Gymnangium undulatum Watson, 2000

Hydrorhiza composed of thick stolons reptant on substrate.

Stems long, to 250 mm, plumose, unbranched, monosiphonic, strongly undulated, bends occurring every 15 - 25 mm along length, divided into internodes by strong, slightly oblique nodes, a tumescence below each node, cauline perisarc very thick, smooth.

Prosegment ahydrocladiate, in hydrocladiate region, hydrocladia close, alternate, directed slightly forwards from opposite sides of stem on a moderately long apophysis with distal hinge joint; two, occasionally three, hydrocladia on internode, if two, proximal in lower third to halfway along internode, upper one just below distal node; sometimes two opposite hydrocladia and on internode. Hydrocladial nodes slightly oblique, marked only by a faint constriction and thinning of perisarc; no intranodal septa.

Hydrothecae saccate, margin tilted obliquely backwards at an angle of c. 40° to hydrocladial axis, a strong septum passing forwards into body from midway along abcauline wall, septum dividing wall into two pronounced convexities, in ventral view septum seen as a shelf with ragged edge; adcauline hydrothecal wall gently convex, a small knot of perisarc at connection with internode. Hydrothecal margin with three pairs of equidistant, deeply scalloped cusps, no unpaired anterior tooth, cusp of middle pair the largest (lateral view), often pointing posteriorly; posterior cusp the broadest, posterior margin of hydrotheca straight between lateral nematothecae.

Mesial nematotheca digitate, jus t overtopping hydrothecal margin, terminal orifice may be entirely absent or with a small orifice on side facing hydrotheca, orifice just above hydrothecal margin or a completely open gutter from apex to hydrothecal margin. Lateral nematothecae small, rectangular to elongate conical, proximally rounded, distal end level with hydrocladium, a large connection with internode and a large terminal orifice. Three cauline nematothecae associated with apophysis: two frontal, large, one in axil and one below apophysis, the one below with two large excavated orifices side by side, axillar one with a single orifice of similar shape, orifices facing distally; rear nematotheca behind axil, smaller, similar to laterals but with wider orifice, facing distally.

Colour: Live colonies grey-brown.