On the Atlantic species of the genus Abyssianira Menzies, 1956 (Isopoda: Asellota: Paramunnidae) Author Doti, Brenda Lía Author Roccatagliata, Daniel text Zootaxa 2006 1252 1 35 journal article 50583 10.5281/zenodo.173029 31f40c8c-9e4b-401d-a751-5fb0e6ee1abb 11755326 173029 Abyssianira lingula n. sp. ( Figs 6–9 ) Material examined Holotype : Stn. WHOI 169A, off Recife, 08º03´S , 34º23´W , 587 m , 21 Feb 1967 , brooding Ψ (AM P68698). Paratypes : Stn. WHOI 167, off Recife, 07º58´S , 34º17´W , 1007 m , 20 Feb 1967 , 1 brooding Ψ (AM P68695), 1 brooding Ψ (AM P72663), 5 ΨΨ (3 of them brooding) and 2 ♂ ɗ (AM P72664); Stn. WHOI 159, off Recife, 07º58´S , 34º22´W , 834 m , 18 Feb 1967 , 1 brooding Ψ and 1 ɗ (AM P68694). Diagnosis Frontal plate tonguelike, distinctly extended beyond article 1 of antennulae. Eyestalks short and distally rounded. Pereon with 4 groups of wartlike elevations. Coxal plates visible in dorsal view on pereonites 5–7 only. Description of brooding female (Body description based on the holotype AM P68698, appendages on paratype AM P68695) Length : 1.47 mm ( Fig. 6 A, B). Body width ~ 0.5 length, widest at pereonite 3. Head length ~ 0.76 width. Frontal plate tonguelike, extending far beyond article 1 of antennulae, slightly concave in dorsal view, length ~ 0.8 width, anterior margin with very large denticles (many broken or folded, Figs 6 A, C; 9A, C). Eyestalks short and distally rounded, longitudinal axis pointing forward at ~ 45º, lateral margin with small denticles (many broken). Pereonites 1, 2, 6 and 7 with a pair of dorsolateral wartlike elevations; those on pereonite 1 and 6 minute; “warts” arrangement as in A. acutilobi n. sp. Coxal plates visible in dorsal view on pereonites 5–7 only. Free pleonite ~ 0.8 length of pereonite 7. Pleotelson width ~ 0.8 length, margins strongly denticulate. Antennula ( Fig. 7 A) article 1 with 2 simple setae; article 2 longest, with 3 BS and 1 simple seta; article 3 ~ twice longer than broad, with 2 simple setae; article 4 ~ 0.6 length of article 3, with 1 BS and 1 simple seta; article 5 glabrous; article 6 with 1 aesthetasc,1 BS and 4 simple setae. FIGURE 6 . Abyssianira lingula n. sp. Holotype brooding female (AM P68698): A, B, habitus in dorsal and lateral views; C, ventral view of the head. Scales = 0.5 mm (B), 0.2 mm (C). A and B share the same scale. FIGURE 7 . Abyssianira lingula n. sp. Paratype brooding female (AM P68695): A, antennula (first article broken); B, antenna (first four articles separated from the remaining ones); C, mandible; D, maxilliped and detail of distal end of its endite (fan setae removed). Scales = 0.1 mm (A, B), 0.05 mm (C, D). FIGURE 8 . Abyssianira lingula n. sp. Paratype brooding female (AM P68695): A–C, pereopods I, II, and VI; D, pleotelson in ventral view; E, uropod. Scales = 0.1 mm (B, E), 0.2 mm (D). A–C share the same scale. FIGURE 9 . Abyssianira lingula n. sp. SEM photographs of a paratype brooding female (AM P72663): A, habitus in dorsal view; B, magnified frame in A showing six setalike shafts emerging from sockets (pointed by arrows) and pores; C, habitus in lateral view. Scales = 0.2 mm (A, C), 0.02 mm (B). Antenna ( Fig. 7 B) article 1 broken off; articles 2 ~ 0.7 length of article 3 and approximately as long as article 4; article 3 slightly longer than wide, but much shorter than fifth; article 5 with 4 simple setae; article 6 ~ 1.19 length of article 5, with 2 BS and 5 simple setae. Flagellum shorter than peduncle, with 7 articles. Left mandible ( Fig. 7 C) incisor process with 5 cusps, lacinia mobilis with 4 cusps, spine row with 3 spines, molar process with teeth and 2 simple setae; palp slightly shorter than body of mandible: article 1 as long as article 2, with 1 simple seta; article 2 with cuticular combs and 2 pectinate setae; article 3 smallest, ~ 0.43 length of article 1, with cuticular combs and 3 setae (at least 2 pectinate). Right mandible as left except for: lacinia mobilis absent, spine row with 4 spines. Maxillula and maxilla similar to those described for A. acutilobi n. sp. Maxilliped ( Fig. 7 D) endite with 2 coupling hooks; distal margin with 3 plumose and 2 pectinate setae; ventral surface with 2 fan setae distally; dorsal surface with 2 pectinate, 1 plumose, and several small simple setae distally. Epipod elongate, ~ 0.8 palp length, width/length ratio ~ 0.5. Palp ~ 0.84 maxilliped axial length, articles 2 (and 3) narrower than endite. Pereopod I ( Fig. 8 A) basis longest article, with 3 simple setae and apparently few translucent denticles. Ischium ~ 0.7 basis length, with 4 simple setae. Merus cupshaped, slightly longer than wide, dorsal margin with 2 simple setae, ventral margin with 1 RS and 3 simple setae. Carpus slightly longer than ischium, dorsodistal corner with 1 simple seta, ventral margin with 2 RS and 3 simple setae. Propodus ~ 0.9 ischium length, dorsal margin with 4 simple setae (1 half length along article and 3 distally), ventral margin with 2 RS and 2 simple setae, anterior surface with 1 simple seta. Dactylus with 4 simple setae near distal end and 2 simple setae between unguis and supplementary claw, unguis ~ 0.9 dactylus length, supplementary claw ~ 0.7 unguis length. Pereopod II ( Fig. 8 B) basis with 3 simple setae. Ischium ~ 0.9 basis length, with 3 simple setae. Merus cupshaped, dorsodistal corner with 2 simple setae, ventral margin with 4 simple setae. Carpus as long as basis, dorsal margin with 1 simple seta half length along article, and 1 BS and 2 simple setae distally; ventral margin with 4 simple setae (2 distal and 2 half length along article). Propodus ~ 0.8 ischium length, with 1 RS, 1 BS and 2 simple setae on distal end, and 3 simple setae half length along article. Dactylus simple setae as in pereopod I; unguis approximately as long as dactylus, supplementary claw ~ 0.4 unguis length. Pereopod VI ( Fig. 8 C) proportions as pereopod II except for: ischium and propodus subequal in length. Chaetotaxy, main differences with pereopod II as follows: basis with 1 BS, propodus with 2 RS and fewer simple setae. Operculum ( Fig. 8 D) subtriangular, width ~ 0.8 length, lateral margins with simple setae, ventral surface with 2 simple setae on distal third. Uropod ( Fig. 8 E) protopod wider than long; endopod with 2 simple setae subapically and 5 BS distally; exopod ~ 0.5 endopod length, with 1 simple seta distally. Distribution Found only from off Recife between 587–1007 m ( Fig. 2 ).

Etymology
From the	Latin	lingula	that	means	small	tongue,	referring	to	the	tonguelike	frontal
plate.

Remarks This species is easily recognizable because of its long and thin tonguelike frontal plate, which extends far beyond the basal article of the antennulae. Abyssianira dentifrons Menzies, 1956 ( Figs 10–13 ) Abyssianira dentifrons Menzies, 1956 : 14 , fig. 6. — Menzies, 1962 : 79 , fig. 61B–H. — Wolff, 1962 : 63 , 265, 301. — Menzies et al. , 1973 : 105 , 125. — Wilson, 1980 : 219 , fig. 2A, B. — Just, 1990 : 403 , 405, 406, 411. — Boyko, 1997 : 4 . Material examined Holotype : Stn. L.G. O . Biotrawl 1, Vema 71, north of the Puerto Rico Trench, 20º3.2´N , 68º21.1´W , 5104–5122 m ( 4983–5001 m after Boyko, 1997 ), 12 Nov 1955 , adult ɗ ( AMNH 11762). Additional material: Stn. WHOI 175, Gay Head– Bermuda transect, 36º36´N , 68º29´W , 4693 m , 29 Nov 1967 , 1 Ψ and 1 juvenile (AM P68696); Stn. WHOI 247A, Argentine Basin, 43º33´S , 48º58´06´´W , 5223 m , 17 Mar 1971 , 1 ɗ (AM P68700) and 4 ΨΨ (2 of them brooding) (AM P72668). Diagnosis Anterior margin of frontal plate slightly convex. Eyestalks short and flat, distally rounded. Antennular article 3 elongate, approximately as long as article 1. Mandibular palp distinctly shorter than body of mandible. Pereon with 4 groups of wartlike elevations. Coxal plates visible in dorsal view on all pereonites. Antennula, antenna and pereopods with setulate setae (SS). Remarks on the holotype (adult male) The holotype is damaged and partially dissected. Only three pereopods are available, two of which were found loose in the vial, and the remaining one (the left pereopod I) was still attached to the specimen. The right mandible, pleopods I, the left pleopods II and III, and the pleotelson were also loose in the vial. The antennulae, right antenna, mouthparts (all except right mandible), and right pleopods II–V were not dissected to keep the specimen as entire as possible. Menzies (1956 , 1962 ) failed to include full illustrations of the pereopods (he only showed the distal articles of the first and fourth pereopods). In order to complete the original description some additional features of the holotype are herein presented. FIGURE 10 . Abyssianira dentifrons Menzies, 1956 . Adult male (AM P68700): A, B, dorsal and ventral views of the head. Holotype adult male (AMNH 11762): C, D, dorsal and lateral views of the anterior part of the body; E, antennula. Scales = 0.2 mm (B, E), 0.5 mm (C). A, B and C, D share the same scale. FIGURE 11 . Abyssianira dentifrons Menzies, 1956 . Holotype adult male (AMNH 11762): A, pereopod I; B, C, two unnumbered pereopods; D, pleotelson; E, uropod; F, pleopod I. Scales = 0.1 mm (A), 0.2 mm (B, C, F), 0.5 mm (D), 0.05 mm (E). FIGURE 12 . Abyssianira dentifrons Menzies, 1956 . Adult male (AM P68700): A, antennula and detail of the first article rotated showing the lateral denticles; B, antenna (first article missing); C, mandible; D, maxilliped. Scales = 0.2 mm (A, B), 0.05 mm (C), 0.1 mm (D). FIGURE 13 . Abyssianira dentifrons Menzies, 1956 . Adult male (AM P68700): A, pereopod I and detail of ventral margin of carpus; B, pereopod II; C, D, pleopods I and II. Scales = 0.1 mm (A), 0.2 mm (B–D). Description of the holotype Length : 2.75 mm (after Menzies, 1956 ). Anterior margin of frontal plate slightly convex ( Fig. 10 C, D). Eyestalks short and flat, distally rounded. Dorsolateral wartlike elevations as in A. acutilobi n. sp. except for “warts” on pereonites 2 and 6 smaller than those on pereonites 1 and 7. Menzies (1956 , 1962 ) did not mention the pereonal wartlike elevations; the four anterior ones are herein depicted in Fig. 10 C, D. Coxal plates visible in dorsal view on all pereonites (as shown in Menzies’ 1956 fig. 6A). Pleotelson width ~ 0.84 length, margins denticulate. Antennula ( Fig. 10 E). As far as can be seen without dissecting: article 1 with 3 SS and 1 row of denticles; article 2 longest, with 3 BS and 2 SS; article 3 elongate, with 2 SS; article 4 shortest, with 1 BS; article 5 glabrous; article 6 with 1 aesthetasc, 1 BS and 5 SS. Right mandible , incisor process with 5 cusps, spine row with 5 spines (not 3 as shown in Menzies’ 1956 fig. 6F), molar process with teeth and 3 setae; palp distinctly shorter than body of mandible (as shown in Menzies’ 1956 fig. 6F); article 1 ~ 0.9 length of article 2, with 1 simple seta; article 2 with cuticular combs and 1 pectinate seta; article 3 smallest, ~ 0.5 length of article 1, with cuticular comb and 2 pectinate setae. Pereopod I ( Fig. 11 A) basis longest article (dislocated), with 3 SS. Ischium ~ 0.9 basis length, with 2 SS. Merus cupshaped, dorsodistal corner with 2 SS, ventral margin with 1 RS and 3 SS. Carpus ~ 0.7 ischium length, dorsal margin with 1 SS, ventral margin with 2 RS, 2 SS and cuticular combs. Propodus ~ 0.57 ischium length, dorsal margin with 1 SS, ventral margin with 2 RS, 4 SS and cuticular combs, anterior surface and ventral margin with cuticular combs. Dactylus with 4 setae near distal end and 2 thick setae between unguis and supplementary claw; unguis ~ 0.7 dactylus length, supplementary claw ~ 0.66 unguis length. Pereopod (?) ( Fig. 11 B) basis missing. Ischium with 4 SS. Merus cupshaped, with 5 SS. Carpus ~ 0.75 ischium length, dorsal margin with 1 SS half length along article, and 2 SS and 1 BS distally; ventral margin with 5 SS. Propodus almost as long as ischium, dorsal margin chaetotaxy as carpus, ventral margin with 9 SS. Dactylus, chaetotaxy as in pereopod I, unguis ~ 0.5 dactylus length, supplementary claw minute. Pereopod (?) ( Fig. 11 C) similar to previous pereopod except for: ischiumpropodus with 1 or 2 less setae, and dactylus without supplementary claw. Pleopod I ( Fig. 11 F) maximum width ~ 0.53 length; lateral lobes at ~ 0.65 of its length from proximal end, each one with 4 simple setae; distal projection with 6 simple setae (not 4 as shown in Menzies’ 1956 fig. 6G); ventral surface with 2 simple setae. Pleopod II , proximal margin of protopod broken in dorsal view (depicted as a straight margin in Menzies’ 1956 fig. 6C), but intact and rounded in ventral view; protopod maximum width ~ 0.37 length (measured from ventral surface), with 8 simple setae on lateral margin. Endopod surpassing protopod. Exopod bilobed. Uropod ( Fig. 11 E) protopod wider than long; endopod with 2 subapical simple setae and 6 distal BS (not 5 BS as shown in Menzies’ 1956 fig. 6I ); exopod ~ 0.5 endopod length, with 2 distal simple setae. Description of adult male from the Argentine Basin (Based on specimen AM P68700) Length : 3.2 mm (the specimen is damage; this measure results from combining the pereon and pleotelson lengths). Body width ~ 0.39 length, widest at pereonite 4. Head length ~ 0.6 width. Frontal plate length ~ 0.61 width, slightly concave in dorsal view, anterior margin slightly convex, with denticles (many teeth broken, Fig. 10 A, B). Eyestalks flat and distally rounded, marginally denticulate, longitudinal axis pointing forward at ~ 35º. Lateral margins of some pereonites with small denticles. Dorsolateral wartlike elevations as in A. acutilobi n. sp. except for “warts” on pereonites 2 and 6 smaller than those on pereonites 1 and 7. Coxal plates visible in all pereonites. Antennula ( Fig. 12 A) article 1 with 1 BS, 5 SS and 1 row of denticles (see detail); article 2 longest, with 3 BS and 3 SS; article 3 elongate, ~ 5 times longer than broad, with 3 SS; article 4 ~ 0.4 length of article 3, with 1 BS; article 5 glabrous; article 6 with 1 aesthetasc, 1 BS and 5 SS. Antenna ( Fig. 12 B) article 1 missing; article 2 broken off; articles 3 and 4 subequal in length, article 3 with 2 marginal denticles; article 4 with 2 SS; article 5 with 2 BS and 3 SS; article 6 ~ 1.28 length of article 5, with 5 BS and 8 SS. Flagellum shorter than peduncle, with 8 articles. Right mandible ( Fig. 12 C) incisor process with 5 cusps, spine row with 5 spines, molar process with several teeth and 4 setae; palp distinctly shorter than body of mandible: article 1 ~ 0.9 length of article 2, with 1 simple seta; article 2 with cuticular combs; article 3 smallest, ~ 0.31 length of article 1, with cuticular comb and 2 pectinate setae. Left mandible with 4cusped lacinia mobilis and molar process with 3 setae, other features as in right mandible. Maxillula as in A. acutilobi n. sp. Maxilla , mesial lobe with 5 simple setae (with a porebearing tip) on distal margin, other aspects as in A. acutilobi n. sp. Maxilliped ( Fig. 12 D) endite with 2 coupling hooks, chaetotaxy similar to those of A. acutilobi n. sp. Epipod elongate, ~ 0.94 palp length, width/length ratio ~ 0.5. Palp ~ 0.89 maxilliped axial length, articles 2 (and 3) narrower than endite. Pereopod I ( Fig. 13 A) basis longest article, with 7 SS on anterior surface and some small lateral teeth. Ischium ~ 0.8 basis length, with 4 SS. Merus cupshaped, dorsodistal corner with 2 SS, ventral margin with 1 RS and 5 SS. Carpus ~ 0.85 ischium length, dorsal margin with 1 SS, ventral margin with 3 RS, 5 SS and cuticular combs. Propodus ~ 0.65 ischium length, dorsal margin with 4 SS (1 at half length along article and 3 distal), ventral margin with 2 RS, 4 SS and cuticular combs. Dactylus with 4 setae near distal end and 2 thick setae between unguis and supplementary claw; unguis ~ 0.6 dactylus length, supplementary claw ~ 0.5 unguis length. Pereopod II ( Fig. 13 B) basis longest article, with 3 SS and many small teeth on both surfaces. Ischium second longest article with 5 SS. Merus cupshaped, dorsodistal corner with 2 SS, ventral margin with 5 SS. Carpus ~ 0.9 ischium length, dorsal margin with 1 SS half length along article, dorsodistal corner with 1 BS and 2 SS, ventral margin with many SS. Propodus ~ 0.8 ischium length, chaetotaxy as carpus. Dactylus setae as in pereopod I, unguis ~ 0.5 dactylus length, supplementary claw minute. Pereopods III–VII missing or remaining bases only. Pleopod I ( Fig. 13 C) maximum width ~ 0.45 length; lateral lobes at ~ 0.67 of its length from proximal end, each one with 6 simple setae; lateral margins with 6 simple setae, distal projection with 12 simple setae. Pleopod II ( Fig. 13 D) protopod maximum width ~ 0.37 length, with 23 simple setae on lateral margin. Endopod surpassing protopod. Exopod bilobed. Comparison between the holotype and specimens from other localities The holotype of A. dentifrons and the five specimens from the Argentine Basin herein studied are badly damaged, while the two specimens examined from the Gay Head– Bermuda transect (Stn. WHOI 175) are in better shape. Regardless of their state of preservation, the “warts” were observed in all these specimens after staining them. Besides, all these specimens have a long antennular third article and setulate setae on their pereopods, a type of seta that is absent in the other Atlantic species of Abyssianira . In addition, the mandibular palp is short both in the holotype and in the Argentine Basin material. This character was not confirmed for the Gay Head– Bermuda transect material to avoid damaging these specimens. The male herein described from the Argentine Basin is larger than the holotype ( 3.2 mm vs. 2.75 mm ), has eyestalk directed somewhat more laterally, a pleotelson less produced, and appendages with a more profuse setation. In his fig. 6A, Menzies (1956) illustrated the holotype (the same drawing but enlarged is reproduced in Menzies’ 1962 fig. 61B). Besides, in his fig. 2A, B, Wilson (1980) showed the habitus and the pleotelson of one specimen collected by the Woods Hole Oceanographic Institution (Stn. WHOI 121, Gay Head– Bermuda transect, 35º49.8´N , 65º10.8´W , 4800 m ; G.D.F. Wilson, pers. comm.). The eyestalks point forward in Menzies’ 1956 fig. 6A but are slightly more lateral in Wilson’s 1980 fig. 2B, in the five specimens from the Argentine Basin ( Fig. 10 A) and in the 2 specimens from the Gay Head– Bermuda transect (Stn. WHOI 175). The head of the holotype is flattened and dislocated (see Fig. 10 D); hence it is not possible to confirm if the angle at which the eyestalks project shown in Menzies’ 1956 fig. 6A is accurate. In Wilson’s 1980 fig. 2B and in the five specimens from the Argentine Basin, the distal part of the pleotelson is slightly less produced than in the holotype . In contrast, in the two specimens from the Gay Head– Bermuda transect (Stn. WHOI 175) herein examined the pleotelson is quite similar to that of the holotype (redrawn by us in Fig. 11 D). The specimens from the Argentine Basin herein studied agree in all points with the diagnosis of A. dentifrons reported above. Although some minor differences have been observed between the holotype and the male herein described, we believe that these variations are not enough to erect a new species. However, since abyssal isopods have a limited power of dispersal, the broad range of distribution of this species (recorded from both the North and South Atlantic) is suspicious. Further individuals, especially males and females from the type locality ( Puerto Rico Trench), are needed to solve this problem. Distribution Recorded from the abyssal plain in both North and South Atlantic ( Fig. 2 ). Originally described from the north of the Puerto Rico Trench in 5104–5122 m ( Menzies, 1956 ). Later on recorded from off the Argentine and SW African coasts, between 4588–5293 m ( Menzies, 1962 ); and from off the North Carolina coast, at 4800 m depth (G.D.F. Wilson, pers. comm.). Some additional specimens, tentatively identified as A. dentifrons , are herein recorded from off North Carolina and Argentina again, at 4693 m and 5223 m depth, respectively. Remarks A. dentifrons is most similar to the Australian species A. bathyalis and A. tasmaniensis but it can be easily distinguished from them by its elongate antennular article 3, which is about 5 times longer than broad. In addition, A. dentifrons has 4 groups of wartlike elevations on the pereon that are apparently absent in A. bathyalis and A. tasmaniensis . These “warts” are hardly noticeable without dyeing the specimen and therefore, they could have been overlooked by Just (1990) in these two Australian species. Abyssianira argentenensis Menzies, 1962 ( Figs 14–19 ) Abyssianira argentenensis Menzies, 1962 : 79 , fig. 61I –L. — Just, 1990 : 403 , 406, 411, 414. — Boyko, 1997 : 4 . Abyssianira argentinensis (incorrect spelling). — Wolff, 1962 : 63 , 265. Material examined Holotype : Stn. L.G. O . Biotrawl 207, Ve ma 15123, northeast of the Malvinas Islands; 50º23.2´S , 47º25.0´W ; 2681 m ; 25 Mar 1959 , 1 Ψ ( AMNH 12134). Additional material: Stn. R1, R/V Aldebarán 200302, off the Río de la Plata estuary, 36º46.9´S , 54º15.2´W to 36º46.6´S , 54º14.7´W , 129 m , 6 Nov 2003 , coll. F. Scarabino, 3 ɗɗ and 1 Ψ (MACNIn 36564), 1 ɗ (MACNIn 36564a), 1 ɗ (MACNIn 36564b), 1 brooding Ψ (MACNIn 36564c), 1 brooding Ψ (MACNIn 36564d), and 1 brooding Ψ and 1 ɗ (MUNHINA 1961); ARA Alférez Sobral , Beagle Channel (Cabo San Pío), 55º03.0´S , 66º37.0´W , 70–80 m , 27 Jan 2003 , coll. D.G. Zelaya, 9 brooding ΨΨ, 5 ♂ ɗ and 4 juveniles (MACNIn 36565). FIGURE 14 . Abyssianira argentenensis Menzies, 1962 . Adult male (MACNIn 36564a): A, B habitus in dorsal and lateral views; C, ventral view of the head. Scales = 0.5 mm (A), 0.2 mm (B). A and B share the same scale. FIGURE 15 . Abyssianira argentenensis Menzies, 1962 . Adult male (MACNIn 36564b): A, antennula; B, detail of the medial margin of article 1 of antennula of another male; C, antenna; D, mandible; E, maxilliped and detail of distal end of its endite (fan setae removed). Scales = 0.1 mm (A, C), 0.05 mm (D, E).

Diagnosis
Frontal plate thin and	convex	in dorsal	view,	gradually slanting forward but not
produced into a horizontal	lamina.	Eyestalks	long,	subcylindrical and distally rounded.

Coxal plates visible in dorsal view on pereonites 1 and 5–7. Coxal plates on pereonite 1 subtriangular and pointing forward, much larger in males than in females. . Abyssianira argentenensis Menzies, 1962 . Adult male (MACNIn 36564b): A, pereopod I and detail of distoventral margin of propodus; B, C pereopods II and VII. Scale = 0.1 mm (A, B). B and C share the same scale. FIGURE 17 . Abyssianira argentenensis Menzies, 1962 . Adult male (MACNIn 36564b): A, ventral view of the pleotelson; B, pleopod II; F, uropod. Brooding female (MACNIn 36564c): C E, pleopods IIIV. Scales = 0.2 mm (A), 0.1 mm (B–F). A, B and D, E share the same scale. Descriptive notes on the type material. Length : 1.5 mm (after Menzies, 1962 ) The holotype is badly damaged; its head is broken and dislocated, and the pleotelson distorted. The antennulae, the left pereopod VI and both pereopods VII are intact. In contrast, of the antennae and the remaining pereopods only basal articles remain. In his fig. 61L Menzies (1962) showed denticles on the frontal margin of the head, the first article of the antennulae and the distal end of the pleotelson. The frontal margin teeth are barely visible (they appear to be broken or folded), whereas those on the antennulae and the end of the pleotelson were not observed. However, since teeth are fragile, the fact that these are missing nowadays in the holotype is not unexpected. FIGURE 18 . Abyssianira argentenensis Menzies, 1962 . Holotype brooding female (AMNH 12134): A, habitus in dorsal view. Brooding female (MACNIn 36564d): B, habitus in dorsal view; C, habitus in lateral view; D, ventral view of pleotelson. Scales = 0.5 mm (A, B), 0.2 mm (D). B and C share the same scale. FIGURE 19 . Abyssianira argentenensis Menzies, 1962 . SEM photographs. Adult male. A, habitus in lateral view. Brooding female: B, habitus in lateral view; C, head in dorsal view; D, anterior margin of frontal plate magnified. Scales = 0.2 mm (A, B), 0.1 mm (C), 0.05 mm (D). Description of adult male (Body description based on specimen MACNIn 36564a, appendages on specimen MACNIn 36564b) Length : 1.43 mm (remaining specimens partially or totally rolled up preventing their measurement, Fig. 19 A). Body width ~ 0.38 length, widest at pereonites 1–3 ( Fig. 14 A). Head length ~ 0.72 width. Frontal plate length ~ 0.72 width, thin and convex in dorsal view, gradually sloping forward but not produced into a horizontal lamina ( Fig. 14 B), anterior margin with denticles ( Fig. 14 C; several rows of overlapped denticles were observed under SEM in a brooding female, see Fig. 19 C, D). Eyestalk prominent, subcylindrical and apically rounded, longitudinal axis pointing forward at ~ 40º, some specimens with dense material inside (light sensory pigment?). Pereonite 1 largest, approximately as long as pereonites 3 and 4 combined. Pereonites 2–4 subequal in length. Pereonites 5 and 6 smallest, and 7 approximately as long as pereonite 3 (or 4). Coxal plates visible in dorsal view on pereonites 1 and 5–7; those of pereonite 1 large, subtriangular and pointing forward, with 1 simple seta and marginal denticles. Free pleonite approximately as long as pereonite 7. Pleotelson length ~ 0.97 width, lateral margins strongly convex and denticulate (11–13 teeth per side). Antennula ( Fig. 15 A) article 1 longest, with 1 BS and 2 simple setae, distomedial margin with denticles (not observed in specimen drawn, see Fig. 15 B); article 2 ~ 0.8 length of article 1, with 3 BS and 4 simple setae; article 3 approximately twice longer than broad, with 2 simple setae; article 4 ~ 0.8 length of article 3, with 1 BS; article 5 glabrous; article 6 with 1 aesthetasc and 3 simple setae. Antenna ( Fig. 15 C) article 1 broken off; article 3 slightly longer than articles 2 or 4; articles 2–4 each with 2 simple setae; article 5 with 2 BS and 3 simple setae; article 6 ~ 1.32 length of article 5, with 3 BS and 10 simple setae. Flagellum shorter than peduncle, with 10 articles. Right mandible ( Fig. 15 D) incisor process with 5 cusps, spine row with 4 spines, molar process with teeth and 2 simple setae; palp as long as body of mandible: article 1 ~ 0.74 length of article 2, with 1 simple seta; article 2 with cuticular comb and 2 pectinate setae; article 3 smallest, ~ 0.63 length of article 1, with cuticular comb and 3 setae (at least 2 pectinate). Left mandible as right except for: spine row with 3 spines and lacinia mobilis with 4 cusps. Maxillula , lateral lobe with 9 spinelike setae (most of them serrate) and 1 simple setae on distal margin, other features as in A. acutilobi n. sp. Maxilla similar to that described for A. acutilobi n. sp. Maxilliped ( Fig. 15 E) endite with 2 coupling hooks, distal margin with 5 pectinate setae; ventral surface with 2 fan setae distally; dorsal surface with 1 pectinate, 1 plumose, and several small simple setae distally. Epipod ovate, ~ 0.6 palp length, width/length ratio ~ 0.7. Palp ~ 1.21 maxilliped axial length; articles 2 (and 3) broad, subequal in width to endite. Pereopod I (Fig. 16A) basis approximately as long as ischium, with 1 RS and 2 simple setae. Ischium with crenate ridge on posterodorsal margin and 2 simple setae. Merus wider than long, with 1 RS and 5 simple setae. Carpus slightly shorter than ischium, dorsal margin with 1 distal simple seta, ventral margin with 2 RS, 4 simple setae and 1 translucent denticulate lamella, anterior surface with cuticular combs. Propodus dorsal margin with 4 simple setae (1 half length along article and 3 distally), ventral margin with 1 RS, 5 simple setae and 1 translucent denticle (see detail), anterior surface with cuticular combs (for the sake of clarity carpus and propodus combs are depicted in full line although they are on the anterior surface). Dactylus with 4 setae near distal end and 2 simple setae between unguis and supplementary claw; unguis approximately as long as dactylus, supplementary claw ~ 0.6 unguis length. Pereopod II (Fig. 16B) basis longest article, with 4 simple setae. Ischium ~ 0.8 basis length, with 4 simple setae. Merus cupshaped, with 6 simple setae. Carpus approximately as long as ischium, dorsodistal corner with 1 BS and 2 simple setae, ventral margin with 1 RS and 5 simple setae. Propodus ~ 0.8 ischium length, dorsal margin chaetotaxy as carpus, ventral margin with 1 RS and 4 simple setae. Dactylus, simple setae as in pereopod I, unguis ~ 0.8 dactylus length, supplementary claw ~ 0.45 unguis length. Pereopod VII (Fig. 16C) ischium subequal to basis; carpus ~ 0.8 and propodus ~ 0.9 ischium length, respectively. Chaetotaxy, main differences with pereopod II as follow: basis with 2 BS; carpus and propodus with several simple setae replaced by RS. Pleopod I ( Fig. 17 A) maximum width ~ 0.54 length; lateral lobes at ~ 0.66 of its length from proximal end, each one with 4 simple setae; lateral margins with 3–4 simple setae; distal projection with 6 setae; ventral surface with 5 setae. Pleopod II ( Fig. 17 B) protopod maximum width ~ 0.44 length, with 6 or 7 simple setae on lateral margin and 4 minute denticles on medial margin. Endopod surpassing protopod. Exopod bilobed and with 3 minute setae at apex. Pleopods III–V as in female (see below). Uropod ( Fig. 17 F) protopod approximately as long as wide; endopod with 3 subapical simple setae and 6 distal BS; exopod ~ 0.6 endopod length, with 2 distal simple setae. Description of brooding female (Based on specimen MACNIn 36564d) As adult male except for: Length : 1.6 mm (for the female depicted in Fig. 18 B, C; other females are partially or totally rolled up preventing their measurement, Fig. 19 B). Body width ~ 0.46 length, widest at pereonite 3 ( Fig. 18 B). Head length ~ 0.36 width; frontal plate shorter than in male, length ~ 0.5 width. Eyestalks, longitudinal axis pointing forward at ~ 12º. Pereonites 1, 3 and 4 subequal in length medially. Coxal plates of pereonite 1 much smaller than in male. Pleotelson length ~ 1.23 width, lateral margins less convex than in male. Pleopod III ( Fig. 17 C) exopod articles not completely separated, distal article with 1 simple seta; endopod width ~ 0.44 length, with 3 plumose setae distally. Pleopod IV ( Fig. 17 D) endopod width ~ 0.47 length, exopod digitiform not surpassing proximal third of endopod. Pleopod V ( Fig. 17 E) shorter than pleopod IV, width ~ 0.6 length. Operculum ( Fig. 18 D) ovoid and pointed distally, width ~ 0.7 length, lateral margins with 9 or 10 simple setae, ventral surface with 6 simple setae. Comparison between the holotype and specimens from other localities Menzies (1962) briefly described A. argentenensis based on a single female, illustrating its habitus, antennula, first pereopod and uropod. In dorsal view, the females herein described show two small subtriangular coxal plates on the first pereonite. These plates were neither observed in the holotype in dorsal view (herein redrawn in Fig. 18 A) nor depicted by Menzies (1962) in his fig. 61L. Nevertheless, if the holotype is slightly rotated, the coxal plate from the right side becomes visible. Since this specimen is badly damaged, however, it is not possible to ascertain if the first coxal plates were visible in dorsal view when the holotype was intact. In addition, Menzies (1962) illustrated the distal end of the pleotelson slightly projecting beyond the uropods. This is an inaccurate observation: when the holotype specimen is laid in a completely horizontal position, the projection is well developed (see the redrawing of the holotype in Fig. 18 A). The holotype and the additional specimens herein studied are subequal in size, except for one brooding female from the Beagle Channel which is remarkably larger (since most of the specimens are rolled up, it is not possible to accurately measure the difference in sizes). Distribution The original record was from off Malvinas Islands at 2681 m . It is now recorded from two shallow waters localities: the eastern mouth of the Beagle Channel at 70–80 m and off the Río de la Plata estuary in 129 m ( Fig. 2 ). Remarks Abyssianira argentenensis is easily distinguished because its frontal plate gradually slants forward and is convex in dorsal view. In contrast, the frontal plates of the remaining species of the genus project forward into a flat (or slightly concave) horizontal lamina. The pereonite 1 and the first coxal plates are much larger in the males than in the females. No other sexual dimorphism was observed. Pereopods I are identical in shape and chaetotaxy in specimens of both sexes and from both localities studied (Río de la Plata estuary and Beagle Channel populations). None of the specimens of A. argentenensis examined under dissecting microscope shows wartlike elevations. However, under SEM a single rough rounded area was observed in the same spot where the other three Atlantic species present a pair of anterior wartlike elevations (see Fig. 19 B).