Hippomedon iugum, Kilgallen, 2009

Kilgallen, Niamh M., 2009, New species of lysianassoid Amphipoda (Crustacea) associated with seamounts, marine canyons and cold seeps of New Zealand, Zootaxa 2298 (1), pp. 1-30 : 12-16

Hippomedon iugum sp. nov.

( Figures 8–10 )

Type material. Holotype male, 11 mm, NIWA 50184 ; allotype female, 8 mm NIWA 42970 , Kaikoura canyon, New Zealand, 42º30.98’S, 173º37.6’E, 1028 m ( TAN0616 /109), collected by van Veen Grab. Paratype female, 11 mm, NIWA 48675 , Kaikoura Canyon, 42º31.39’S, 173º37.14’E, 1014 m ( TAN0616 /108) collected by van Veen Grab.

Other material examined. 1 female, 8.5 mm, NIWA 19530 , Mahina Knoll , Bay of Plenty, North Island, New Zealand, 37º21.63’S, 177º06.63’E, 590 m ( TAN0413 /135) collected by van Veen Grab.

Etymology. The name ‘iugum’ is taken from the Latin, meaning ‘ridge’ or ‘rising’, and alludes to the large flange on the peduncle of uropod 2.

Diagnosis. Eyes apparently absent. Lateral cephalic lobes subacute to acute, with tips very narrowly rounded. Upper lip produced very slightly in front of epistome. Mandible molar columnar, strongly triturative, palp attached distally. Maxilla 1 outer plate with setal-teeth in a modified 6/5 arrangement. Gnathopod 1 subchelate; palm extremely acute, slightly convex; dactylus slightly over-reaching palm. Gnathopod 2 carpus twice as long as propodus with broad posterodistal lobe. Pereopod 7 basis posterodistal margin slightly excavate, not produced distally. Epimeron 1 anterodistal corner producing small up-turned spine. Uropod 2 peduncle producing large flange.

Description of male holotype. Antenna 1 shorter than antenna 2; flagellum article 1 distinctly elongate, with strong 2-field callynophore. Antenna 2 more than half of body length. Calceoli present on both antennae. Eyes apparently absent. Lateral cephalic lobes acute, with tips very narrowly rounded. Epistome and upper lip separate, upper lip produced very slightly in front of epistome. Mandible incisor smooth; lacinia mobilis present on left mandible only; setal row present; molar columnar and strongly triturative; palp attached distally. Maxilla 1 outer plate with setal-teeth in a modified 6/5 arrangement; palp present. Maxilla 2 inner plate without oblique setae. Maxilliped palp 4-articulate, article 4 well developed, without unguis; outer plate large, reaching beyond halfway of palp article 3, with strong, well-developed apical robust setae; inner plate well developed, greater than half the length of outer plate.

Coxae 1–3 posterodistal corner with slight notch with inserted seta. Gnathopod 1 subchelate; coxa 1 large, about as long as coxa 2, subrectangular, with straight anterior margin; basis with sparse setal fringe; ischium short, only slightly longer than broad; carpus long, subequal in length to propodus; propodus margins subparallel, palm extremely acute, slightly convex, minutely denticulate and defined by robust setae at posterodistal corner; dactylus slightly over-reaching palm, without accessory tooth. Gnathopod 2 minutely subchelate; carpus twice as long as propodus, with broad posterodistal lobe. Pereopod 3 merus weakly expanded anterodistally and subequal in length to carpus; carpus slender, subequal in length to propodus; propodus slender, without blunt, locking robust setae at posterodistal corner. Pereopod 4 coxa with poorlydeveloped posterodistal lobe, lobe slightly crenated on posterior margin; merus weakly expanded anterodistally and longer than carpus; carpus slender, subequal in length to propodus; propodus slender, without blunt, locking robust setae at posterodistal corner. Coxa 5 equilobate, posterior lobe broader than anterior lobe; pereopod 5 basis slightly longer than broad, crenate posteriorly; merus moderately expanded posterodistally, subequal in length to carpus; carpus robust, stocky, shorter than propodus; propodus slender, with blunt robust setae along anterior margin; dactylus long, more than half the length of the propodus.

Pereopod 6 basis slender, distinctly longer than broad, posterior margin slightly crenate; merus barely expanded posteriorly, shorter than carpus; carpus subequal in length to propodus; propodus slender; dactylus long, about 0.65 the length of the propodus. Pereopod 7 basis longer than broad, crenate posteriorly, posterodistal margin slightly excavate, not produced; merus not expanded, distinctly shorter than carpus; carpus slender, very slightly longer than propodus; propodus slender, with blunt robust setae along anterior margin; dactylus long, about 0.65 the length of the propodus. Gills from gnathopod 2 to pereopod 7.

Epimeron 1 anterodistal corner acute, producing small spine. Epimeron 2 posterodistal corner quadrate. Epimeron 3 posterodistal corner produced, forming a large upturned spine. Urosomite 1 weakly produced dorsodistally. Uropod 1 peduncle subequal in length to rami; rami subequal. Uropod 2 peduncle shorter than rami, producing large flange which overhangs the rami; rami subequal, inner ramus without marginal constriction. Uropod 3 peduncle shorter than rami; rami subequal in length, outer ramus 2-articulate, article 2 short, inner ramus extending past article 1 of outer ramus, both rami with dense fringe of plumose setae. Telson distinctly longer than broad, deeply cleft.

Female (sexually dimorphic characters; based on allotype female). Antenna 2 less than 40% of body length; calceoli absent.

Length. Male 11 mm; females 8–11 mm.

Distribution. New Zealand: Kaikoura canyon and the Mahina Knoll, Bay of Islands; 590–1028 m.

Ecology. A bathyal species associated with sediments of canyons and seamounts.

Remarks. The flange on the uropod 2 peduncle of this species is a highly diagnostic character that seems to be shared only by a small number of other Hippomedon species , all of which are found only in New Zealand waters. Of those, H. incisus K.H. Barnard, 1930 , differs from the present species by the incised posterodistal corner of epimeron 3 at the base of the spine, and the presence of eyes. Hippomedon manene Lowry & Stoddart, 1983 , is also quite similar, but lacks the posterior lobe on the gnathopod 2 carpus, the excavate posterior margin of pereopod 7, and the small up-turned spine on the anterodistal corner of epimeron 1. Hippomedon whero Fenwick, 1983 , differs from H. iugum in the shape of both the lateral cephalic lobes and the 1 st urosomite. Finally, both H. manene and H. whero have eyes with crystalline lenses, and the males of those species exhibit one large, over-developed calceolus on the 1 st flagellar article which is absent in H. iugum sp. nov.

Figures

FIGURE 8. Hippomedon iugum sp. nov., holotype, male, 11 mm, NIWA 50184: A, habitus; B, antenna 1 (broken at flagellum); C, antenna 2 (broken at flagellum) D, maxilla 1; E, left mandible; F, left maxilliped palp and outer plate; G, maxilliped inner plates; H, maxilla 2. Scale bars: a = 2 mm (fig. 8A); b = 0.4 mm (figs 8B, C), 0.2 mm (figs 8D–H).

FIGURE 9. Hippomedon iugum sp. nov., holotype, male, 11 mm, NIWA 50184: A, gnathopod 1; B, gnathopod 2; C, pereopod 3; D, pereopod 4. Scale bar, 0.4 mm.

FIGURE 10. Hippomedon iugum sp. nov., holotype, male, 11 mm, NIWA 50184: A, pereopod 5; B, pereopod 6; C, pereopod 7; D, telson. E, uropod 3; F, uropod 2; G, uropod 1. Scale bars: a = 0.4 mm (figs 10A–C); b = 0.2 mm (figs 10D–G).