Ventromma bellarensis

Colonies comprising pinnate unbranched stems to 8 mm high interspersed with taller branched stems to 50 mm high.

Hydrorhiza consisting of narrow tubular, weakly rugose stolons reptant on substrate; on branched stems the stolons joining to become polysiphonic tubes of stem; unbranched stems monosiphonic.

Branched stems fan-shaped with one to three upwardly directed branches; stems and branches fascicled, the polysiphonic tubes decreasing in number distally to about two thirds the distance along branch then becoming monosiphonic. Internodes of monosiphonic stems and branches long, cylindrical, slender, nodes transverse, rarely a secondary node close to the primary, a nematotheca between hydrocladium and node.

Hydrocladia alternate on stem and branches, one distal on internode, given off from an upwardly directed shoulder-like apophysis, distal node of apophysis transverse, abcauline wall of apophysis curving smoothly up from internode, adcauline wall shorter with a small partial septum and a tiny hydrostatic pore flush with perisarc. Hydrocladia slender, curving gracefully away from stem, bearing up to 10 hydrothecae. Hydrocladium beginning with a short quadrangular athecate segment with transverse to weakly oblique distal node, succeeding internodes hydrothecate, cylindrical, the first shorter than succeeding ones; nodes transverse to slightly oblique, diameter of internode increasing distally to accommodate hydrotheca; no intranodal septa.

Hydrotheca seated on distal third of internode but not reaching node, shallow cup-shaped, wider than deep, adcauline wall closely adpressed to but not fused to internode, abcauline wall extending in a smooth concave curve up from internode. Margin circular, transverse to hydrothecal axis, sloping obliquely back from internode, often a shallow indentation behind margin, no eversion of rim; a wide foramen connecting base of hydrotheca with internode.

Hydranth large, with c. 16 long tentacles armed with large nematocysts, hypostome annular, wide. Hydrothecate internode with two identical small nematothecae, one median inferior about halfway to two thirds along internode, the other median superior, in front of hydrothecal margin, distal end just reaching node.

Nematothecae bithalamic, scoop-shaped, basal chamber short, indistinct, adcauline side closely adpressed to internode, abcauline wall convex, margin open; cauline nematothecae of same shape and size as those on hydrocladia. Male and female gonothecae on different colonies, borne thickly on lower region of stems and branches, rare on upper monosiphonic regions, borne on a short, thick pedicel not well demarcated from body of gonotheca, point of insertion of pedicel often obscured by polysiphonic tubes of stem; gonothecae variable in shape, barrel-shaped to weakly conical, expanding from pedicel, walls faintly to distinctly rugose, perisarc thick, margin truncate, transverse, closed by a slightly depressed or domed operculum torn aside at release of contents; mature female gonophore packed with ova, surmounted by an aureole of large nematocysts.

Cnidome of two categories of nematocysts:

i) large microbasic mastigophores, capsule ovoid, 12–13 x 6–7 μm, shaft 7–8 μm, finely spinous, tubule very long, distributed throughout coenosarc and gonophores; easily discharged.

ii) smaller ?microbasic mastigophores, capsule elongate, 7–8 x 3 μm, in tentacles of hydranth; difficult to discharge.

Colour of colony buff to pale brown, gonophores white.

The species is named for the type locality on the Bellarine Peninsula.

In the field, Ventromma bellarensis can be mistaken for Plumularia setaceoides Bale, 1882 which grows in similar habitat, but the recurved, well separated hydrocladia and pale buff colour of the colonies of V. bellarensis distinguishes it from the lacy white colonies of P. setaceoides.

When removed from water V. bellarensis exudes long sticky strands of mucous containing discharged nematocysts.

Holotype: NMV F171365, fertile colony, alcohol preserved; two microslides from holotype; from floating pontoon, depth 0.5 m, in Clifton Springs boat harbour, Victoria, Australia.

Victoria, Australia.

Subtidal.