Hydrobiologia 453/454: 427–430, 2001. R.M. Lopes, J.W. Reid & C.E.F.Rocha (eds), Copepoda: Developments in Ecology, Biology and Systematics. © 2001 Kluwer Academic Publishers. Printed in the Netherlands. 427 A new species of Acontiophorus Brady, 1880 (Copepoda: Siphonostomatoida) from Ushuaia, Argentina R. Johnsson Universidade de São Paulo, Depto Zoologia, Inst. Biociências Caixa Postal 11461, 05422-970 São Paulo, Brazil E-mail: johnsson@ib.usp.br Key words: Copepoda, Siphonostomatoida, Asterocheridae, Acontiophorus, Argentina, Patagonia Abstract A new species of Acontiophorus belonging to the family Asterocheridae was found in Ushuaia, Patagonia, southern Argentina. The new species is distinctive in having 9-segmented antennule, five setae on terminal endopodal segment of leg 1, seven elements on terminal segment of leg 1 and leg 4 exopods, and characteristic setation on maxillule lobes. This combination of characteristic features does not occur in any other species of the genus. The genus Acontiophorus is recorded for the first time from the South Atlantic. Introduction So far, studies of siphonostomatoids associated with invertebrates in the South Atlantic Ocean have been restricted to the western coast, primarily in southeast- ern and northeastern Brazil (Alvarez, 1988; Johnsson, 1997/1998, 1998, 1998a/1999, 1998b/1999; Johnsson & Bustamante, 1997). Except for the studies of Eiselt (1965), who revised the material from the ‘Gauβ Sta- tion’ (66 ◦ S, 90 ◦ W – Bellingshausen Sea, Antarctica) collected by the Deutsche Südpolar Expedition (1901- 1903), there are no records of siphonostomatoids in the Southern Ocean. A new Acontiophorus (Siphonostomatoida: As- terocheridae) was collected in Ushuaia (54 ◦ S, 68 ◦ W), Southern Argentina, in a coastal area under the influ- ence of the cold Falklands Current. This is the first record of the genus in the South Atlantic Ocean. Results Taxonomy Acontiophorus ushuaiensis n. sp. (Figs 1 and 2) Material examined. Female holotype from Bahía Ensenada in Ushuaia, Argentina, found in the sediment of a cirriped sample collected by J. Cal- cagro on 22 October 1998. This material is deposited in the Museu Nacional / Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil (MNRJ 14019). Description Female. Body (Fig. 1a) cyclopiform, prosome en- larged and slightly flattened dorso-ventrally; urosome cylindrical. Length (excluding caudal setae) 768 μm; greatest width 423 μm, 1.8 times longer than wide. First two pedigers showing pointed epimera. Ratio of length–width of prosome 1.3:1. Ratio of length of prosome to that of urosome 2.8:1. Genital double-somite (Fig. 1b) 86 × 104 μm, ratio of length–width 0.8:1, rounded anteriorly, and with acute posterior corners. First abdominal somite 39 × 58 μm, ratio of length – width 0.7:1; with acute posterior corners. Anal somite 46 × 51 μm, ratio of length to width 0.9:1, with 5 setules around anal area. Caudal rami 55 × 20 μm, ratio of length to width 2.7:1, bearing 6 setae. Seta I absent; setae II–VII, 97, 143, 264, 270, 130 and 65 μm long, respectively. All setae plumose. Inner margin of caudal ramus covered with setules. Antennule (Fig. 1c) 150 μm long and 9- segmented. Length of segments 1–9, measured along their posterior margins: 37 (23 along anterior margin), 32, 11, 12, 15, 6, 10, 17 and 11 μm, respectively. Formula for armature: 1, 7, 1, 1, 6, 1, 1 + spine, 3