Sertularia trigonostoma
Hydrorhiza tubular, entwining substrate.
Stems to 85 mm long, pinnate, lax, monosiphonic, cauline diameter decreasing distally, perisarc thick; proximal region of longer stems ahydrocladiate, shorter stems entirely hydrocladiate; cauline internodes indistinct.
Hydrocladia alternate, arising at an angle of c. 40° to cauline axis, longest in proximal stem region (up to 10 mm long on tallest stems), becoming shorter distally; hydrocladium inserted on long apophysis with distinct transverse distal node; two hydrothecae between each hydrocladium on same side, another in axil. Hydrocladial internode with two hydrothecae, node transverse to slightly oblique, distinct.
Hydrothecae biseriate, subopposite, seated on front of internode, subopposite, slightly separated on proximal region of hydrocladium, adcauline walls adjoined distally along hydrocladium. Hydrotheca longer than wide, adcauline wall convex, free part very short, abcauline wall straight to faintly sinuous or concave, contiguous with internode, base of adcauline wall with an inturned knot of perisarc, no true floor to hydrotheca, connection with hydrocladium wide. Margin thickened with two lobate lateral cusps and a much smaller adcauline cusp, aperture elliptical, opercular remnants obscuring most margins.
Hydranths with no caecum; approximately 16 tentacles.
Gonotheca immature, balloon-shaped to irregularly cylindrical, inserted without pedicel between two cauline hydrothecae in mid to upper stem region, expanding a little apically to a blunt distal end, gonophore an aborted hydranth, perisarc extremely thin.
Colour: Pale yellow-brown.
Tropical Indo-Pacific and Red Sea.
Subtidal, on rocky substrates and epizoic on polychaete tubes.