SEM study of species of Oswaldella Stechow, 1919 (Cnidaria, Hydrozoa, Kirchenpaueriidae), with an annotated checklist of the species of the genus Author Molinero, A. González Author Peña Cantero, A. L. text Zootaxa 2015 4052 4 401 441 journal article 10.11646/zootaxa.4052.4.1 93f6d085-8e63-4d1e-89c1-f950128b2a61 1175-5326 245887 47EF6E9D-9064-4899-B3DD-276FF7C969EB Oswaldella laertesi Peña Cantero, 2007 ( Figs 9 , 16 I, 18A, 20A) Material examined. New Zealand Antarctic Expedition TAN0802: Stn 17 , Tangaroa , 9 February 2008 , 73°07'47''S , 174°19'23''E (Ross Sea, Cape Adare), 321 m ( NIWA 35536); Stn 117 , Tangaroa , 21 February 2008 , 72°35'41.99''S , 175°20'53"E (Ross Sea, Cape Adare), 479– 475m ( NIWA 37198). Description. Polysiphonic, unbranched stems, up to 100 mm high, irregularly divided into internodes. Three longitudinal rows of cauline apophyses ( Fig. 9 F). Cauline apophyses with five to six nematophores: three to four axillary ones, provided with distinct, collar-shaped nematotheca, and two extra nematophores, each emerging through ‘mamelon’ ( Fig. 9 B, F). FIGURE 7. Oswaldella grandis Peña Cantero, Svoboda & Vervoort, 1997 . A, hydrocladial branching and disposition of hydrothecae; B–C, cauline apophysis with axillary nematophores and ‘mamelons’ (upper view); D–E, unforked hydrocladial internodes (D frontal view, E lateral view); F, detail of mesial inferior nematotheca. Scale bar: 400 µm (A), 100 µm (D–E), 40 (B–C) and 20 µm (F). FIGURE 8. Oswaldella incognita Peña Cantero, Svoboda & Vervoort, 1997 . A, hydrocladial branching and disposition of hydrothecae; B–C, cauline apophyses showing axillary nematophores; D–E, unforked hydrocladial internodes (D lateral view, E frontal view); F, detail of mesial inferior nematotheca. Scale bar: 200 µm (A), 100 µm (C–E) and 20 µm (B–F). FIGURE 9. Oswaldella laertesi Peña Cantero, 2007 . A, hydrocladial branching and disposition of hydrothecae; B, cauline apophysis showing axillary nematophores and ‘mamelons’; C–D, unforked hydrocladial internodes (C lateral view, D frontal view); E, detail of mesial inferior nematotheca; F, cross section of stem showing arrangement of cauline apophyses; G, gonotheca. Scale bar: 400 µm (A), 200 µm (C–D, F–G), 80 µm (B) and 8 µm (E). Hydrocladia much branched ( Fig. 9 A), with up to third-order hydrocladia. First hydrocladial internode bifurcated, with two similar prongs ( Fig. 9 A). Mesial inferior nematophore emerging from strongly marked swelling at proximal third of internode ( Fig. 9 C); with well-developed nematotheca ( Fig. 9 C–E). Hydrotheca placed on middle of internode or at its distal half ( Fig. 9 C–D). Hydrotheca low, as high as wide. Adcauline hydrothecal wall with small, but distinct, free portion. Abcauline wall straight ( Fig. 9 C). Hydrothecal aperture circular, perpendicular to long axis of internode ( Fig. 9 C–D); rim even, sometimes with slight adcauline elevation. Male gonotheca fusiform, with sub-terminal oval aperture ( Fig. 9 G). Remarks. The most remarkable result from the SEM study of this species is the finding of undescribed, distinct, collar-shaped nematothecae in the axillary nematophores of the cauline apophyses. Variability in the number of axillary nematophores, found in Oswaldella laertesi and in certain other species of the genus (e.g. Oswaldella niobae ), could be due to fusion.