Redescription of Tripoma arboreum Hirohito , 1995 ( Hydrozoa : Campanulinidae ) from the Tasman Sea with notes on quasi-parasitism of the species *

A study of the hydroid fauna from deep water seamounts in the Tasman Sea south of Tasmania, Australia, and from the Norfolk Ridge, north of New Zealand, found a species of the family Campanulinidae that has led to critical review of the systematic status of the genus Tetrapoma Levinsen, 1893, type Calycella quadridentatum Hincks, 1874 and Tripoma Hirohito 1995, type Tripoma arboreum Hirohito, 1995. Bouillon (1985) redefined the genus Tetrapoma Levinsen, 1893, limiting it to stolonal colonies with nearly cylindrical, pedicellate hydrothecae with four triangular opercular valves distinctly demarcated from the margin. The gonotheca was unknown. Hirohito (1995) further modified the generic description of Tetrapoma to accommodate erect, arborescent colonies from Sagami Bay, Japan. Although his specimens were fertile he did not include the gonosome in the generic description. Nor was his concept of a hydrothecal operculum of four flaps, not demarcated from the hydrothecal body, in accord with the generic diagnosis of Tetrapoma. We have examined a paratype colony of Tetrapoma fasciculatum Hirohito 1995 (NSMT-Hy R: 3010, alcohol preserved), a microslide preparation No. 6212 from the same colony, four microslide preparations No. 5139, 5143, 5145, from the holotype colony of Tetrapoma fasciculatum (NSMT-Hy R: 3002), and the holotype specimen (NSMT-Hy R: 3009, alcohol preserved) and three microslide preparations (No. 6206, 6207, 6211) of Tripoma arboreum Hirohito 1995, loaned by the Showa Memorial Institute, Japan. The heavily fascicled lower branches of the preserved specimen of Tripoma arboreum bear many SCI. MAR., 64 (Supl. 1): 249-254 SCIENTIA MARINA 2000


INTRODUCTION
A study of the hydroid fauna from deep water seamounts in the Tasman Sea south of Tasmania, Australia, and from the Norfolk Ridge, north of New Zealand, found a species of the family Campanulinidae that has led to critical review of the systematic status of the genus Tetrapoma Levinsen, 1893, type Calycella quadridentatum Hincks, 1874and Tripoma Hirohito 1995, type Tripoma arboreum Hirohito, 1995. Bouillon (1985) redefined the genus Tetrapoma Levinsen, 1893, limiting it to stolonal colonies with nearly cylindrical, pedicellate hydrothecae with four triangular opercular valves distinctly demarcated from the margin.The gonotheca was unknown.Hirohito (1995) further modified the generic description of Tetrapoma to accommodate erect, arborescent colonies from Sagami Bay, Japan.Although his specimens were fertile he did not include the gonosome in the generic description.Nor was his concept of a hydrothecal operculum of four flaps, not demarcated from the hydrothecal body, in accord with the generic diagnosis of Tetrapoma.
The heavily fascicled lower branches of the preserved specimen of Tripoma arboreum bear many

Description
Hydrorhiza a small mass of tubes; hydrocauli erect, stiff, arborescently branched untidily more or less in one plane, branches heavily fascicled to distal region, polysiphonic tubes narrow, parallel to entwining.Distalmost branches monosiphonic, internodes long and slender, nodes a faint constriction in perisarc above apophysis or absent altogether; apophyses of branch distal on internode, moderately long, with a deep transverse distal node.Hydrothecae strictly alternate in monosiphonic branch region, given off in one plane; a hydrotheca in axil of each branch.Hydrotheca borne on a distinct pedicel merging into base of hydrotheca with none to three deep constrictions, hydrotheca deep, perisarc thin (Tasmanian seamount specimens) to moderately thick (Norfolk Ridge specimens), more or less tubular, curved, widening a little from base to about halfway along length, adcauline wall gently convex, abcauline wall slightly concave to straight, a scarcely visible straight or dish-shaped pseudodiaphragm supporting hydranth, often a ring of desmocytes just above.Margin with an operculum of four delicate valves without crease-line.
Gonothecae large, thickly scattered throughout polysiphonic parts of stem and branches, laying along branch or in axil of two branches, cocoon-like, one side adnate to branch.Developing gonotheca flask-shaped, perisarc relatively thin, mature gonotheca cocoon-like, embedded in a meshwork of more or less parallel or entwined fascicular tubes.Empty gonotheca circular to bun-shaped in transverse section with a moderately thick, smooth inter-nal rind of perisarc surrounded by a tight single or double row fascicular tubes.Orifice terminal, facing outwards or upwards, but frequently obscured, shape variable from subcircular to vaguely quadrangular or scoop-shaped, sometimes surrounded by a short raised collar composed of several perisarcal rings.Operculum a sheet of tissue torn aside at release of contents.
Gonophores eumedusoid but too poorly preserved for detailed description.

Colour
Lower stems buff to yellow-brown, upper stems and monosiphonic branches colourless to transparent, gonophore (preserved) cream coloured.

Remarks
The opercular valves of the hydrotheca are fragile, most of those of the seamount specimens having collapsed during preservation; the perisarc of the Norfolk Ridge hydrothecae is somewhat thicker and the valves are thus better preserved (Fig. 2C).In some hydrothecae there is an almost imperceptible thickening of the hydrothecal wall at the point of attachment of the very thin, dish-shaped pseudodiaphragm (Fig. 2A).Although Hirohito (1995) reported no diaphragm in his material from Sagami Bay, the same structure together with desmocytes is clearly visible in some hydrothecae in the paratype microslides of Tetrapoma fasciculatum.
It is extremely difficult to define the true shape of the gonothecal orifice, most being either damaged in mature specimens, obscured by overgrowth of the fascicular tubes or abraded.The few immature or reasonably intact mature ones suggest a vaguely, subcircular to scoop-shaped rim covered by a very thin operculum fragmented at emergence of the gonophore.There is no evidence in the present material of the four opercular valves described for the gonotheca of T. fasciculatum by Hirohito (1995) nor is there any clear evidence of any such structure in Hirohito's specimens.
Quasi-parasitism of Tripoma arboreum Bale (1915) reported epizootic hydrothecae which he ascribed to the genus Lafoea growing on Acryptolaria arboriformis (Ritchie, 1911) from Babel Island in Bass Strait, southern Australia.He also (1915) described but did not figure a gonotheca on Acryptolaria arboriformis (Ritchie 1911) from deep water off southern Australia, assuming it to be that of Acryptolaria arboriformis.
We have also examined the holotype specimen of Acryptolaria arboriformis (Ritchie 1911) (AM Y257, wet preserved) loaned by the Australian Museum.The much broken colony is about 80 mm high, branching in many directions, both stem and branches being heavily fascicled by thin parallel and entwined tubes.A few of the less heavily fascicled distalmost branches consist of parallel tubes bearing long, tubular Acryptolaria arboriformis hydrothecae.The remainder of the colony is almost completely invested in gonothecae of Tripoma arboreum.These impart a lumpy appearance to the stems and branches, and together with the surficial meanderings of polysiphonic tubes impart the "frolicsome" appearance of the fasciculations which puzzled Ritchie (1911).The outward-facing, sub-circular orifices of most gonothecae are probably what Ritchie mistook for openings from the polysiphonic tubes to the exterior of the branch.A few small hydrothecae, distinctly those of T. arboreum, are also present on the lower regions of several branches.Transverse sections made by us of a branch and gonotheca of the type specimen show that the tubes of T. arboreum cannot be distinguished from those of A. arboriformis (Fig. 1D).Transverse sections of 252 J.E. WATSON and W VERVOORT   Acrytolaria arboriformis (Ritchie, 1911).Scale bar: 2A-F, H-J, 0.5 mm; 2G, 2 mm.
We therefore conclude that Tripoma arboreum is capable of adopting two habits of growth: one as independent, arborescent colonies which commence life on invertebrate or inert substrate as in the Tasmanian seamount and Norfolk Ridge colonies.The second, quasi-parasitic habit, seen in the Bass Strait and Sagami Bay Acryptolaria colonies is one in which the Tripoma stolons become indistinguishably intergrown with the polysiphonic tubes of the host, mimicking its structure and possibly eventually smothering it.
Several examples of epizootism, where juvenile hydroid colonies develop epizootically on colonies of other hydroids then become independently growing, erect colonies are known to us: e.g.colonies of Halecium delicatulum Coughtrey, 1876 from the Strait of Gibraltar (Ramil & Vervoort 1992) and the Tasman Sea (W.V. unpub.), and in southern Australia, Aglaophenia tenuissima (Bale 1915) on Gymnangium tubulifer (Bale, 1915) (see Bale (1915)), Sertularella pinnata (Lamouroux, 1816) on Nemerstesia procumbens (Spencer, 1891) (see Gordon et al 1998) and Gymnangium longirostre (Kirchenpauer, 1876) on Aglaophenia divaricata (Busk, 1852) (J.E. W. unpub.).Millard (1973) described cases of auto-epizootism in which a species may develop on and within the hydrocaulus of the same species.None of these cases however describe the present situation of quasi-parasitism in which one species invades the hydrocaulus of another species, adopting its structure but without necessarily killing its host.

FIG. 1
FIG. 1. -A, Colony of Tripoma arboreum Hirohito, 1995 (TM K1708) from Tasmanian seamounts.B, Distal end of branch of same colony.C, Distal end of branch of colony from the Norfolk Ridge.D, Transverse section of branch and gonotheca from seamount colony TM K1708.Scale bar:1A, 1 cm; 1B, C, D, 0.5 mm.
branches of the Tasmanian seamount specimens of T. arboreum not associated with A. arboriformis are indistinguishable from those of the type of A. arboriformis, indicating that independent colonies of T. arboreum have the same internal structure as the host species of the quasi-parasitic form.Examination of the less heavily fascicled parts of branches of the preserved holotype colony and microslide specimen of Tripoma arboreum Hirohito 1995 reveals biseriate rows of hydrothecae (neither described nor figured by Hirohito) which are clearly those of Acryptolaria arboriformis(Ritchie, 1911)  (Fig.2J).

Redescription of Tripoma arboreum Hirohito, 1995 (Hydrozoa: Campanulinidae) from the Tasman Sea with notes on quasi-parasitism of the species* hydrothecae
Hirohito's 1995 F.BOERO, A. MIGOTTO and J.M. GILI (eds.), some pedicellate and others partially immersed in the polysiphonic tubes of the branches as described by Hirohito.The few reasonably well preserved pedicellate hydrothecae have the same range of dimensions as those of T. fasciculatum and clearly show four, not the three opercular valves characterised by Hirohito as diagnostic of the genus Tripoma.We therefore conclude that Hirohito's genus Tripoma cannot possibly be separated from Tetrapoma as defined inHirohito's 1995 paper,  Tetrapoma fasciculatum Hirohito, 1995: 95, fig.27a-c, pl. 5, fig.D.  Tripoma arboreum Hirohito, 1995: 98, fig.28a-e, pl.6, fig. A.