Obelia bidentata Clarke, 1875

Watson, Jeanette E., 2011, New species, new records and redescriptions of Thecate hydroids (Cnidaria: Hydrozoa: Leptothecata) from Southern Australia, Zootaxa 3122, pp. 1-36 : 26-28

Obelia bidentata Clarke, 1875

Fig. 12 A,B

Obelia bidentata Clarke, 1875: 58 , pl. 9, fig. 2.– Cornelius, 1975: 260, fig. 2.– Millard, 1978: 195.– Millard, 1980: 131.– Cornelius, 1983: 155, fig. 1.– Cornelius, 1982: 113.– Bouillon, 1984: 108.– Izquierdo, García-Corrales & Bacallado, 1986: 91, fig. 8.– Calder, 1991a: 70, fig. 37. Calder, 1991b: 2068.– Ramil & Vervoort, 1992: 241, fig. 68b.– Cornelius, 1992: 254.– Boero & Bouillon, 1993: 265.– Calder, 1993: 68.– Cornelius, 1999: 6.– Medel & Vervoort, 2000: 46, fig. 12.– Watson, 1994: 67.– Vervoort & Watson, 2003: 424, fig. 103F. – Galea, 2009: 23 , fig. 5G.– Galea, 2010a: 9 , fig. 2I.

Obelia spinulosa Bale, 1888: 756 , pl. 12, figs 5–7.

Material examined. NMV F171369, microslide, colony from hull of boat, Western Port, Victoria, coll: J. Watson, 20/09/1970. Material in author’s collection: McLure reef, Queensland, coll: L. Zann 13/8/1972, no depth data. Flinders Islet, Port Kembla New South Wales, on alga Zonaria , depth 12 m, coll: J. Watson, September, 1976. Port Kembla Harbour, New South Wales, on Styela plicata , coll: J. Watson, February, 1991.

Description. Colonies infertile. Hydrorhiza reptant on substrate, stolons tubular, crumpled, perisarc thin. Young stems monosiphonic, older stems polysiphonic, branched, branching opposite in distal part of colony. Stem internodes long, cylindrical with 3–5 proximal annulations.

Hydrothecae terminal on pedicels of variable length, annulated throughout, a thickening of internode at junction of stem and pedicel. Hydrotheca conical, walls smooth, expanding evenly from base to margin, basal chamber distinct, rectangular to quadrangular, merging smoothly into hydrotheca, distal wall of basal chamber transverse to slightly concave. Margin of hydrotheca dentate with c. 16 sharp cusps separated by shallow embayments.

Hydranth with c. 12 long, thin tentacles.

Remarks. Bale (1888) was uncertain whether the species he described as Obelia spinulosa may in fact be Obelia bidentata but kept them separate because O. bidentata is polysiphonic and known to grow to a height of 15 cm whereas his specimen was monosiphonic (although his figure shows incipient fasciculation) and was less than 12 mm in height. His material was probably a young colony of O. bidentata . While most colonies in the present collection are also small and monosiphonic, a colony from Western Port, Victoria was 20 mm high eight weeks after larval settlement, the stem by then having four polysiphonic tubes. The species seems to be somewhat variable and although widely distributed in southern and southeastern Australia there are too few records of fresh material for a detailed overview including that of the cnidome.

Distribution. O. bidentata is distributed world-wide. The present records extend its range from southern Australia to southern Queensland.

Hydrorhiza, width 80–96

Stem

internode length 600–800 diameter 80–96 length of pedicel 120–480

Hydrotheca

length, diaphragm to margin 456–544 diameter at margin 200–240 diameter at diaphragm 80–96 length of marginal cusp 32–40

Figures

FIGURE 12. Obelia bidentata. A, part of stem showing alternate pedicels. B, hydrotheca. Scale bar: A, B 0.5 mm.