Metacrangonyx dhofarensis sp. nov.

(Figs 2–10)

Material examined. Collected by Jan H. Stock and co-workers, 6 April 1996. Dahariz north, Salalah, Dhofar Governorate, Oman. Station 96-108B in Stock et al. (1997). Ministry of Water Resources well no. 202/304; closed, square, 1.5x 1.5 m well with electric pump; water surface at 7.5 m, water column 0.5 m; UTM coordinates (Datum unknown) 0197614/1884244 (ca. 17º01’21’’N 54º09’35’’E). Holotype: preparatory female (oostegites developed, but non-setose) 4.37 mm; completely dissected and mounted on single slide [ZMA Amph. 206112]. Paratypes: one preparatory female 4.36 mm; two males, 3.80 and 3.58 mm, one of them with G2 and P3&4 dissected and preserved on single slide; three juveniles, one of them possibly female [ZMA Amph. 206113]. In addition, two preparatory females were also examined but were accidentally lost.

Al Hafah, Salalah. Covered well in urban area, with electric pump. GPS coordinates (Datum WGS84): N 17.00278º, E 54.10885º. Water surface at 3 m, water column 0.3 m. 27 specimens, both sexes, preserved in ethanol vial. Collected by authors, 5 March 2012.

Diagnosis. Metacrangonyctid with hypertrophied coxal plate IV and reduced plates I-III. Coxal plate VII lacking anteroventral lobe. Mandibular palp wanting. Proximal segment of peduncle of antennule provided with two hypertrophied robust setae on dorsolateral margin. Telson longer than broad. Sexual dimorphism manifested on presence of series of long simple setae on posterior margin of propodus of male pereiopod IV.

Etymology. Species name refers to its area of distribution, apparently limited to the Dhofar Governorate in the Sultanate of Oman (SE Arabian Peninsula).

Description of adult female. Body (Fig. 2) compact, with integument of somites and limbs densely covered with blunt setules (sketched only in some figures). Head with hardly developed lateral lobes; post-antennal sinus shallow, hardly noticeable. Epimeral plates (Fig. 9 A) unarmed, each with pointed posteroventral corner; ventral margin of plate I straight, margins of plates II-III evenly convex; posterior margin of plate I convex, those of plates II-III straight.

Antennule (Figs 2; 3A) peduncle segments non-elongated, progressively shorter towards distal; proximal segment provided with two long and stout robust setae on dorsolateral margin, distal seta attaining about half length of segment. Main flagellum longer than peduncle, up to 12-articulate, with one aesthetasc per article at most. Accessory flagellum 1-articulate, slightly overreaching distal margin of proximal article of main flagellum.

Antenna (Figs 2; 3B) gland cone slender, slightly overreaching distal margin of third peduncle segment; medial surface of latter segment with triangular outgrowth provided with two setae; peduncle segments 4-5 progressively longer towards distal; segment 4 only 3.1 times longer than broad; segment 5 1.1 times as long as segment 4. Flagellum short, up to 6-articulate, shorter than peduncle segments 4-5 combined.

Labrum (Fig. 4 A) globose, ordinary. Paragnaths (Fig. 4 B) with no trace of inner lobes.

Mandibles devoid of palp. Left mandible (Fig. 4 C, D) with 6-denticulate incisor and 5-denticulate lacinia; spine row composed of 5 stout denticulated elements; row of 5 long spinulose setules running subparallel to spine row as figured; row of 4 secretory pores placed adjacent to spine row as figured; molar triturative, columnar, with three short plumose setules implanted close to each other on distal margin; molar seta reduced. Right mandible (Fig. 4 E) with 6-denticulate incisor and complex, bifid lacinia provided with marginal patch of setules; spine row composed of 4 elements plus row of only three spinulose setules; rest of appendage similar to left counterpart except for longer molar seta.

Maxillule (Fig. 5 A) coxal endite with row of 10 short, plumose marginal setae. Basal endite with only 9 rakelike robust setae. Endopod 2-segmented, distal segment with 5 heterogeneous setae distally.

Maxilla (Fig. 5 B) inner lobe with row of 10 facial setae; distal half of medial margin with ca. 8 sparsely set, stiff setae with pointed tip; ca. 9 setae with blunt bifid tip placed terminally on lobe. Outer lobe with two rows of setae plus isolated, short simple seta distally; one of setae of distal row stouter than rest.

Maxilliped (Fig. 5 C) basal endite subrectangular with distolateral angle produced into pointed process; distal margin with 6 heterogeneous setae, one of which simple, another one pinnate, both with rounded tip; remaining 4 distal setae short, robust, blunt, flattened, one of which more slender than rest; submarginal row of 7 setae running subparallel to distal margin and distal half of medial margin of endite as figured. Ischial endite (Fig. 5 D) expanded with 10 short robust setae along distal and distomedial margins, distolateral seta more slender than rest; three more distal robust setae denticulated. Rest of limb as figured.

Pereiopodal coxal plates I-III reduced, much shorter and narrower not only than plate IV, but also than plates V-VI, and equalling plate VII in length (see Fig. 2). Plate IV broadly expanded, with deeply excavated posterior margin; plate adorned with submarginal row of simple setae (Fig. 7 C). Plate V with well-developed, evenly rounded anteroventral lobe provided with simple seta (Fig. 8 A). Anteroventral lobe on plate VI reduced, finger-like (Fig. 8 C). Plate VII devoid of anteroventral lobe (Fig. 8 E). Coxal gills present on G2 to P6, ellipsoid and stalked (Figs 6 C; 7A, C; 8A, C). Oöstegites on G2 to P5, ordinary (Figs 6 C; 7A, C; 8A).

Gnathopod I (Fig. 6 A) reduced compared to rest of pereiopods; all segments bearing patch of spinules on medial surface as figured. Carpus elongated, longer than propodus. Propodus (Fig. 6 B) subrectangular, twice as long as broad; palm angle lined with only one medial and two lateral unicuspid flagellate robust setae; palm margin short, oblique, slightly convex with microspinulate surface.

Gnathopod II (Fig. 6 C) slender, with carpus elongated, longer than propodus; both segments with posterior margin poorly armoured. Propodus not expanded, more slender than carpus, 2.8 times as long as broad with subparallel margins; two stout flagellate setae pointing outwards, one of them hypertrophied, implanted close to palm margin on outer (= lateral) surface of segment (Fig. 6 C, D; also visible in Fig. 2); palm angle placed at 68% of maximum length of segment, marked by reduced flagellate unicuspid robust seta on medial side and similar but larger seta on lateral side (Fig. 6 D); palm margin short, oblique, straight, with microspinulate integument and devoid of robust setae (Fig. 6 D). Posterior margin of dactylus densely microspinulate (Fig. 6 E).

Pereiopods III & IV (Fig. 7) about equal in length, ordinary, each with unguis about two-thirds as long as corresponding dactylus.

Pereiopods V-VII (Fig. 8) each with broadly expanded basis with strongly overhanging posterodistal angle. Basis of P5 with anterior and posterior margins strongly convex conferring segment subcircular aspect. P5 and P7 subequal in length, P6 longest. Nails of P5-P6 subequal in length, nail of P7 shorter; each with dactylus ca. 2.5 times as long as corresponding unguis (compare Fig. 8 B, D and F). Marginal armature of robust setae on pereiopods variable in number (cf. Fig. 2).

Pleopods (Fig. 9 B) biramous, protopods devoid of marginal setae, each with two retinacles. Proximal seta on inner margin of proximal article of endopod modified, shorter than rest, with smooth, bifid distal portion.

Uropods progressively shorter towards posterior, tip of U1 reaching tip of U2 (Fig. 9 C). Uropod I (Figs 9 D; 2) with exopod much shorter than endopod, both rami shorter than protopod. Latter with one flagellate robust seta at each posterodistal corner plus two flagellate robust setae on lateral and medial margins. Exopod devoid of marginal armature, with 4 unequal robust setae terminally, two of which reduced. Endopod with 2-3 robust setae along medial margin; tip of segment with 5 flagellate robust setae, two of which reduced, plus reduced smooth seta.

Uropod II (Figs 9 E; 2) with exopod much shorter than both endopod and protopod; exopod longer than protopod. Protopod with one flagellate robust seta at each posterodistal corner plus another one about midway of lateral margin. Terminal armature of rami as in U1 except for absence of reduced smooth seta on endopod tip. Endopod with 0-2 flagellate robust setae along lateral margin. Exopod with 0-1 flagellate robust seta along lateral margin and 2-3 flagellate robust setae along medial margin.

Uropod III (Fig. 9 F) uniramous with no trace of endopod; exopod longer than protopod. Protopod with simple seta midway of inner margin. Exopod lanceolate, with two flagellate (only 1 in smaller specimens) robust setae along inner margin and flagellate robust seta on tip.

Telson (Fig. 9 G) triangular, about 1.2 times longer than broad, with two simple and two penicillate setae subterminally on dorsal surface, and one lateral penicillate seta at each side.

Description of male. Males are apparently smaller than females and show a modified setation (long setae vs. short setae in female) on the posterior margin of carpus and propodus of P4 (Fig. 10 C). The gnathopod II palm (Fig. 10 A, B) is rhomboidal, with the palm angle placed at about 32% length of segment; the palm margin is smooth, undulate, and lined by a series of 10 smooth, unicuspid strong robust setae on outer side, and a series of approximately 10 short simple setae with blunt tip on medial side; there is one short bifid robust seta close to palm angle on medial surface of segment, and two long flagellate setae pointing outwards implanted nearby on outer (= lateral) surface; the posterior margin of palm is short and unarmed.