Obelia longissima (Pallas, 1766)

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 : 51-52

Obelia longissima (Pallas, 1766)

Figs. 54, 55

Sertularia longissima Pallas, 1766: 119 .

Obelia flabellata .— Segerstedt, 1889: 12, 25.— Jäderholm, 1909: 62, pl. 5, fig. 16.

Obelia longissima .— Jäderholm, 1909: 63, pl. 5, fig. 14.— Östman, 1982a: 156; 1982b: 228; 1983: 7; 1999: 19.—Cornelius, 1990: 559.

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

Type locality. “Oceanus Europaeus” (Pallas 1766: 120).

Museum material. Tjärnö, floating dock at Sven Lovén Centre for Marine Sciences , 58°52’33.68”N, 11°08’43.65”E, <1 m, 07.ix.2010, collected manually from dock, one colony, 9.5 cm high, without gonothecae, ROMIZ B3889 .

Remarks. The validity of Obelia longissima (Pallas, 1766) was confirmed by Östman (1982a, b, 1987, 1999), who showed that it differed from O. dichotoma ( Linnaeus, 1758) in cnidome and in acid phosphatase band patterns. According to Cornelius (1990, 1995), O. longissima can be distinguished from O. dichotoma in having: (1) long, monosiphonic, regularly branched stems that are often dark or even black in colour; (2) branches over a given part of the stem of essentially the same length; (3) hydrothecae round instead of polygonal in cross-section; (4) hydrothecal margin with shallow, wavy cusps rather than a series of lobes. Nevertheless, young, old, or damaged specimens can still be difficult to identify based on morphology alone. The colonies examined here, rather old, somewhat damaged, and overgrown with other hydroids, corresponded most closely to O. longissima . Examination of nematocysts from tissue of museum material (ROMIZ B3889) revealed the presence of F l - type isorhizas ( Fig. 55b, c ), diagnostic of the species ( Östman 1982b). A-type b-rhabdoids (microbasic b-mastigophores) were also present.

A comprehensive overview of this species was given by Cornelius (1990). It is frequent in Danish waters ( Kramp 1935b, as Laomedea longissima ) and in the Oslofjord, Norway ( Christiansen 1972), as well as western Sweden ( Jägerskiöld 1971).

Reported distribution. West coast of Sweden.—From Strömstad area to south of Falkenberg ( Jägerskiöld 1971: 63).

Elsewhere.—North Atlantic-Arctic from polar waters to the Mediterranean Sea in Europe, and to South Carolina in North America ( Schuchert 2001a; Bouillon et al. 2004; Calder 2004).

Figures

FIGURE 55. Obelia longissima, nematocysts, ROMIZ B3889: a, A-type b-rhabdoids (microbasic b-mastigophores); b, Fl-type isorhiza; c, Fl-type isorhiza.