Ischnomesus tasmanensis, 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 : 291-295

publication ID	https://doi.org/ 10.24199/j.mmv.2003.60.27
persistent identifier	https://treatment.plazi.org/id/671587C8-307A-5511-FF36-71EFFEDCC75A
treatment provided by	Felipe
scientific name	Ischnomesus tasmanensis
status	sp. nov.

Treatment

Ischnomesus tasmanensis View in CoL sp. nov.

Figures 7–11 View Figure 7 View Figure 8 View Figure 9 View Figure 10 View Figure 11

Material examined. Holotype. Australia, Vic., 76 km S of Point Hicks (38˚29.33´S, 149˚19.98´E), 1840– 1750 m, sandy mud, fine shell, WHOI epibenthic sled, G.C.B. Poore et al., RV Franklin, 26 Oct 1988 (stn SLOPE 69 ), NMV J20280 View Materials (immature male, 7 mm).

Paratypes. Australia, type locality, NMV J40684 View Materials (1 male, 7 mm; 1 female, 8 mm) .

Other material. Australia, Vic., 67 km S of Point Hicks (38˚23.95´S, 149˚17.02´E), 1277– 1119 m, fine mud, WHOI epibenthic sled, G.C.B. Poore et al., RV Franklin, 25 Oct 1988 (stn SLOPE 67 ), NMV J20279 View Materials (2 fragments). Type locality, NMV J40685 View Materials (4 fragments) .

Diagnosis. Body covered in short setae, about 6 times as long as wide. Pereonite 1 with pair of 2 frontal-facing, long anterolateral spines and 2 stout dorsal spines. Pereonites 2–4 each with pair of stout dorsal spines; pereonites 2–4 each with pair of anterolateral spines; pereonites 5–7 each with pair of posterolateral spines. Pereonites 4 and 5, about 4 and 6 times the length of pereonite 2 respectively, pereonite 5 with 7 pairs of protrusions extending from each lateral margin, most terminating with long, thick simple setae. Pleotelson longer than broad, lateral margins with 3 large spines. Antenna 1 of 6 articles, with long simple setae present on articles 2 and 3. Antenna 2 article 1 fused to head, with 3 spines; articles 2–4 with numerous simple setae. Mandible without palp. Maxilliped palp article 2 widest; epipod with 3 simple setae. Pereopod 1 carpus with 2 long robust setae and several robust flagellate setae on inferior margin. Pleopod 3 exopod over half epipod length, superior margin with numerous fine simple setae and 9 simple setae. Uropod elongate, with article 2 1.4 length of article 1, both articles with many simple setae of different lengths, terminating with 3 long simple setae.

Description of immature male holotype. Body length 5.8 times maximum width of pereonite 3 (excluding spines). Body cuticle calcified and brittle, with numerous scattered simple long setae. Head+pereonite 1 length 0.8 times maximum width (excluding spines), with 2 long anterolateral spines that do not reach beyond front of head and 2 stout dorsal spines set posteriorly on pereonite. Although fused, head differentiated from pereonite 1, maximum width 0.7 times width of pereonite 1 (excluding spines), with pair of dorsal tubercles. Ratio of lengths of pereonites 2: 3: 4: 5: 6: 7: pleonite 1: pleotelson, 1.0:1.0:4.2:5.5:1.3:1.0:0.3:3.2. Pereonite 1 widest, pereonites 2–4 width subequal at widest point, pereonites 5–7 decreasing in width posteriorly Pereonites 2–4 with 2 anterolateral and 2 stout dorsal spines. Pereonites 5–7 with 2 posterolateral spines. Pereonite 5 with 7 pairs of long, thick, simple setae extending from small projections of cuticle. Pleotelson length 1.2 times width (excluding spines), elliptical, with 3 pairs of posterior-facing lateral spines extending from margin; posterior margin dorsal surface with small projection each side of median large projection; smaller projections are insertion point for uropod; ventral surface with triangular medial extension, not extending past margin.

Antenna 1 not complete, article 1 short, rectangular, distal end rounded, with 3 simple setae; article 2 elongate, 5.3 times as long as article 1, bearing 16 simple setae of different lengths and distally, 1 brush seta; article 3 2.9 times as long as article 1, with 12 simple setae of different lengths; article 4 1.0 times as long as article 1, with 1 simple seta and distally, 1 brush seta. Antenna 2 not complete, article 1 fused to head with 3 robust setae and 1 simple seta; article 2 with 11 simple setae; article 3 0.5 times as long as article 2, with 2 simple setae; article 4, damaged, portion present with 40 simple setae.

Mandible incisor process and lacinia mobilis each with 4 cusps; spine row of 7 spines; molar wide, rectangular, with row of 6 medial simple setae. Maxilla 1, both lobes with numerous fine simple setae on both margins; mesial lobe 0.7 times width of lateral lobe; with 1 short dentate and 7 simple distal setae; lateral lobe with 8 simple setae on distolateral margin, 12 distal, strong, dentate setae and few proximal microtrichs. Maxilla 2 lateral lobe 1.2 times width of middle, with many rows of microtrichs on superior margin and 6 simple setae on inferior margin; middle lobe with few microtrichs; both lateral and middle lobes with 4 long distal pectinate setae of different lengths; mesial lobe 2.4 times width of lateral lobe, with numerous fine simple setae and microtrichs on surface, 5 simple setae and 2 long pectinate setae medially set on inferior margin, and distally, 3 dentate setae, 2 long robust simple setae, 2 small pectinate and 11 simple setae of various lengths. Maxilliped coxa small, rectangular; basis length 2.1 as long as wide at widest point, endite with 3 coupling hooks, distally with 6 simple setae, 6 pectinate setae and 3 fan setae, flap with margin of fine simple setae. Palp narrowing, 3.2 times length of basal endite; article 1 squat and rectangular, with 1 simple setae on superior margin; article 2 1.4 times as wide as basal endite at maximum width, 1.6 times wider and 2.3 times as long as article 1, with 1 simple setae on superior and 4 simple setae inferior margin; article 3 width equal with article 2 at widest point, 2.0 times as long as article 1, with 1 simple seta on superior and 6 simple setae on inferior margin; article 4 1.7 times as long as article 1, much narrower than article 3, with 2 long simple setae on distal inferior margin; article 5 length equal to 4, with 2 simple setae and terminating with 2 robust simple setae; posterior face of palp, article 1 with 1 additional simple setae, article 2 with 2, article 3 with 2, article 4 with 2 and no additional setae on article 5. Epipod elongate, length 2.8 times width, 1.3 times basis length, with 3 long simple setae and microtrichs on superior margin.

Pereopod 1 basis 0.1 body length; ratio of lengths of articles, basis to dactylus, 1.0:0.7:0.3:0.8:0.7:0.4; basis with 7 simple setae, all except 1 on inferior margin; ischium with 8 simple setae of various lengths; merus with 2 long simple and 1 short robust setae on distal superior margin and 2 simple and 2 robust setae of different lengths on inferior margin; carpus wider proximally, superior margin with 2 long and 1 short simple setae and inferior margin with 1 short simple seta, 1 short and 2 long robust setae and 7 flagellate robust setae; propodus width subequal throughout, with 9 long simple setae on superior margin and inferior margin with 6 simple setae of different lengths interspersed between 7 flagellate robust setae; dactylus with 4 simple setae in inferior margin and clump of 4 simple setae on distal superior margin

Pereopod 2 basis 0.2 body length; ratio of lengths of articles, basis to merus (carpus–dactylus missing) same as in pereopod 1; basis with 7 simple setae and 1 medial brush seta; ischium with 11 simple setae; merus with 6 simple setae and 1 distal flagellate robust seta; part of carpus intact, with 1 simple seta and 5 robust flagellate setae.

Pereopod 3 basis 0.2 body length; ratio of lengths of articles, basis to dactylus, 1.0:0.8:0.4:1.0:0.5:0.3; basis with 17 simple setae, and superior margin also with 2 fork and 1 brush setae; ischium with 13 simple setae; merus with 6 simple setae; carpus with at least 8 simple setae and 13 flagellate robust setae, 3 of these on distal margin; propodus with at least 11 simple setae and inferior margin with 6 long and 5 short alternating flagellate robust setae on inferior margin; dactylus with 4 small simple setae on distal superior margin.

Pereopods 4–7 missing.

Pleopod 1 length 3.5 times proximal width; lateral margins each with 4 simple setae; ridge overlapping dorsal lobes at 0.9 length, with 6 simple setae on left and 7 on right; small distolateral horns extend from dorsal lobes. Penes triangular, 0.2 length of pleopod 1. Pleopod 2 sympod elongate, length 2.6 times maximum width, with 12 long, distal, simple setae; exopod short, roughly oval, length 0.2 times sympod length; stylet 0.8 length of sympod undeveloped. Pleopod 3 exopod 0.9 times endopod length with fine simple setae on both margins and 9 simple setae on superior margin. Pleopod 4 slightly elongate, length 1.8 times width.

Uropod (from male paratype) of 2 articles, elongate and slender; article 1 with at least 11 long simple setae; article 2 length 1.4 times article 1, narrower, with 21 simple setae and 3 long simple setae distally.

Female. Pereopod 1 thicker and more setose than in male; ratio of lengths of articles, basis to dactylus, 1.0:0.5:0.7:0.6:0.6:0.3; basis with 15 simple setae; ischium with 10 simple setae of different lengths; merus with 7 simple setae, 2 short robust setae and 2 long robust setae; carpus similar in shape but more robust than in male, with at least 6 simple setae on superior margin, inferior margin with 9 robust flagellate setae, 2 short robust setae, 2 long robust setae and at least 5 simple setae; propodus with 21 simple setae of different lengths on superior margin and 3 simple setae on surface, inferior margin with 11 robust flagellate setae and 4 long simple setae; dactylus similar to that in male although with 5 simple setae on inferior margin and clump of 3 on distal superior margin.

Distribution. South-eastern Australia, eastern Bass Strait; 1119–1840 m depth.

Etymology. For the Tasman Sea.

Remarks. This is the second species of Ischnomesus reported from the Tasman Sea off south-eastern Australia. The first, I. anacanthas Wolff, 1962 , was described from only the anterior half of one specimen. The first four pereonites of I. anacanthus are totally devoid of both lateral and dorsal spines. Of other species, I. tasmanensis most resembles I. birsteini Wolff, 1962 (the anterior half of the type specimen was too damaged for an accurate description) and I. spaercki Wolff, 1956 , both from the Kermadec Trench. All three species have long simple setae on lateral projections of the cuticle on pereonite 5, the pleotelson with posterior-facing lateral spines extending from margin and the second uropod article terminated with long simple setae. However, Ischnomesus tasmanensis can be distinguished from those two species by its pleotelson, which has three lateral spines on each margin and lacks spines on the dorsal surface. It can be further distinguished from I. spaercki by a pair of short dorsal spines on each of the first four pereonites as opposed to I. spaercki having a pair of dorsal spines on pereonite 1, 4 dorsal spines on pereonites 2 and 3, and short dorsal spines on pereonites 6 and 7.