Monostaechas quadridens (McCrady, 1859)

Colonies comprising bunches of many stems to 100 mm high.

Hydrorhiza a tangle of stolons; stolons tubular, rugose, of same diameter as stems.

Stems straight, monosiphonic, slender, lower stem region ahydrocladiate, internodes long, cylindrical, nodes transverse, varying from obscure to distinct, usually a slight tumescence below node; proximal internodes without nematothecae; internodes further up stem with up to six nematothecae in a double row. Stems sparingly to much branched, primary branches opposite to alternate, distal on internode, arising from a long tubular, upwardly directed apophysis, distal node of apophysis transverse to weakly oblique, a nematotheca in axil and one cauline nematotheca below branch; primary branching from the stem dichotomous or trichotomous; when dichotomous, branches opposite, if trichotomous the third branch inserted between the first two; branches displaced towards front or back of stem.

Primary branch internodes long, slender, ahydrothecate, with a row of up to five nematothecae; secondary and tertiary hydrocladia (branches) given off from distal apophyses on internode, a nematotheca in axil; tertiary branching more common in distal stem region. Proximal internode of hydrothecate hydrocladium with a long oblique proximal node passing below hydrotheca; secondary and tertiary hydrocladia usually beginning with a moderately long athecate internode with oblique distal node and two or three adaxillar nematothecae.

Hydrocladial internodes alternately hydrothecate and ahydrothecate; proximal node of hydrothecate internode strongly oblique, passing below hydrotheca, distal node transverse to weakly oblique, indistinct, athecate internode with one or two equidistant median nematothecae Hydrothecate internode short, with a centrally placed hydrotheca occupying much of internode, hydrotheca seated at an angle of c. 45° to internode, deep cup-shaped, adcauline wall straight, half wall free of internode, abcauline wall straight to weakly convex, slightly thickened, a small foramen connecting with internode at base of wall. Margin circular, transverse to hydrothecal axis, rim slightly everted, perisarc thin.

Hydranth with c. 20 tentacles. One median nematotheca on ahydrothecate internode two-chambered, base stout, adcauline side of cup foreshortened and adpressed to internode, median inferior nematotheca same, cup not reaching base of hydrotheca, twin lateral nematothecae movable, tall, splayed outwards on a long pedicel; base of nematotheca conical, about same length as pedicel, cup just overtopping hydrothecal margin, shallow, wide, reduced on side facing hydrotheca; median superior nematotheca single-chambered, small, ovoid, scoop-shaped, dorsally convex, tucked below adcauline wall of hydrotheca.

Cauline nematothecae on branch internodes and gonothecae, same as twin laterals but without pedicel, a smaller one, sometimes absent, in axil of branches and hydrocladia.

Male and female gonothecae borne prolifically on same stems and sometimes on same hydrocladium. Male much smaller than female, kidney-shaped, tapering to base, walls smooth, inserted in internode on a pedicel of two segments beside a hydrotheca, a nematotheca at base, aperture terminal, small, circular. Female gonotheca large, kidney-shaped, tapering to base, walls smooth, inserted in internode beside a hydrotheca on a pedicel of three segments, two nematothecae at base; aperture terminal, large, circular, oblique, closed by a slightly domed operculum that ruptures to release a single planula larva.

Cnidome of two categories of nematocysts:

i) large microbasic mastigophores, capsule oval, 18–20 x 7 μm, shaft 17–20 μm long, with large spines, tubule 45 μm long, moderately thick, with small spines; abundant in nematothecae.

ii) small isorhizas, capsule 7 x 3 μm, tubule c. 40 μm long; abundant in tentacles.

Perisarc of hydrocaulus moderately thick, walls of hydrotheca and gonotheca thin.

Colour of colonies dark purplish grey in life; male gonophore dark brown, female brown to creamy yellow.