Gymnangium prolifera  

Branched colonies up to 150 mm high and 80 mm wide.

Simple pinnate and branched monosiphonic stems, arising from a tangled hydrorhiza of tough stolons. Branching irregular in one or two orders in one plane, branches directed obliquely upward; pro segments of stem occupying lower third to one half of stem. On branched stems each branch originating from a claw-like knot of stolons ("pseudostolon") tightly wrapped around stem beside a hydrocladium. Pseudostolon without coenosarcal connection to stem or branch; pseudostolon flattened adcaudally, upper side inflated; at point where it becomes free of branch, pseudostolon proximally constricted into a false apophysis, distally with two to four pairs of opposite abcaudal perforations, pseudostolon then becoming a new branch. Stem and branch internodes short, wide, nodes distinct, transverse to slightly oblique, two subopposite hydrocladia per internode on stem and proximal region of branches, gradually becoming alternate and reducing to one per internode distally.

Hydrocladia to 10 mm long, on a short apophysis with transverse distal node, hydrocladium directed slightly upward, internodes fairly short; nodes distinct, transverse to weakly oblique, each with a small distal shoulder; no internal septa.

Hydrotheca seated on top of internode at an angle of ca. 40° to internode; semi-ovoid, widening from base to margin, basal connection with internode surrounded by ragged denticles; margin (lateral view) with a long blade-shaped anterior cusp and three pairs of evenly spaced cusps, anterior pair tall, upright, pointed, frequently obscured by anterior cusp, second and third pair low, rounded, embayments between shallow; in anterior view, first cusp bifid with scoop-shaped central depression, posterior pair rounded.

Median nematotheca tubular, variable in length, often shorter than hydrotheca in proximally, becoming longer distally along hydrocladium; nematotheca where long, erect, curving away from hydrotheca to above margin; nematotheca narrowing distally, terminal aperture narrow, circular, a secondary aperture above hydrotheca. On proximal hydrocladial internode nematotheca truncated below hydrothecal margin and with larger secondary aperture than succeeding nematothecae. Lateral nematotheca flask-shaped, base wide, directed obliquely downwards from hydrotheca, terminal aperture small, circular, a wide secondary aperture connecting with hydrotheca. Three cauline nematothecae, one close below and one on front of internode above apophysis and one behind at base of apophysis; frontal nematothecae with two or three apertures.

Gonothecae crowded along stem, conical, apex truncate, closed by a sheet of tissue.

Perisarc of stem and branches very thick, thinning a little on hydrocladium and hydrotheca, perisarc of gonotheca thin.

Colour: Stems and hydrocladia of simple colonies dark shining brown, branched colonies honey brown; pseudostolons paler yellow-brown.