , a nominal species originally based on a mix of at least two species and representing a possible threat to H. articulosum or H. sessile , was resolved by Cornelius (1975a). A lectotype was chosen that links it to Halecium muricatum ( Ellis & Solander, 1786) .

Halecium plumosum Hincks, 1868 bears some resemblance to H. articulosum , but the original account is scarcely sufficient for identification. Unfortunately, material studied by Hincks (1868) was also sterile. Cornelius (1975a: 409) concluded that this hydroid resembled “...specimens referable to H. sessile sensu Norman ...” The similarity between H. plumosum and H. sessile was noted again more recently ( Cornelius 1998). Halecium plumosum is regarded here as a nomen dubium, although it was maintained as valid by Schuchert (2011). Material from the Oslofjord identified as H. plumosum by Bonnevie (1899) was examined by Christiansen (1972). He considered the identity of the specimen doubtful but close to H. sessile .

Halecium articulosum is much less abundant than H. halecinum in European waters, and records of it in southern Scandinavia are limited ( Kramp 1935b, 1938; Christiansen 1972; Checklist herein). Kramp and Christiansen, and Jäderholm (1909) earlier, noted that it had been confounded with other species and was likely more widely distributed on the boreal coast of Europe than indicated in the literature.

Reported distribution. West coast of Sweden.—Koster to middle Kattegat ( Jägerskiöld 1971).

Elsewhere.—North Atlantic from Iceland to Belgium ( Christiansen 1972) in the east, and from the Grand Banks of Newfoundland to Long Island Sound ( Fraser 1944) in the west.

FIGURE 30. Halecium halecinum : part of branch with hydrothecae and female gonotheca, ROMIZ B3897. Scale equals 0.5 mm.

Figures

FIGURE 29. Halecium articulosum: part of branch with hydrothecae, ROMIZ B3887. Scale equals 0.5 mm.