New Mysida (Crustacea) in the genera Amblyops and Pseudomma from the Iceland Basin Author Meland, Kenneth Author Brattegard, Torleiv text Zootaxa 2007 1628 43 58 journal article 10.5281/zenodo.179324 8d9d8b76-bfd9-46ef-b118-58fcc94c7825 1175-5326 179324 Pseudomma antarcticum Zimmer, 1914 Pseudomma antarcticum Zimmer 1914 :389 . Pseudomma antarcticum Tattersall 1955 :94 . Pseudomma antarcticum Ledoyer 1990 :41 . Material examined “Discovery”, st. 182 ( Tattersall, 1955 ), 1 adult female (21.5 mm), 1 immature male ( 20 mm ) NHML- 1958.11.19.515-19. BIOICE 2844, 1 immature female ( 8 mm ), 1 immature female (thorax), IMNH-2196. Stn BIOICE 2853, 1 immature female ( 12 mm ), IMNH-2197. Stn BIOICE 2856, 1 adult female ( 14 mm ), 1 immature female ( 13 mm ), IMNH-2198. Stn BIOICE 2859, 1 male (abdomen), 1 immature female ( 10 mm ), IMNH-2199. Description Pseudomma antarcticum was first decribed by Zimmer (1914) for the reception of an immature female from the East Antarctic and additional records were later reported from the South Shetlands ( Tattersall 1955 ) and the Weddell Sea ( Ledoyer 1990 ). Since previous descriptions have concentrated on presenting diagnostic characters within the Pseudomma we give a full description of both male and female P. antarcticum . Carapace ( Fig. 7 A) with anterior margin evenly rounded and anteriorly produced lateral corners. Ocular plate ( Fig. 7 A) extending to mid-portion of first segment of antennular peduncle; plate deeply cleft, anterior median portion of plate evenly rounded, antero-lateral margins finely serrated with 20–25 stout setae. Antennal scale ( Fig. 7 B) 3.0 times longer than broad; distal terminal denticle on outer margin consisting of one spine; apex extends slightly beyond terminal denticle, without suture, outer margin of apex armed with five setae. Left mandible setal row consisting of three hirsute spines and right mandible setal row consisting of nine entire spines. Distal segment of mandible palp ( Fig. 7 C) with five enlarged proximal ventral setae, medial margin with row of eight to ten setae, dorsal margin with eight setae. Maxillule ( Fig. 7 D), apex of coxal lobe armed with three strong setae bordered by three smaller setae placed distal-posterior, distal-ventral, and distal-anterior; anterior lateral margin armed with two setae in medial region; ventral surface and posterior lateral margin supporting seven to eight setae; ventral surface of maxillule basis supporting three setae, posterior lateral margin set with small spinose setae, apex supporting 14 strong cuspidate setae. Maxilla ( Fig. 7 E) with three setae on proximal inner margin of endopod in Antarctic specimens and five in North Atlantic specimens; exopod supporting 20 lateral setae in North Atlantic specimens and 25 in Antarctic specimens; coxa with up to one seta on dorsal surface, coxal surface entire, lateral margin of coxa armed with two rows of setae, dorsal row consisting of one large and five smaller setae. FIGURE 7. Pseudomma antarcticum Zimmer, 1914 . A, E, North Atlantic immature female (13 mm); B, C, North Atlantic female (14 mm); D, Antarctic male (20 mm). A, anterior portion of cephalon and cephalic appendages. B, antennal scale. C, mandible palp. D, maxillule. E, maxilla. Pseudomma antarcticum Zimmer, 1914 . A–C, Antarctic immature male ( 20 mm ); D, North Atlantic female, ( 14 mm ); E, North Atlantic immature female ( 13 mm ). A, male genital organ. B, first male pleopod. C, fourth male pleopod. D, uropod. E, telson. First and second thoracic appendages formed as maxillipeds ; nail armed with six denticles, first maxilliped dactylus fringed with five to seven large setulate setae on each lateral margin in Antarctic specimens and four to five lateral setulate setae in North Atlantic specimens,; second maxilliped with short unarmed nail, dactylus fringed with eight to nine large setulate setae on each lateral margin in North Atlantic specimens and 12 to 13 in Antarctic specimens. Third to eighth thoracic appendages take on the form of long and slender pereopods. Female marsupium consists of three pairs of oostegites arising from the sixth to eighth pereopods, increasing in size posteriorly. Male genital organ (Fig. 8A) extending beyond eigth pereopod, bearing one apical seta and several lateral setae. Sixth abdominal somite two times longer than fifth. Pleopods of male biramous; first pleopod with unsegmented endopod and 12-segmented exopod (Fig. 8B); second pleopod damaged; third to fifth pleopod with 11-segmented exopod; third and fourth pleopod endopods 10-segmented (Fig. 8c), fifth pleopod endopod 7- segmented. Male specimen immature leaving uncertainty in assessing modified setae, no enlarged setae on third pleopod, fourth pleopods bear five enlarged setae arising from five distal segments of endopod (Fig. 8C). Female pleopods uniramous, taking on the form of unsegmented plates and set with apical and ventral surface setae. Uropod endopod (Fig. 8D) with one ventrally placed strong spinose seta on inner margin near statocyst, outer margin of endopod armed with 13 large setae, and 15 small setae. Telson (Fig. 8E), lateral margins entire, apex truncate and armed with three to four pairs of spinose setae and one pair of median plumose setae. Distribution Pseudomma antarticum has previously been known to be widely distributed in Antarctic waters, recorded from the South Shetlands down to 810 m depths, the Weddell Sea at 705 m and also the east Antarctic ( 65°15’S , 80°0’E ) at 3425 m . With our new records at 1800–2300 m depths in the north-eastern part of the Iceland Basin P. antarcticum seems to be distributed throughout the deeper benthic environment of the Atlantic Ocean. Remarks The Pseudomma antarcticum specimens from the North Atlantic are smaller in size than those described from the Antarctic , 14 mm compared to up to 21.5 mm in South Shetland specimens, but we also note Zimmer’s type specimen from the east Antarctic , a young female with undeveloped oostegites measuring 14 mm , which supports a considerable length distribution in P. antarcticum . Considering the indication of such a remarkable geographical range for P. antarcticum we compared the Iceland basin specimens with the South Shetland material and found that the only characters distinguishing north and south P. antarticum are differences in setae numbers on the maxilla. In the Antarctic specimens there are three setae on the proximal inner margin of the endopod and the exopod supports 25 lateral setae; in the Iceland Basin specimens we register five and 20 setae, respectively. Similar variation in proximal endopod setae is also observed in P. b e l g i c a e from the Ross Sea and P. a r m a t u m from the South Orkneys. With comparable variation in maxilla setation in other “ Antarctic ” species, we can not justify erecting a new species for our Iceland Basin specimens. P. a n t a rc t i c u m bears closest resemblance to P. surugae from Japan in both having a telson with no lateral spines and three to four pairs of apex spines. However, they are easily distinguished by the ocular plate in P. antarcticum supporting strong antero-lateral spinose setae, whereas in P. surugae the ocular plate margins are entire. With the extended distribution of P. antarcticum to include the north Atlantic, the morphologically similiar species of P. islandicum from the Iceland Basin and Antarctic Pseudomma , and the northern extension of Amblyops trisetosa and A. spinifera ; a hypothesis of widely distributed benthic mysid species in the Atlantic Ocean deep-sea is supported.