Ischnomesus justi, Merrin & Poore, 2003

Merrin, K. L. & Poore, G. C. B., 2003, Four new species of Ischnomesidae (Crustacea: Isopoda: Asellota) from off south-eastern Australia, Memoirs of Museum Victoria 60 (2), pp. 285-307 : 296-300

publication ID	https://doi.org/ 10.24199/j.mmv.2003.60.27
persistent identifier	https://treatment.plazi.org/id/671587C8-3077-5515-FC94-722EFBDCC655
treatment provided by	Felipe
scientific name	Ischnomesus justi
status	sp. nov.

Treatment

Ischnomesus justi View in CoL sp. nov.

Figures 12–14 View Figure 12 View Figure 13 View Figure 14

Material examined. Holotype. Australia, NSW, 54 km ESE of Nowra (34˚52.72´S, 151˚15.04´E), 996– 990 m, mud, fine sand, fine shell, WHOI epibenthic sled, G.C.B. Poore et al., RV Franklin, 22 Oct 1988 (stn SLOPE 53 ), NMV J20306 About NMV (female, 5 mm).

Paratype. Tasmania, eastern Bass Strait, 87 km ENE of North Point, Flinders I. (39˚28.2´S, 148˚52.4´E), 841 m, muddy sand, naturalists’ dredge, G.C.B. Poore, HMAS Kimbla , 29 March 1979 (stn BSS 37 ), NMV J20309 About NMV (1 immature male, 5 mm) .

Other material. Tasmania, 48 km ENE of Cape Tourville (42˚00.25´S, 148˚43.55´E), 1264– 1130 m, gravel with lumps of sandy mud aggregate, WHOI epibenthic sled, G.C.B. Poore et al., RV Franklin, 30 Oct 1988 (stn SLOPE 81 ), NMV J20307 About NMV (1 male, 1 female). Eastern Bass Strait, 87 km ENE of North Point, Flinders I. (39˚28.2´S, 148˚52.4´E), 841 m, muddy sand, naturalists’ dredge, G.C.B. Poore, HMAS Kimbla , 29 Mar 1979 (stn BSS 37 ), NMV J20308 About NMV (1 female) . Paratype locality, NMV J40690 About NMV (1 fragment). Type locality, NMV J40689 About NMV (2 females, 1 fragment) .

Diagnosis: Body smooth, about 6 times as long as wide. Pereonite 1 bearing pair of stout anterolateral spines. Pereonites 2–4 anterolateral margins and pereonites 5–7 posterior margin rounded, lacking spines. Pereonites 4 and 5 elongate, about 2 and 3 times length of pereonite 2 respectively. Pleotelson paddle-like, proximally with wide proximal keel. Antenna 1 with 6 articles, article 2 with several long flagellate setae. Antenna 2, article 1 not fused to head; articles 2 and 3 with 1 long robust seta on distolateral margin. Mandible with palp. Maxilliped palp article 2 widest; epipod broad, rounded, with no setae. Pereopod 1 carpus inferior margin with only long pectinate and simple setae. Pleopod 3 exopod small, length less than half of endopod. Uropod of single article, inserted at about 90˚ in pleotelson with 4 simple setae.

Description of female holotype. Body length 5.8 times maximum width of pereonite 3. Body cuticle smooth, calcified and brittle, with few short simple setae scattered over pereonites. Head+pereonite 1 curved around in lateral view, with 2 short anterolateral spines. Ratio of lengths of pereonites 2: 3: 4: 5: 6: 7: pleonite 1: pleotelson, 1.0:1.3:2.2:2.9:0.9:0.8:0.3:1.6. Widths of pereonites 1 (including spines) to 3 equal; pereonites 4–7 widths decreasing (at widest part) posteriorly. Pleotelson long, paddle-like in dorsal view, length 1.3 times widest point; apex broadly rounded, with angular posterolateral corners.

Antenna 1 article 1 conical, bearing 1 short simple seta and 1 distal brush seta; article 2 elongate, 4.4 times as long as article 1, with 6 simple setae, 4 long flagellate setae and 1 brush seta; article 3 1.8 times length of article 1, with 2 distal simple setae; article 4 0.6 times length of article 1, with 3 simple setae; article 5 0.8 times length of article 1, with 1 simple seta; article 6 length equal to article 5, with 2 short simple setae and 4 simple setae of different lengths distally. Antenna 2 not complete; article 1 broad and short, with 1 simple seta; article 2 with longer lateral margin, 1.4 times length of article 1, with 1 simple seta and 1 distal lateral robust simple seta; article 3 elongate, 2.1 times length of article 1, with 2 simple setae and 1 distal lateral long robust seta; article 4 quadrangular, 1.5 times length of article 1, with 3 simple setae.

Mandible body with 1 simple seta near palp; incisor process with 5 cusps; lacinia mobilis with 4 cusps; spine row of 5 spines; molar rectangular, angled tightly towards spine row, with microtrichs and cusp on proximal margin; palp article 1 with 1 simple seta; article 2 1.7 times as long as article 1 with many microtrichs and 2 pectinate setae on inferior distal margin; article 3 0.5 times as long as article 1 with microtrichs on the inferior margin and terminating with 1 pectinate seta. Maxilla 1 with mesial lobe 0.6 times width of lateral lobe, with numerous fine simple setae on superior margin, and distally 2 simple setae, 1 pectinate seta and 1 dentate seta; lateral lobe with many fine simple setae on face and superior margin, distally 2 simple and 12 strong, dentate setae. Maxilla 2 lateral lobe wider than middle, with a few fine simple setae, 19 simple setae and distally, 1 long pectinate and 3 long simple setae; middle lobe 0.8 times width of lateral lobe, with 2 simple setae on inferior margin and distally, 1 long pectinate and 2 long simple setae; mesial lobe 2.0 times as wide as lateral lobe with many fine simple setae covering face, inferior margin with 7 simple setae, medially 2 long pectinate setae, and distally 3 small pectinate, 1 dentate and 13 simple setae. Maxilliped coxa large, rectangular; basis 1.7 times as long as widest point, curved over on inferior margin with fringe of fine simple setae, 2 simple setae and endite with 3 coupling hooks, and distally 3 fan and 7 simple setae. Palp tapering, 2.6 times length of basal endite; palp article 1 shortest, wide, rectangular, bearing 4 simple setae; article 2 wide, triangular, 0.9 times maximum width of basal endite, 3.0 times length of article 1, with 3 simple and 1 robust setae; article 3 same width as widest point of article 2, 2.1 times length of article 1, with 1 robust and 11 simple setae; article 4 narrower than article 3, 1.4 times length of article 1 with 6 simple setae; article 5 twice as long as wide, 1.8 times length of article 1, with 4 simple setae and 1 thick distal seta. Epipod with broad rounded tip, length 2.9 times width and 1.2 times basis length.

Pereopod 1 basis 0.2 body length; ratio of lengths of articles, basis to dactylus, 1.0:0.4:0.2:0.4:0.4:0.2; basis with 5 simple setae; ischium with 7 simple setae of various lengths, all distal; merus with 3 simple setae on distal superior margin and medially, at least 4 simple and 2 long pectinate setae; carpus width subequal throughout, with 3 long and 2 short simple setae on distal superior margin and inferior margin with at least 7 simple and 13 pectinate setae of different lengths, with small fringed scale-setae between; propodus width subequal throughout, superior margin with 11 long simple setae and inferior margin with 4 robust flagellate setae, at least 20 simple setae of various lengths and with small fringed scale-setae between on margin; dactylus with 8 short simple setae.

Pereopod 2 basis 0.2 body length; ratio of lengths of articles, basis to dactylus, 1.0:0.8:0.4:0.7:0.3:0.2; basis with 7 simple and 1 distal short flagellate seta; ischium with 5 simple setae and at least 5 short flagellate setae; merus with at least 9 simple setae of different lengths; carpus with at least 16 simple setae of various lengths, at least 3 robust flagellate setae, 1 of these on distal inferior margin and 1 brush seta on distal superior margin; propodus with 20 simple setae and 5 short robust flagellate setae on inferior margin; dactylus with 4 simple setae.

Operculum length 1.3 times maximum width; heart-shaped, margin with small simple setae, 4 slightly longer simple setae on posterior margin. Pleopod 3 exopod small and slender, 0.3 times endopod length; endopod elongate, rounded distal margin. Pleopod 4 elongate, length 2.7 times maximum width.

Uropod of 1 article, straight, 0.4 times length of pleotelson; sitting up in socket almost 90º degrees to body, with 4 simple setae.

Immature male. Pereopod 1 more slender than in female; ratio of lengths of articles, basis–dactylus, 1.0:0.3:0.2:0.5:0.4:0.2; basis with 6 simple setae; ischium with 3 long simple setae; merus with 2 long simple setae on distal superior margin and medially 2 long simple setae and 2 long pectinate setae; carpus width subequal throughout, with 2 long simple setae on distal superior margin, inferior margin with at least 20 pectinate setae of various lengths and small fringed scale-setae between; propodus with 17 simple setae of different lengths and inferior margin with 4 fringed scale-setae and 3 short, robust flagellate setae; dactylus with 5 simple setae.

Pleopod 1 length 2.6 times maximum width; completely fused, with no ornamentation, margin slightly compressed, widens towards distal end and curves inwards, indenting in the middle. Pleopod 2 sympod length 2.6 times maximum width, with 1 small simple seta; exopod rounded distally, 0.1 times as long as sympod; stylet, undifferentiated, fused with endopod, rounded distally. Pleopod 3 exopod short, length 0.4 times that of endopod; endopod distal margin coming to a wide point. Pleopod 4 elongate, length 2.0 times width.

Distribution. South-eastern Australia, from Nowra, New South Wales, to Cape Tourville, Tasmania; 841–1264 m depth.

Etymology. For Jean Just, distinguished isopod taxonomist, who sorted and made preliminary identifications of the material on which this study is based.

Remarks. Although the uropods of Ischnomesus justi have only one article, the species displays all the other diagnostic characters that characterise Ischnomesus . Ischnomesus justi is similar to I. calcificus Menzies and George, 1972 and I. simplex Menzies and George 1972 (both species from the Peru – Chile Trench) and I. roseus Wolff, 1962 (from the Gulf of Panama and Eastern Pacific Ocean, off Costa Rica). There are many characters that link these four: pereonites 2–4 rounded, with pereonite 1 having either small spines or tubercles; I. calcificus , I. justi and I. roseus all have a threearticled mandibular palp (not illustrated in I. simplex ) with two pectinate setae on the distal inferior margin; and pereopod 1 in both I. calcificus and I. justi has scale-setae on the inferior margin of the carpus and propodus. The uropods are undescribed for I. calcificus and I. roseus , and are of two articles in I. simplex . I. justi is distinguished from the others by the uropod having a single article, pereonites 1–4 being broadly rounded, and the pleotelson with only slight lateral bulges and an anterior keel.