Sertularella mytila sp. nov. Fig. 11 A–E Material examined. Holotype, NMV F171367, fertile colony, alcohol preserved; two microslides from holotype colony, colony, Clifton Springs jetty piles on mussels, depth 0.5 m, coll: J. Watson, 01/01/2010. Paratype, NMV F171368, microslide, Clifton Springs jetty piles on mussels, depth 1 m, coll: J. Watson, 23/01/06. Description. Hydrorhiza reptant on substrate, stolons cylindrical, rugose; numerous single hydrothecae arising from stolon, interspersed with unbranched stems bearing 3–8 hydrothecae; exceptionally, some longer stems with up to 16 hydrothecae. Proximal stem region deeply corrugated with up to seven twists, stem internodes thereafter slender, variable in length, inclined or straight, internodes fairly smooth to corrugated, inflated behind hydrotheca, nodes oblique, often indistinct. Hydrothecae biseriate, one on each internode, facing slightly forward, size and shape somewhat variable, barrel-shaped to almost cylindrical, widest in proximal third, tapering distally, floor concave with a distinct circular foramen, abcauline wall straight or curving slightly away from internode, with two to five ridges, adcauline wall convex above floor, convexity weakening distally, margin with four sharp equidistant cusps separated by shallow embayments, three large submarginal cusps, the largest below the adcauline marginal cusp, the other two below opposite marginal cusps; operculum of four flaps. Hydrorhiza, diameter 56–76 Internode length to floor of proximal hydrotheca 200–400 length succeeding internode(s) 320–544 diameter of annulated section 68–80 Hydrotheca length, base to margin 320–376 length adnate adcauline wall 120–180 maximum diameter 168–192 diameter at margin 100–120 width at floor 136–152 Gonotheca length including pedicel 900–1040 maximum width 600–680 width at orifice 184–272 Gonotheca ovoid to almost barrel-shaped, widest in lower third to mid-section, inserted into internode on a smooth, short pedicel opposite a hydrotheca, wall of gonotheca with up to 10 broad ridges, distalmost ridge rounded into a low shoulder, orifice on a short cylindrical neck with four equidistant sharp terminal spines. Mature female gonophore containing large ova. Perisarc of hydrorhiza and hydrocaulus thick. Colour of younger colonies white, older colonies brown, gonophores creamy yellow. Remarks. There are no very small species of Sertularella with corrugated hydrothecae that match the present material. Species considered were: Sertularella robusta Coughtrey, 1876 b, Sertularella stolonifera Vervoort & Watson, 2003 and Sertularella microgona (von Lendenfeld, 1884). S. robusta and S. microgona (probably = S. robusta) are species of much larger dimensions commonly occurring in southern Australia and New Zealand. S. stolonifera, presently known only from New Zealand, also is considerably larger in dimensions. This very small species preferentially colonises shells of mussels (Mytilus galloprovincialis) and and crustose bryozoans on mussel shells in shallow water on jetty pilings. Colonies are reproductive over the summer months (November to February). Etymology. Named for the mussel Mytilus on which it grows.

Published as part of Watson, Jeanette E., 2011, New species, new records and redescriptions of Thecate hydroids (Cnidaria: Hydrozoa: Leptothecata) from Southern Australia, pp. 1-36 in Zootaxa 3122 on pages 24-26, DOI: 10.5281/zenodo.203966