In absence of the gonosome, S. decipiens can be distinguished from other members of the species group by the longer hydrotheca with distal outward bend , the small submarginal shelf at the point of flexure and the low inclination of the margin to the hydrocladial axis.
Hydrorhiza reptant on substrate, tubular, smaller in diameter than hydrocaulus.
Stems to 30 mm high, stiffly pinnate, monosiphonic, perisarc thick, sometimes secondary branching; stem thecate almost from base, divided into short internodes, nodes oblique, deep, tilted parallel with hydrocladium below; three hydrothecae on internode, one axillar, one below hydrocladium and one opposite.
Hydrocladia to 12 mm long, given off alternately at an angle of c. 60° halfway up cauline internode from a short apophysis, usually marked by a distinct constriction in perisarc, constriction deeper on proximal side of hydrocladium. Hydrocladial internodes may or may not be present, but if present, bearing three to six hydrothecae on each side, node an oblique constriction in perisarc.
Hydrothecae subopposite to almost opposite, immersed in internode, cauline hydrothecae well separated, close, but not in contact on hydrocladium, margin of one reaching about halfway along adcauline wall of that opposite; hydrotheca more or less tubular, narrowing a little to margin, adcauline wall shallowly convex, usually completely adnate but sometimes a very short free part; abcauline wall almost straight and parallel to hydrocladial axis, bending outwards just below margin, a small she lf of perisarc projecting into hydrotheca at point of cladial axis, bending outwards just below margin, a small shelf of perisarc projecting into hydrotheca at point of flexure; ratio of length of abcauline to adcauline wall 1:1.4-1.8, base of hydrotheca flat to slightly convex, a knot of perisarc projecting downwards into internode from base of wall. Margin tilted upwards at an angle of 25°- 30° to hydrocladial axis with four low, equidistant, sharp cusps, one adcauline, one abcauline and two lateral, separated by wide, shallow embayments; operculum of four valves. Margin not replicated.
Gonothecae borne on hydrorhiza and lower stem, long, sausage-shaped, straight or curved, perisarc very thick, pedicel variable in length, crumpled to almost straight, body of gonotheca encircled by 8-10 deep corrugations usually deepest about middle, fading out proximally but some gonothecae merely undulated throughout; aperture a shallow, transverse depression surrounded by four low cusps, operculum of four valves. Female gonophore ovoid, containing many large ova.
Colour: Live colonies pale yellow.
Northern Australia and Indonesia.
Subtidal, epizoic on polychaete tubes.