Amphinema dinema ( Péron and Lesueur, 1810 )

Watson, Jeanette E., 2015, Five athecate hydroids (hydrozoa: anthoathecata) from south-eastern australia, Memoirs of Museum Victoria 73, pp. 19-26 : 20-21

Amphinema dinema ( Péron and Lesueur, 1810)

Figure 2A−G

Oceania dinema Péron and Lesueur, 1810: 346 .

Perigonimus serpens Allman, 1863: 10 .

Stomotoca dinema . − Mayer, 1910: 109, pl. 9, figs 8−10, pl. 10, figs 1−4.

Amphinema dinema . – Rees and Russell, 1937: 62, figs 1−4.− Russell, 1953: 180, pl. 10, figs 1, 2, 4, pl. 11, figs 1, 3, text-fig. 89.− Kramp, 1959: 117, fig. 109.− Kramp, 1961: 93.− Kramp, 1968: 42, fig. 108.− Goy et al. 1991: 109, fig. 24.− Schuchert, 1996: 63, fig. 36.

Material examined. NMV F202871 , Clifton Springs jetty ruins, Port Phillip, fertile colony on mussel Mytilus galloprovincialis , depth 0.2 m, coll: J. Watson, 10/12/2008. Material fixed in 5% formalin later preserved in 70% ethanol .

Description (from live material). Colony stolonal, hydrorhiza reptant on substrate, stolons tubular, faintly rugose to smooth, perisarc relatively thick. Hydrothecal pedicels scattered along hydrorhiza, hydrocaulus erect, cylindrical, unbranched, of same diameter at base as hydrorhiza, diameter increasing distally, perisarc thin with several obscure corrugations above base, continuing upwards for variable distance, corrugations gradually becoming indistinct then fading out. Hydranth long spindle-shaped, extending well above hydrotheca, hypostome high-domed, quadrate when viewed from above, with 8-10 long tentacles in a single indefinite whorl held out stiffly below hypostome.

Medusa buds globular, arising on short, corrugated pedicels along hydrorhiza. Prior to release one tentacle emerges, followed by a second tentacle one to two hours later. At release, medusa deep bell-shaped to hemispherical with two opposite tentacles on large tapering bulbs, manubrium cylindrical in lateral view, quadrate in dorsal, umbrella without apical projection, small nematocysts sparsely scattered over bell, velum a broad shelf, radial canals thin, a small wart at base of each canal. After four days in the laboratory, medusa remained of same shape and size as at release; no apical projection, manubrium with four indistinct lips, a small marginal wart between radial canals. After five days, manubrium extending almost halfway down bell.

Cnidome comprising two categories of nematocysts:

(i) microbasic euryteles, capsule narrowly elongate, 7−10 x 2.5−3.5 µm (discharged), shaft thin, 7−10 µm, thread very long; abundant in tentacles of hydranth and medusa and scattered over bell of medusa; easily discharged.

(ii) desmonemes, almond shaped, 4−6 x 2.5−3.5 µm (undischarged) on hydranth.

Hydrorhiza pale brown, hydrocaulus, body of hydranth and tentacles translucent white, stomach region below tentacles orange. Umbrella of medusa and radial canals colourless, tentacle bulbs and manubrium orange-brown; colour fading after five days.

Remarks. The hydrothecae vary considerably in length within and amongst the colonies, shorter hydrothecae having a slightly thicker and more visible perisarc while longer ones usually have a thinner, sometimes almost invisible perisarc. The perisarc is often lightly coated with fine sediment. The hydranth is very active, bending from side to side when disturbed; one was observed capturing and ingesting a small amphipod. When the hydranth is extended the tentacles are scattered over the distal body region but when contracted they appear as an indefinite whorl.

The tentacles of the medusa are remarkably long, about 20 times the length of the bell. The medusa is very active, swimming with rapid jerking movements.

Medusae of Amphinema dinema were originally reported from the English Channel and later from the plankton of north-eastern Australia ( Kramp 1968). It was found in New Zealand ( Schuchert 1996) so it was only a matter of time before also being recorded from southern Australia.

The present material agrees in most respects with Schuchert’s (1996) description of the hydroid and early medusa stages of the New Zealand species but differs in: i) the manubrium of the newly released medusa is only one third the height of the bell, ii) the manubrium is yellow, not red as in the New Zealand material, iii) there was no indication of development of an apical projection in medusae up to five days old. While the apical projection is considered an important diagnostic character, in the present case it may have been due to retardation of growth in the laboratory. Despite these differences and until further material is available for study I refer the species to Amphinema dinema .

Distribution. North-eastern Australia, India, Mediterranean Sea, north-western Europe, east coast of North America, New Zealand. This is the first record of Amphinema dinema from southern Australia.