Holotype: NMV, G2496-microslide (KOH cleared preparation); G2497-preserved material, remainder of holotype colony. Penguin Island, Tasmania, 15m deep on sponge and rock [43°25'S, 140°20'E].
Paratype: NMV G2498-microslide; Penguin Isl., Tasmania, 15 m deep on sponge and rock.
H. luteum displays several unusual morphological features. One is the presence of supplementary nodes, so well defined that the stems could almost be described as being alternately athecate and thecate; another feature is the extraordinarily shallow hydrophore, which seems to offer negligible support to the very bulky hydranth. The extreme thinning of the wall of the internode in the fenestration below the hydrophore must produce a serious structural weakness of the hydrocaulus. This is, however, offset by the support given to the hydrophore by the very strong wedge of perisarc extending across the base of the hydrophore from the stem.
Colonies to 2.5 cm high, growth arborescent, main stem strongly fascicled, woody and stiff. Branching more or less in one plane, the ultimate branches in anyone part of the colony all directed anteriorly; in other parts they may face in other directions. Stem internodes of variable length, 0.40-0.65 mm, expanding distally, with 1, occasionally 2, extra nodes. Nodes distinct, oblique, parallel in each internode, sloping alternately left and right in adjacent internodes; perisarc indistinctly internally ridged. Width at proximal node, measured parallel to node, 0.12-0.14 mm.
Hydrophore seated on distal third of distal segment of internode, well below node. Hydrophore sessile, very flat and shallow, walls thin and strongly constricted just above diaphragm, abcauline wall more concave than adcauline. Margin circular, 0.14-0.16 mm in diameter, with outrolled rim. Depth from margin to punctae, 0.015-0.02 mm; depth from margin to diaphragm, 0.025-0.035 mm. Diaphragm very strong, with a distinct ring of punctae above, and a wedge shaped thickening of the perisarc of the internode below. Below the wedge the wall of the internode thins into a large circular fenestration from which the apophysis of a branch may arise.
Hydranth large with an annular hypostome and 25-28 tentacles.
Colour: bright yellow.
Southern Australia (Tasmania and Victoria).
Subtidal, on invertebrates and rocky substrates.