Tripoma arboreum Hirohito, 1995

Watson, Jeanette E., 2018, Some Hydroids (Cnidaria, Hydrozoa) from the Great Australian Bight in the collection of the South Australian Museum, Zootaxa 4410 (1), pp. 1-34 : 7-9

Tripoma arboreum Hirohito, 1995

Figure 3A–D

Tetrapoma fasciculatum Hirohito, 1995: 95 , fig. 27a–c, pl. 5, fig. D.

Tripoma arboreum Hirohito, 1995: 98 , fig. 28a–e, pl. 6, fig. A.— Watson & Vervoort, 2001: 249, fig. 1A–D, fig.2 A–J.

Material examined. SAM H2341, preserved colony; three microslides ( SAM H2574, H2575, H2576). PIRSA Stn IMP4c.

Description. Infertile colony comprising a tangled mass of hair-like stems. Hydrorhizal stolons tubular, of same diameter as stems. Hydrocauli monosiphonic, internodes cylindrical, with up to three orders of straggling branches to 70 mm long, some branches ending in corrugated anastomoses.

Cauline internodes moderately long, increasing in diameter distally, a distinct transverse to slightly oblique node above apophysis, sometimes one or two supplementary nodes above primary one. Apophysis moderately long and broad, distal node transverse with a distinct rim.

Hydrothecal pedicel with one to five, typically two or three deeply corrugated segments, distal segment saucershaped, supporting hydrotheca. Hydrothecae alternate, long, tubular, slightly asymmetrical, adcauline wall slightly convex, abcauline wall straight to weakly concave. Margin with four delicate sharp cusps separated by moderately deep wide embayments, operculum of four triangular valves.

Colony colourless (preserved material) perisarc of stems thick, perisarc of hydrotheca very thin.

A. Stem from holotype colony, SAM H2 341. B, monosiphonic branch. C, hydrotheca showing margin and putative submarginal crease-line. D, broken base of hydrotheca.

Stem

internode length 600–760 width at node 152–192 Pedicel

length of distal segment 80–96 diameter distal segment 152–160 Hydrotheca

length, base to margin 440–472 diameter of margin (estimated) 200 length of opercular valve 120–144 Remarks. The hydrocauli are so intergrown that it cannot be ascertained whether one or several colonies are present; for the present description it is assumed to be one colony. The fragile hydrothecae are so crushed and broken that the marginal diameter and the number of opercular valves could only be inferred by examination of several hydrothecae (diagram, Fig. 3C). Some hydrothecae appear to have a marginal crease-line but as others show no evidence of this structure it may be from damage during collection. Many hydrothecae are broken off at about one third the distance up from the pedicel. As there is no evidence of a diaphragm or pseudodiaphragm at the point of breakage it may simply be a point of structural weakness in the hydrotheca. In contrast to the very thin hydrothecal perisarc, that of the stems is relatively thick, showing many horizontal internal stress fractures.

In discussing the taxonomic status of Tripoma Hirohito, 1995 and Tetrapoma Levinsen, 1893 , Watson & Vervoort (2001) recognised Tripoma as having four opercular valves and a marginal crease line. The present material is morphologically similar to Tripoma arboreum Hirohito, 1995 in the tubular apophysis, segmented pedicel, dimensions and morphology of the hydrotheca, an operculum of four valves but shows no evidence of a crease-line. Also, in contrast to the robust fascicled arborescent colony of T. arboreum described by Watson & Vervoort (2001) from the Tasmanian seamounts, the present material is a tangled mass of lax monosiphonic cauli. Such differences in habit may prove to be yet another expression of the growth plasticity of T. arboreum discussed by Watson & Vervoort (2001).

Distribution. Japan, south-east of Tasmania; a new record for the Great Australian Bight.