Benthic hydroids (Cnidaria, Hydrozoa) from the Ross Sea (Antarctica) collected by the New Zealand Antarctic expedition BioRoss 2004 with RV Tangaroa Author ÁLVARO L. PEÑA CANTERO text Zootaxa 2017 2017-07-14 4293 1 1 65 journal article 32747 10.11646/zootaxa.4293.1.1 dc2674e2-e260-4b6e-ad3a-731d610bccc5 1175-5326 828475 6FF96B5C-1F80-47ED-9962-19603DCBF550 Staurotheca gracilis sp. nov. ( Figs 3 B, 10D–H) Material examined. Stn 53, one stem 60 mm high (holotype, NIWA 115614); Stn 139, one monosiphonic stem fragment 35 mm long (paratype, NIWA 115615). Description ( Holotype ). Stem 60 mm high ( Fig. 3 B), basally polysiphonic, irregularly branched in approximately one plane. Branches narrow at origin ( Fig. 10 D), basally more or less perpendicular to previous one, strongly curving upwards later. Branches repeatedly branched (up to sixth order present) ( Fig. 3 B), with anastomose. A few perisarc constrictions irregularly distributed along stem and branches. Hydrothecae typically arranged in decussate verticils of three, resulting in six longitudinal rows of hydrothecae. Decussate pairs also present in some parts ( Fig. 10 D). Hydrotheca elongate ( Fig. 10 D–H), roughly cylindrical, almost completely adnate to branch, and with mushroom-shaped diaphragm. Measurements (in µm). Hydrothecae : abcauline wall 700–800, free part of adcauline wall 30–60, adnate part of adcauline wall 800–930, adcauline wall 830–980, diameter at aperture 230–260, maximum diameter 330–350, diameter at diaphragm 300–310. Cnidome : larger microbasic mastigophores 22–24 x 5–6, smaller microbasic mastigophores 9 x 2. Remarks. This species is easily distinguishable from other species of Staurotheca by the shape and size of the hydrotheca, which is relatively high and thin. Among the known species of Staurotheca , only six species ( S. affinis , S. dichotoma , S. juncea , S. multifurcata , S. undosiparietina , and S. vanhoeffeni ) are characterized by having mushroom-shaped diaphragm and hydrothecae arranged in decussate verticils and almost completely immersed into the branches. Staurotheca gracilis sp. nov. also clearly differs from S. affinis , S. juncea , S. multifurcata , and S. undosiparietina by the presence of anastomoses among the branches. In this character, however, Staurotheca gracilis sp. nov. is similar to S. dichotoma and S. vanhoeffeni . Staurotheca gracilis sp. nov. differs from these two species by the shape of the hydrotheca, which has, in frontal view, a more or less constant diameter along its length, and is distinctly thinner (the hydrotheca is more robust in S. dichotoma and S. vanhoeffeni ), having a smaller diameter at the aperture (e.g., diameter at the aperture is 273–332 µm in S. dichotoma and 273–312 µm in S. vanhoeffeni , cf. Peña Cantero et al. 1997b ). Concerning the number of longitudinal rows of hydrothecae, S. juncea and S. multifurcata have up to 12, S. undosiparietina up to ten and S. affinis and S. vanhoeffeni up to eight. Only S. dichotoma has been described having up to six longitudinal rows; it is characterized by having verticils of three hydrothecae or decussate pairs. Even when a few decussate pairs have been observed in the material of S. gracilis sp. nov. , they cannot be considered as characteristic as in S. dichotoma , in which whole colonies with decussate pairs have been described. Ecology and distribution. Material collected at depths between 197 and 236 m off Moubray Bay and Cape Hallet. Etymology. The specific name gracilis refers to the gracile, elongate shape of the hydrothecae.