Macroarthrus victoriae, Hendrycks & Conlan, 2003

Hendrycks, EA & Conlan, KE, 2003, New and unusual abyssal gammaridean Amphipoda from the north-east Pacific, Journal of Natural History 37, pp. 2303-2368 : 2339-2343

Macroarthrus victoriae sp. nov.

(figures 15, 16)

Type material

H  : X, 15.7 mm, (appendages on two slides), CMNC 2002-0013 , Sta. 439, Cup# 4, 34°50.97∞N, 122°59.79∞W, 4050 m, 27 July 1990 .

Description

H  , female, CMNC 2002-0013.

Head: slightly longer than peraeonites 1 and 2 combined, midcephalic margin with a bulge at the mid-point; eye absent; rostrum short, acute; lateral cephalic lobe rounded. Antenna 1: very long, about 1.5×body, peduncular articles 1:2: 3 in ratio of 14:21:2, article 1 equal to head length, distally with a small tooth, article 2 long with lateral and medial setae and a small, distomedial tooth, article 3 very short with a small, distolateral, scale-like lobe; accessory flagellum very long, length about 0.3×primary flagellum, 18-articulate; primary flagellum about 92-articulate, articles with fine setae anterodistally, calceoli absent. Antenna 2: about 0.3×length of antenna 1, peduncular article 3 long, reaching end of peduncular article 1 of antenna 1, peduncular articles 4–5 with brush setae, peduncular article 4 long, length 1.2×peduncular article 5; flagellum short, about 0.3×peduncle length, 12-articulate, articles with distal setae, calceoli absent.

Peraeon: smooth. Pleon. smooth, segments elongate, longer than peraeon, pleonite 1>2>3. Coxae: shallow; coxae 1–4 about equal in depth, 0.5×as deep as wide, posterior margin of coxa 4 weakly excavate; coxae 5–6 subequal, about 0.4×as deep as wide; coxa 7 very shallow, 0.25×as deep as wide, ventral margin straight. Urosomite 1: with a small posterodorsal, bidentate tooth. U rosomite 2: with a small, posterodorsal tooth and subapical setule.

Upper lip: ventral margin shallowly incised, lobes symmetric. Lower lip: outer lobes broad, setulose, mandibular lobes narrow; inner lobes absent. Mandible: incisors slightly asymmetric, margins broad, straight, anterodorsal corner with a rounded tooth, anteroventral corner with a sharper tooth, right incisor with a tooth proximal to the anterodorsal corner; left lacinia mobilis broad, about 0.7×incisor, four- to five-cusped; right lacinia absent; accessory spine row with two to three stout spines and a proximal tuft of setae; molar absent; palp three-articulate, long, length 1.6×mandible, article 1 very short, wider than long, article 2 long, length 1.5×article 3, strongly setose posterolaterally, article 3 with 14 posterolateral and two apical setae. Maxilla 1: inner plate ovate, with four apical plumose setae; outer plate rectangular with eight smooth spine-teeth; palp two-articulate, longer than outer plate, medial margin strongly sloping with 11 apical spines and 13 marginal setae. Maxilla 2: inner plate triangular, broader than outer plate, with long plumose setae medially and apically; outer plate narrow, truncate apically with three long plumose setae. Maxilliped: inner plate nearly reaching base of palp, conical, with one apical spine; outer plate rounded, nearly reaching base of article 2 of palp, with 10 strong, apical and medial spines and three medial setae; palp four-articulate, long and raptorial, article 2 very long, length 3.4×article 1 and 1.4×article 3, both articles medially setose, article 3 with long, slender spines medially, dactylus long, slender, length 0.6×article 3.

Gnathopods: dissimilar in size and form. Gnathopod 1: basis with setose anterodistal margin, with medial setae; ischium and merus short, ventral margin of merus setose, distomedially with six spines; carpus about equal to propodus, not lobate, ventral margin lined with long setae, with medial setae; propodus stout, tapering distally, weakly subchelate, palm margin long, length 2.8×remaining ventral margin, palm lined with long setae and clusters of short pecinate, comb-like setae; dactylus long, sickle-like, as long as ventral margin. Gnathopod 2: basis with cluster of setae anterodistally; ischium as above; carpus long, length 1.6×propodus, with long setae on ventral margin that increase in length distally, with medial and distal setae, carpus not lobate; propodus stout, short, length 0.6×carpus, strongly subchelate, palm oblique, with long setae and clusters of short, pectinate, comb-like setae, ventral margin equal to palm and lined with clusters of setae; dactylus slightly longer than palm and straighter than dactylus of gnathopod 1.

Peraeopods 3–4: very slender, basis long, length 13×width, anterior margin with fine setae; ischium extremely elongated, almost as long as basis, length 25×width; merus, carpus, propodus and dactylus in ratio of 20:16:19:8. Peraeopod 5: basis deep, rectangular, 3.8×width, posterior margin with setae; merus, carpus and propodus in ratio of 31:24:16, anterior margins with pairs of long stout setae; dactylus strong, about 0.8×propodus. Peraeopod 6: subequal to peraeopod 5; basis deep, rectangular, 3.4×width, posterior margin with submarginal setae; merus, carpus and propodus in ratio of 34:22:15, anterior margins with pairs of long, stout setae; dactylus as above. Peraeopod 7: elongate, slender, about as long as body and about 1.5×as long as peraeopod 6; basis subovate, deep, 2.1×width, posterior margin with long, submarginal setae; merus, carpus and propodus in ratio of 45:41:46, margins with fine setae; dactylus straight, about 0.3×propodus.

Gills: on peraeopods 2–7. Brood plates: long, narrow, lacking brood setae, on peraeopods 2–5. Pleopods: powerful, peduncles and rami long. Epimeron 2–3: with a small, posteroventral tooth, tooth on 3 the larger.

Uropod 1: very long, length 2.5×urosome length, peduncle long, length 2.8×inner ramus, dorsolateral and dorsomedial margins strongly spinose, ventral margin with short setae; outer ramus 0.8×inner, inner and outer ramus with both margins strongly spinose. Uropod 2: as above except, peduncle very long, length 3.3×outer ramus; inner ramus missing. Uropod 3: peduncle very short, length 0.2×outer ramus; rami subequal, very long, length 5.2×peduncle; outer ramus twoarticulate, medial margin of article 1 with long plumose setae; article 2 minute, length less than 0.1×article 1, medial margin of inner ramus with long plumose setae. Telson: short, length 1.2×width, dorsolateral margins with one spine, margins parallel, cleft 55%, lobes widely divergent, acute distally. Male unknown.

Etymology The species is named in honour of the first author’s daughter, Victoria.

Condition

All appendages present. The left antenna 1 was dissected off instead of the right. The right antenna 1 has some of the distal articles of the flagellum broken off. The left uropod 2 is broken along the peduncle and the right is missing the inner ramus. The left uropod 3 has both rami broken off distally. The non-setose brood plates indicate that this female is not fully sexually mature.

Remarks

The extreme elongation of the ischium of peraeopods 3 and 4 is a remarkable and unique character, an autapomorphy never before reported in the Amphipoda . At a casual glance, Macroarthrus appears to resemble many of the bathyal/abyssal eusirids, especially in the elongated peraeopod 7 and overall habitus. Most likely, this species is an epibenthic or demersal predator, living on and above soft sediments. Some morphological adaptations for this life style consist of the very long first antenna, strong, raptorial gnathopods, the very powerful maxilliped palp and the elongated peraeopods. Prey organisms may be detected, caught rapidly and thrust towards the mandibles and maxillae, in a manner similar to that shown for Eusirus perdentatus by Klages and Gutt (1990). The extremely elongated peraeopods 3 and 4 and peraeopod 7 may be spread widely and employed as stilts, to prevent M. victoriae from sinking into soft sediments, in a similar manner to deep-water eusirids (Bousfield and Hendrycks, 1995). The strongly developed pleon, powerful pleopods and uropod tail fan are indicators of rapid swimming. Peraeopods 5 and 6 are strongly spinose anteriorly and they could be employed as a collecting basket, when feeding in the water column, trapping prey items in a similar manner to that of a dragonfly capturing mosquitoes (Bousfield and Hendrycks, 1995).

Distribution North-east Pacific off Point Conception, California in 4050 m depth.