Pseudingolfiella possessionis Smet, 2015, n. sp.

Pseudingolfiella possessionis n. sp.

Type locality. Crozet archipelago, Île de La Possession, Pointe du Bougainville, in submerged marginal moss of small running brooklet, 46.43766 S – 51.86153 E.

Material examined. Twenty-four males and 36 females from the type locality, and several specimens from other localities on the island.

Holotype: male in a permanent glycerine glass slide mount (Royal Belgian Institute of Natural Sciences, Brussels, IG 32762, INV. 131001).

Paratypes: female in glass slide mount (RBIN,S INV. 131002); 3 males (RBINS, INV. 131003) and 3 females (RBINS, INV. 131004) each in vial, 80% ethanol-glycerine.

Etymology. The species name is derived from its type locality, Île de La Possession, Crozet archipelago.

Diagnosis. Terminal aesthetasc on accessory flagellum antenna 1; calceolus on antenna 2 in male; 3 setae on article 3 of mandibular palp; labium inner lobes apparently present; 7 distal setae on outer plate maxilla 2; 4 distal setae on inner plate maxilla 2; incisor and lacinia mobilis each with 5 teeth; inner plate maxillipeds with 6 setae, outer one 4; spine on posterior margin dactylus; propodus pereiopod 5 with strong median articulated seta; pleopod 3 of female with 2nd vestigial article; posterolateral margin of external ramus uropod 1 and 2 in male undulate with mediolateral notch, posteromedian margin smoothly curved; distal end of external ramus uropod 1 and 2 in male rounded; external ramus uropod 2 distinctly smaller than internal one in female; uropod 3 uniramous; article 1 uropod 3 with 2 robust flagellate setae; dorsomedian lobe of telson with terminal spine, longer than lateral lobes; coxal gills on coxal plates 2–6; sternal gills on pereionites 1–7, single, 2−6 protruding.

Description. Small-sized species, body length 1.6–3.5 mm. Body shape as typical for the genus, vermiform, elongate, weakly compressed laterally (Figs 2 A, 6A); unpigmented.

Head (Figs 2 A, 6B) slightly shorter as pereionites 1 and 2 combined; inferior antennal sinus shallow with rounded angle; eyes absent; rostrum short; 6 lateral and 2 frontal setules; cephalic lobe more or less roundedrectangular.

Antenna 1 (Fig. 2 C) longer than antenna 2, about 1/4th body length. Peduncular articles 1, 2 and 3 with length ratios 1.0: 0.6: 0.4; article 1 shorter than head length; article 1 with setae all over, article 2 with setae in distal half, article 3 with distal setae only. Primary flagellum with 4 articles bearing distal setae, article 1 shortest, articles 2–4 of equal length; article 4 with small terminal aesthetasc, articles 2 and 3 with long, stalked cylindrical aesthetasc distally (Figs 2 C, 7A). Calceoli lacking. Accessory antenna (2E, F, 7B) distally at medial surface of peduncular article 3, composed of 2 articles; basal article broad with 3–4 distal setae; distal article narrow with 3 terminal setae and minute aesthetasc; accessory antenna shorter than first article of primary antenna in female (Fig. 2 E) and as long or slightly longer in male (Figs 2 F, 7B).

Antenna 2 (Fig. 2 D) stout, length 0.7 × length of antenna 1. Peduncular articles 1–4 equally broad, article 5 narrower; articles 1‒ 3 with few setae, articles 4 and 5 with numerous setae in distal half, especially situated distoventrally; gland cones distinct, flattened-conical, narrow and parallel-sided in lateral view. Flagellum 4- articulate, all articles with distal setae; article 1 with 4 robust bifurcate setae; articles 2 and 3 with 2 robust bifurcate setae; article 1 spineless with small terminal aesthetasc. Male with single calceolus (Figs 6 E, F) at medial side of article 1 of flagellum, composed of short stalk continuous with flattened oval lobe showing 10 transversal welldefined bands; distal margin of plate 3 and 4 with 2 minute setae, plate 5 with lenticular slit and emerging minute seta; surface of stalk and posterior of lobe granulated; calceoli lacking in female.

Pereionites (Fig. 2 A, Figs 4 A −G) dimensions increasing posteriorly, slightly smaller than pleonites; ventral margin of pereionites 4–7 with broad and shallow indentation; all pereionites with 4–6 minute facial setae, and a single small marginal seta at the posterolateral corners of pereonites 5–7 and anteroventral corners 6 and 7. Coxal plates (Figs 4 A −G) small, wider than high; plates 5–7 with dominant anterior lobe; plates 1–4 each with an anterior and posterior marginal seta; plates 5–7 with posterior seta only.

Pleonites (Fig. 2 A) dorsal margin postero-medially with pair of setae; all pleonites with up to 5 minute facial setae; a single ventral marginal seta, and 1 (pleonite 1) or 2 (pleonites 2, 3) setae at posterior margin.

Urosomites (Fig. 2 A) dorsal margin 1–3 posteriorly with a pair of setae; urosomites 1and 2 with 3 and 1 minute facial seta(e); urosomite 1 with single seta at ventral margin.

Labrum (Fig. 2 G) anterior margin rounded with shallow, broad median protruding part, setae absent.

Mandibles (Figs 2 I − K): left mandible (Fig. 2 J) with strong pars incisiva composed of 5 stout teeth, 2nd tooth largest; lacinia mobilis slightly smaller than pars incisiva, with 5 stout teeth; 4 robust serrate setae in row, and series of fine setae. Right mandible (Fig. 2 K) pars incisiva with 5 strong teeth, the two medial ones largest; lacinia mobilis finely dentate distally, with a large cut-off cusp medially, and a smaller one of similar build mediodistally; a single robust serrate seta and a series of delicate setae in row; pars molaris shallow, a rounded protuberance. Mandibular palp (Fig. 2 I) triarticulate, articles 1 and 2 broad, 2 broadest; length ratios of articles 1, 2 and 3, 1.0: 1.6: 1.1; article 1 unarmed; article 2 with 2 setae; article 3 with minute distal process and 3 subdistal setae.

Maxilla 1 (Fig. 3 A) inner plate small, rounded, weakly elongate, apparently unarmed; outer plate about twice longer than inner plate, with 7 robust serrate (4) or bifurcate (3) distal setae with swollen basis, a group of 8 smaller simple medial setae, and a row of 4 fine subdistal setae. Palp 2-articulate, projecting beyond outer plate, articles of equal length; article 1 unarmed; article 2 distally with 3 long and strong setae and a single short fine seta, medially with row of 4 fine subdistal setae.

Maxilla 2 (Fig. 3 B) with large outer plate and smaller inner plate; outer plate with 7 setae at distal margin, inner plate with 4 setae at medial corner of distal margin.

Maxilliped (Figs 3 C, D) inner and outer plate weakly developed; inner plate reaching basal 1/4−1/3 of palp article 1, with 2 (male) or 3 (female) short robust distal setae, a single stout subdistal seta, and a slender medial seta. Outer plate reaching middle of palp article 1, with long slender and 2 shorter robust distal setae, and stout long subdistal seta medially. Palp 4-articulate, articles broad; articles 1, 3 and 4 of similar length, article 2 ~1.5 × longer; article 1 with 1 medial seta in both sexes, and 1 lateral subdistal seta in female (Fig. 3 C) which is absent in male (Fig. 3 D); article 2 with 1 subdistal external seta and 4 subdistal setae medially; article 3 with pronounced, more or less triangular distal lobe with weakly serrate distal margin, 2 simple external setae laterally at the base, and an oblique longitudinal crest along medial margin with 3 robust serrate and 4 simple medial setae distally; article 4 with terminal claw, and 1 simple external seta basally, and 2 medial setae distally.

Labium (Fig. 2 H) lateral lobes well developed, rounded, closely spaced, distal margin with minute setae; inner lobes apparently present, shallow, unarmed; posterior expansions of lateral lobes diverging.

Gnathopod 1 (Fig. 3 E) basis with 1 subdistal seta posteriorly; ischium very short, rectangular with 1 bifid distal seta at posterior margin; merus with arched posterior margin bearing 3 subdistal setae (2 bifurcate, 1 simple); carpus triangular, short, length ~0.4 × length of propodus, with 2 distal and 2 subdistal bifurcate setae at posterior margin; propodus highest near proximal 1/3rd; palm lined with: a denticulate lamella (Fig. 7 C), 2 bifurcate robust setae and 2 long slender setae near proximal 1/3rd, a series of 5 submarginal curved setae in row set at more or less similar distance and extending from proximal 1/3rd towards apex, and a short stout subdistal seta; anterior margin with 2 setae distally and 2 at 1/3rd from apex; dactylus curved, anterior margin with 1 seta basally, posterior margin near mid-length with short stout spine, posterior to spine a single slender seta, anterior to it 3 stiff setae.

Gnathopod 2 (Fig. 3 F) similar in size and shape to gnathopod 1; differences concern: the basis which is somewhat more slender than in gnathopod 1, the ischium with 2 distal setae at posterior margin instead of 3, and the propodus with 2 long ventral bifurcate setae (occasionally distal one replaced by robust bifurcate seta) between proximal margin and palmar angle setae (absent in gnathopod 1).

Pereiopods (Figs 5 A −F) fairly short, of similar morphology, very poorly armed, pereiopods 3, 4 and 6 of similar length, pereiopod 5 shorter, pereiopod 7 longer, propodus increasing in length from pereopod 3 to 7. Pereiopods 3 and 4 almost identical; basis basally constricted, posterior margin with single median and subdistal seta, anterior margin with single subdistal and occasional single median seta; ischium with 1 posterodistal seta; merus with 1 anterodistal seta and 2 (pereiopod 3) or single (pereiopod 4) posterodistal setae; carpus with 2 subapical posteromarginal setae; propodus with 1 submedian posteromarginal seta, 2 stout unequal posteroapical setae, and 2 dorsoapical setae (one simple, one plumose); dactylus ~3/4 length of propodus, 1 subdistal simple posteromarginal seta, 1 plumose submedian anteromarginal seta. Pereiopod 5 with relatively shorter and broader basis; basis with 1 subdistal posteromarginal seta, 1 submedian anteromarginal seta and 2 anteroapical setae; ischium with 1 anteroapical seta; merus with 1 posteromarginal and 2 anteromarginal setae distally; carpus with 2 anterodistal setae of unequal length, 1 short, strong median posteromarginal articulated seta, 2 unequal bifurcate robust posteroapical setae on projection of posterior carpus margin; propodus with 1 submedian posteromarginal seta, 2 tiny anterodistal setae and 2 robust posterodistal setae of unequal length; dactylus ~1/2 length of propodus, 1 submedian anteromarginal plumose seta and 1 subdistal posteromarginal seta and some short tiny setae. Pereiopod 6 setation similar as in pereiopod 5, except for the ischium bearing 2 distal setae and the carpus lacking the strong median posteromarginal articulated seta; dactylus (Fig. 5 F) almost 1/2 length of propodus, setation similar to pereiopod 5. Pereiopod 7 setation similar as in pereiopod 6, except for the basis bearing 1 anterodistal seta and 3 posteromarginal setae; dactylus ~1/2 propodus length.

Coxal gills (Figs 4 C, 6C, D) on pereionites 2–7, paired; gills 2–6 ovate with fairly well demarcated basal stalk, extending beyond coxal plates; coxal gills 7 reduced (occasionally absent), more or less rounded-conical, basal stalk absent, completely covered by coxal plates.

Sternal gills (Figs 2 B, 6C, D) on pereionites 1–7, single, rounded-conical, basis demarcated by circular marks on medial sternal surface; gills 2–6 well developed, but 2 less high.

Pleopods 1–3 (Figs 4 H −M, 7D −G) of similar general morphology, uniramous, elongate consisting of a single gutter-shaped segment (pleopods 1–2), or two segments (pleopods 3) with long proximal segment and small distal segment bearing single terminal seta; pleopods 1–3 occasionally with tiny seta at anterior margin. Pleopod 3 sexually dimorphic: distal segment vestigial with very short tiny terminal seta in female (Fig. 4 J, 8A); distal segment short and distinct in male, terminal seta long, stout and kinked near mid-length (Figs 4 M, 7E).

Uropod 1 (Figs 5 G, I, 7F, G, 8A) biramous, sexually dimorphic. Peduncle conical, somewhat longer in female than in male, with long distal and medial seta at posterior margin; a smaller subdistal seta in male. Inner ramus of similar shape and length in both sexes, elongate conical, weakly curved, with subdistal cavity showing long seta. Outer ramus almost equally long in both sexes but differing in shape. Outer ramus of female (Figs 5 G, 8A), slightly shorter than inner one, with smoothly curved margins and subdistal cavity with long seta. Outer ramus of male (Figs 5 I, 7F, G), slightly shorter than inner one, with smoothly curved posteromedial margin, and undulate posterolateral margin with broad shallow lateral indentation near mid-length; inner surface with long slender seta slightly distal of basis and at c. 1/3 from basis, near mid-length a series of 3–4 setules, a short robust seta and a stout bifurcate seta; outer surface with short stout bifurcate seta near subdistal undulation.

Uropod 2 (Figs 5 H, J, 8A) biramous, sexually dimorphic. Peduncle weakly conical to cylindrical, about equally long in both sexes, with single long distal seta. Inner ramus of similar shape in both sexes, elongate conical, weakly curved, terminating in blunt tip with sub-distal cavity with short seta in female, or ending in a more or less acute tip with sub-distal small seta in male. Outer ramus of female (Figs 5 H, 8A) very short, c. 1/5 length of inner ramus, with subdistal seta. Outer ramus of male (Fig. 5 J) long, c. ¾ length of inner ramus; posterolateral margin undulate with broad, shallow indentation near mid-length; inner surface with a long slender seta slightly distal of basis and at c. 1/3 from basis, near mid-length a series of 3–4 setules and a short robust seta, and a stout bifurcate seta near mid-length; outer surface with stout bifurcate seta near subdistal undulation.

Uropod 3 (Figs 4 N, 5K, 8D) uniramous, similar in both sexes; peduncle slightly longer than broad, bearing 3 long distal setae and 1 short subdistal seta medially; exopodit composed of 2 tapering articles; basal article with pronounced external triangular distal lobe, flanked by 1 long distal external seta and 1 flagellate robust external distal seta, a second flagellate robust internal seta at ¾ from the basis; distal article shorter, conical, with 3 subdistal simple setae.

Telson (Figs 4 N −Q, 8C) of similar shape in female (Fig. 4 P) and male (Fig. 4 O), larger in female; uncleft, cordiform, slightly longer than wide; distally with mediodorsal triangular projection showing more or less undulate lateral margins, and terminating in a short spine usually accompanied by lateral subdistal spines (Figs 4 Q, 8B); ventrally a very shallow, broad median indentation; lateral lobes shorter than mediodorsal projection; laterally from mediodorsal projection 2 robust setae with long lateral branches; a pair of two dorsolateral plumose setae at 2/3 from basis, and 2 plumose setae at distal margin.

Cuticle: simple micropores with knob or more or less short denticle at the anterior edge are ubiquitous, and organized in straight to crescent small arrays forming more or less parallel rows (Figs 8 E, F); knobbed micropores predominate on the ventrolateral body surface, denticulate ones occur especially on the dorsolateral surface. A localized spiniferous field is present ventromedially at the distal end of the peduncle of uropods 3 (Fig. 8 F). Several types of setal structures are present which will be the object of another study by SEM.

Size: body length 1.61–3.50 mm. Females 1.64–3.50 mm, average 2.78 mm (N=34), males 1.61–3.21 mm, average 2.42 mm (N=22).

Distribution and biology. To date Pseudingolfiella possessionis n. sp. was only found in freshwater samples from Île de La Possession. It occurred in the extensive moss-beds along slow running brooklets, and very occasionally in the benthic layer. All samples have not been examined yet, but from the preliminary results it appears distributed all over the island. Basic physical and chemical information of the waters in which P. possessionis n. sp. was encountered is as follows: water temperature 7.7–13.9 °C, pH 6.5–7.4, conductivity 59–88 µS cm -1, total alkalinity titre 0.3–0.6 mmol L -1, oxygen 13.4–13.9 mg O2 L -1, Cl - 7.8–19.5 mg L -1, Na+ 4.550–6.579 mg L -1, K+ 0.485–0.837 mg L -1, Ca2+ 0.571–3.073 mg L -1, Mg2+ 1.154–1.667 mg L -1, hardness 0 mg CaCO3 L -1. Summarizing, the species was found in circum-neutral, well-oxygenated, weakly buffered and very soft waters with low mineralization. The accompanying moss fauna was rather rich and diverse comprising Nematoda, Rotifera, Cladocera, Copepoda, and the enigmatic Limnognathia maerski Kristensen & Funch, 2000 (Phylum Micrognathozoa).

Examination of the gut content revealed loricae and trophi of rotifers (Cephalodella, Lecane, Lepadella, Proales) as well as remains of copepods, indicating that P. possessionis n. sp. is carnivorous, and might be a predator or scavenger.

Reproduction is syngamic with males being common as shown by a sex ratio (♀♀/♂♂) of 1.5 in the population of the type locality.