Akanthophoreus nanopsenos Bamber & Bird, sp. nov.

Akanthophoreus nanopsenos Bamber & Bird, sp. nov.

Figures 3–4

Material examined. Holotype: 1 female (NHM.2008.4759), station RFI 1, off the coast of Israel, 31º53.640’N 034º33.069’E, 57 m depth, fine mud; 14 September 2006. Allotype: 1 male, (slide-mount, NHM.2008.4760), station RFI 7, 31º48.809’N 034º28.892’E, 62 m depth; 0 3 May 2007. Paratypes: 4 females (NHM.2008.4761–4767), 3 females, 1 neuter, dissected, same sample as holotype; 1 female (N H M.2 0 0 8. 4 7 6 8), s t a t i o n R F I 2, 3 1º 5 3. 3 2 8’ N 0 3 4º 3 2. 8 5 3 ’E, 5 8 m d e p t h; 2 f e m a l e s (NHM.2008.4769–4770), station RFI 3, 31º53.842’N 034º33.300’E, 57 m depth; 1 female (NHM.2008.4771), station RFI 4, 31º54.193’N 034º33.241’E, 59 m depth; 1 female (NHM.2008.4772), station RFI 5, 31º53.633’N 034º33.251’E, 55 m depth; 1 female (NHM.2008.4773), station RFI 7, 31º48.809’N 034º28.892’E, 62 m depth; all 14 September, 2006. 14 specimens (TAU AR 28415), RFI 2; 2 specimens (TAU AR. 28416), RFI 3; 5 females (TAU AR 28417), RFI 4; 11 females (TAU AR 28418), RFI 7; all 0 3 May 2007. 22 females (NHM.2008.4774–4783), RFI 1; 63 specimens (NHM.2008.4784–4793), RFI 2; 52 specimens (TAU AR 28419), RFI 3; 33 specimens (TAU AR 28420), RFI 5; 87 specimens (NHM.2008.4794–4803), RFI 7; all 20 October 2007. All fine mud.

Other material. 80 specimens, RFI 4; 20 October 2007. Fine mud.

Diagnosis: very small Akanthophoreus, female 8 times as long as wide, without long setae on basis and endite of maxilliped; articles of antenna peduncle and pereopods short, chela robust, truncate. Male with compact antennular flagellum segments, stout chela, dactylus and unguis of the anterior pereopods short, those of the posterior pereopods long; uropod endopod 3-segmented.

Description of female: body (Figure 3 A) elongate, slender, 1.2 mm long, 8.2 times as long as wide. Cephalothorax subrectangular, tapering towards anterior, 1.2 times as long as wide, longer than any pereonite, with slight rounded rostrum, naked; eyes absent. Six free cylindrical pereonites; pereonites 1 and 5 subequal in length and 0.7 times as long as cephalothorax, pereonites 2 and 3 longer, pereonite 4 longest, pereonite 6 shortest (all pereonites respectively 1.3, 1.1, 1.1, 0.9, 1.1 and 1.4 times as wide as long). Pleon of five free subequal pleonites bearing pleopods, each pleonite with one midlateral seta on each side. Pleotelson (Figure 3 I) pentagonal, short, 3 times as long as pleonite 5, 1.5 times as wide as long.

Antennule (Figure 3 B) of four articles, proximal article 2.5 times as long as wide, with two outer setae; second article 1.3 times as long as wide, about half length of first, with one inner distal and two outer setae, one longer than article width; third article two-thirds length of second with simple distal inner seta and two outer penicillate setae; distal article slender, just longer than third article, with five distal setae and single aesthetasc.

Antenna (Figure 3 C) of six articles, articles 1 to 3 subequal, compact, proximal article naked, second and third articles with single dorsodistal setae; fourth article longest, 1.4 times as long as articles 1 to 3 together, with ventrodistal seta; fifth article 0.4 times as long as fourth, with two distal setae; sixth article (flagellum) minute with three distal setae.

Labrum (Figure 3 G) apically rounded, finely setose. Left mandible (Figure 3 E) with slender, simple lacinia mobilis, pars incisiva crenulate; pars molaris slender, tapering, with few distal spinules; right mandible as left but without lacinia mobilis. Labium (Figure 3 D) bilobate, simple. Maxillule (Figure 3 F) with nine distal spines, palp short with two distal setae. Maxilla not seen. Maxilliped (Figure 3 H) endites naked, with slightly crenulated distal margin; basis naked; palp first article naked, second and third articles with three inner and one outer distal setae; fourth article with two inner, two distal and one outer marginal setae, one inner submarginal seta.

Cheliped (Figure 3 K) basis slender, 2.25 times as long as wide; carpus 1.2 times as long as wide with paired, short ventral setae and no shield; propodus longer than wide, fixed finger short with lamellate cutting edge, two ventral and three inner setae; dactylus with outer margin smooth, cutting edge naked.

Pereopod 1 (Figure 3 L), coxa simple with seta; basis arcuate, three times as long as wide; ischium compact with single seta; merus as long as carpus, propodus 1.3 times as long as carpus; merus, carpus and propodus each with single ventrodistal spine, carpus with dorsodistal spine; dactylus with distinct, slender unguis, both together as long as propodus. Pereopod 2 (Figure 3 M) similar to pereopod 1, but carpus with three distal spines, propodus with additional subdistal ventral spine. Pereopod 3 as pereopod 2.

Pereopod 4 (Figure 3 N) basis slender, 3.5 times as long as wide; merus with ventrodistal seta and spine; propodus with outer-distal and one dorsodistal and two ventrodistal spines, all shorter than dactylus; dactylus with slight ventral groove, unguis short. Pereopod 5 as pereopod 4. Pereopod 6 (Figure 3 O) as pereopod 4, but propodus with three dorsodistal spines.

Pleopods (Figure 3 J) all alike, with naked basis, endopod and exopod both with proximal plumose seta on ventral margin well-separated from remaining seven distal plumose setae, all setae about as long as rami.

Uropod (Figure 3 I) basis naked, rami slender; exopod of two segments, almost as long as proximal endopod segment, one distal seta of exopod much shorter than other; endopod of two subequal segments, setose as figured.

Description of male: body (Figure 4 A) more compact and smaller than female, allotype 0.7 mm long, 4.7 times as long as wide. Cephalothorax proportionately shorter, 0.85 times as long as wide. All pereonites much wider than long, pereonites 1 to 3 subequal, very short, five times as wide as long; pereonites 4 and 5 subequal, 1.4 times as long as pereonite 3, pereonite 6 longest, 1.6 times as long as pereonite 3. Pleonites subequal, as long as, or slightly longer than, pereonite 6. Pleotelson elongate, about 1.5 times as long as last pleonites.

Antennule (Figure 4 B) compact. Proximal peduncle article 1.7 times as long as wide; second article 0.6 times as long as first and with one outer seta; third article short, one-third as long as second article, with two ventrodistal setae. Flagellum of four segments, first segment very short, bearing about ten aesthetascs, second to fourth articles progressively longer, each longer than wide, bearing ten, two and one aesthetascs respectively, fourth article with three distal setae.

Antenna (Figure 4 C) similar to that of female, but proximal peduncle article half length of second article, second and third articles twice as long as wide.

Mouthparts atrophied; only maxilliped palp seen.

Cheliped (Figure 4 D, E) slightly stouter than that of female, chela more slender, dactylus proportionately much longer and overreaching terminal spine of fixed finger; inner face of propodus with comb of ten spines

Pereopod 1 (Figure 4 F) similar to that of female, but merus, carpus and propodus more slender, unguis proportionately shorter. Pereopods 2 and 3 similar to pereopod 1. Pereopods 4 (Figure 4 G), 5, and 6 (Figure 4 H) again similar to but much more attenuate than those of female, but dactylus plus unguis much longer and more slender, three or more times as long as propodus.

Pleopods (Figure 4 I) all similar, similar to but more slender than those of female, detached proximal seta much closer to seven distal setae, all setae plumose and more than twice as long as rami.

Uropod (Figure 4 J) endopod three-segmented, first segment naked, second segment with one inner-distal seta, third segment with four distal and one sub-distal setae; exopod two-segmented, second segment with two unequal distal setae.

Etymology: from the Greek nanos — a dwarf, and psenos —bald (alluding to the lack of certain setae normally characteristic of the genus).

Remarks. The clarification of the various taxa once incorporated within the old “dustbin” genus Leptognathia sensu lato, in terms of both generic and familial affiliations, is proving complex and is far from resolution as yet. The present species resembles most closely species of Akanthophoreus, in terms of its mandibular, antennal, pereopodal, pleopodal and uropodal morphology, but departs from the most recent diagnosis of that genus (Bird, 2007) in the lack of setae on the basis and endite of the maxilliped; these setae are present in A. longiremis (Lilljeborg, 1864), the only other species of Akanthophoreus recorded previously from the Mediterranean. A. nanopsenos sp. nov. also differs from A. longiremis in its more truncate chela, in the proportions of the antennular and antennal articles, and in having less slender pereopods, as well as being a much smaller animal.

Males of this family are rare and infrequently recorded; indeed, the recorded sex-ratios in collections of species of Akanthophoreus (also Araphura, et al.) where males are found are of the order of hundreds to one (e.g. Holdich & Jones, 1983), and in most species males are unknown. That only one male of A. nanopsenos was found in the present material in a total of 385 individuals is therefore typical. With its small size and the similar morphology of antennae, pereopods, uropods (other than features of sexual dimorphism), there can be little doubt that the male is of the same species as the eleven females found in the same sample, and the only species of the genus or subfamily recorded in the area.

The present male shows many similarities to that of A. gracilis (Krøyer, 1842) (e.g. Sars, 1899, pl.12, as A. longiremis), including the morphology of the antenna and the three-segmented uropod endopod. However, the chela is stouter than that of A. longiremis, the antennular flagellum segments more compact, and the dactylus and unguis of the anterior pereopods proportionately shorter while those of the posterior pereopods are proportionately longer.

Akanthophoreus nanopsenos sp. nov. was only found in the finer mud sediments of the RFI stations, at between 55 and 62 m depth. It is one of the smallest species of the genus known: its discovery probably reflects the merit of using 0.25 mm mesh sieving for these diminutive peracarids.