Sertularia mertoni Stechow & Müller, 1923

Hydrorhiza tubular, reptant, and strongly adherent to substrate.

Stems to 20 mm high, monosiphonic, pinnate, arising at irregular intervals from hydrorhiza, pinnate, monosiphonic, rather lax, straight or slightly flexuose, lower part of stem athecate, moderately long, perisarc thick, ending in a strong hinge joint. Stem internodes thereafter short, nodes transverse, distinct, internodes becoming narrower distally.

First two hydrocladia above hinge joint usually opposite, hydrocladia thereafter alternate, inserted above node at an angle of 70 - 80° to stem axis on short apophyses with transverse distal node varying from a mere constriction to a deep collar; node followed by a short athecate internode with one, sometimes two, deep, distal hinge joints; hydrocladium thecate, inflated beneath hydrothecae, nodes narrow, internodes bearing up to four pairs of hydrothecae.

Hydrothecae biseriate, opposite, seated on front of internode, long, tubular, adnate adcauline walls conjoined, free adcauline wall c. one third length of adnate wall, curved outwards, abcauline wall partly adnate, straight proximally, concave distally, floor of hydrotheca straight to faintly rounded. Hydrothecal margin elliptical (ventral view) with two long, pointed lateral cusps shifted to adcauline side and a minute adcauline cusp, a small submarginal knot of perisarc on abcauline wall, operculum of two valves, adcauline valve with a central rupture line. A cauline hydrotheca in axil of hydrocladium, one above, almost level with distal node and one opposite, halfway along internode. Perisarc of hydrocladia and hydrothecae thin.

Gonothecae grouped on proximal stem, large, barrel shaped, borne on a short, stout pedicel, body faintly corrugated to almost smooth, perisarc moderately thick, orifice distal, circular, inside a short, slightly everted collar, a row of denticles within, operculum a thick, dome-shaped plug.

Colour. Stems white to almost colourless, gonophores white to pale pink.