Macrorhynchia ambigua Watson, 2000

Hydrorhiza a knot of tough stolons entwining substrate. Stems stiff, to 100 mm high, heavily fascicled, single stems branched irregularly alternate to subopposite, or stems may be branched fan-wise from hydrorhiza, secondary branching rare; polysiphonic tubes of stem parallel, running a long branches, but distal part of branches monosiphonic; nodes if present, transverse, prosegment ahydrocladiate with a central row of nematothecae or oval foramen representing former sites of cauline nematothecae.

Hydrocladia frontal on axial tubes of stem and branches, short , alternate one per internode, inserted on apophysis with transverse to slightly oblique distal node and a reduced, central nematotheca. Hydrocladiate internodes short, nodes transverse, indistinct, marked by a notch in perisarc; internode with two strong septa, one almost vertical below intrathecal septum, the other sloping distally from base of lateral nematotheca.

Hydrothecae not immersed in internode, ventricose, abcauline wall and proximal adcauline wall strongly convex, base flatter, posterior adcauline wall free of internode, a slightly oblique intrathecal septum passing almost halfway into hydrotheca from proximal intranodal septum. Margin inclined at an angle of 10 - 20° to internode with a prominent, blunt abcauline cusp passing downward into hydrotheca as a wide hatchet-shaped wedge; margin with a pair of low, central lobes, sometimes reduced to mere undulations, in frontal view, margin oval with smoothly everted rim , perisarc of margin very thin.

Mesial nematotheca reaching abcauline cusp, usually completely adnate, widening to orifice but with a short narrow neck seen only in frontal view. Lateral nematotheca on proximal hydrocladial internodes fairly short, lengthening a little distally a long internode, tubular, reaching node but not reaching hydrothecal margin, orifice circular, connection with internode large. A cauline nematotheca at base of hydrocladial apophysis, small, like proximal hydrocladial laterals but with shorter orifice, facing upwards and outwards. Two large, tubular cauline nematothecae flanking hydrocladium, terminal orifice deeply excavated, a tumid mamelon with circular orifice between.

Phylactocarp replacing a branch hydrocladium, borne on a short apophysis, adcauline wall of basal internode with a reduced nematotheca; next internode long, upwardly curved below an unmodified hydrotheca, followed by a long internode with hydrotheca with twin lateral nematothecae and slightly reduced mesial nematotheca, a single long nematotheca similar to laterals above; phylactocarp thereafter directed upwards almost parallel to axis of branch, bearing up to eight internodes, nodes transverse, with two (sometimes one or three) opposite nematothecae.

Nematothecae short, stubby, widest about middle, terminal orifice circular, a pore without collar at junction with internode. A single gonotheca borne frontally on a short pedicel on second hydrothecate internode, gonotheca large, lenticular, perisarc very thin and transparent.

Colour: live colonies brown, gonophore cream.