Machatrium tropex (Bolstad and Kensley, 1999) Bolstad and Kensley, 1999

Bruce, Niel L. & Buxton, Chad, 2013, Review of the marine isopod crustacean genus Hansenium Serov and Wilson, 1995 (Asellota: Stenetriidae) from tropical Australia and Papua New Guinea, with description of a new genus, Zootaxa 3664 (4), pp. 445-478 : 474-477

Machatrium tropex (Bolstad and Kensley, 1999) , comb. nov.

(Figures, 19, 20)

Hansenium tropex Bolstad and Kensley, 1999:165 , fig. 1–2.

Material examined. 3 (4.0; juv. 2.5, 2.4 mm), 2 Ƥ, (ovig. 3.4, 3.1 mm), Madang, Papua New Guinea, barrier reef south of Wongat Island, 5.1411ºS, 145.8267ºE, 3 May 1989, coral rock on buttress, 21–25 m, coll. N.L. Bruce & M. Jebb (MTQ W34006).

Description of male. Body length 4.1 as greatest width; pereonite 1 medial width 2.6 length; pleotelson length 1.0 width.

Cephalon length 0.6 width; lateral margins acute, sparsely setose; antennal spines acute; eyes with 12–18 ommatidia, ommatidia orange, arranged in two parallel rows.

Antennula length 0.8 cephalon width; article 1 length 1.5 width; article 2 length 1.8 width; article 3 length 1.9 width; article 4 length 0.2 width; flagellum with 13 articles.

Mandible left spine row with 4 spines; palp article 1 distolateral margin with 2 long setae; article 2 with submarginal row of 6 short serrate setae. Maxillula lateral lobe apex with 9 serrate setae; mesial lobe apex with 2 serrate setae. Maxilla mesial lobe mesial margin with 5 serrate setae. Maxilliped basis length 2.5 width, width 1.0 endite width; endite mesiodistal margin with 4 serrate setae, distal margin with 5 fan setae; palp article 2 length 0.9 basis width; epipod length 3.7 width, width 1.0 basis width, apex acute, mesial margin with 7 regularly spaced setae.

Pereopod 1 basis length 3.5 width, submarginal mesial ridge with regularly spaced setae along length. Ischium length 1.8 width; superodistal margin not produced, concave, densely covered with long setae; mediodistal margin with 2 long setae. Merus angular, length 0.7 width, length 0.9 carpus length; inferior margin densely covered with setae; inferodistal margin not produced; superodistal margin slightly produced, apex acute, densely covered with long setae. Carpus length 1.0 width; distal margin convex, without short setae; carpal process long, length 3.8 width, extending two thirds to propodal palm articulation, straight, spatulate; inferior margin smooth, straight, densely setose, 1 row of stiff setae, 2 rows of long serrate setae; superior margin slightly convex, densely covered with setae. Propodus inferior half of article flattened, length 3.5 width; inferior margin length 0.7 superior margin length, densely covered with rows of long and serrate setae; superior margin sparsely setose, setae regularly spaced, restricted to distal half of margin, superior mesial surface densely covered with long, bi-denticulate setae; propodal palm parallel, margin with 2 medial teeth, row of long setae, row of short setae on each side of margin, cluster of setae at articulation, terminal end with single RS. Dactylus 3.7 times as long as basal width, length 1.0 propodal palm length, curved or robust, distal margin setose, setae regularly spaced along entire length, opposing margin with regularly spaced rows of setae and serrate setae, mesial surface lightly setose.

Pleopod 1 protopod length 0.9 width, lateral margins with 1 seta, posterior lobes without pair of RS, surface setae absent; rami lateral margins with regular setae along distal half of margin, apex with clustered setae, ventral surface with setae. Pleopod 2 protopod length 3.5 width; lateral margin with 1–2 setae; endopod proximal segment length 0.5 protopod length, without setae; appendix masculina length 1.5 endopod proximal length, 0.8 protopod length, lateral margin without distal groove, apex concave, depression fringed with setae, interior pocket with setae present, 3 anteriorly projecting distolateral spines, 3 clusters of short setae distal to spine, mesial margin without setae. Pleopod 3 endopod apex with 4 plumose setae. Pleopod 4 exopod apex with 7 plumose setae. Pleopod 5 apex with 4 plumose setae.

Female. Pereopod 1 basis length 3.6 times as long as wide, mesial submarginal ridge with regular short setae; ischium superodistal process developed, acute; merus superodistal process developed, acute; carpus inferodistal process rounded; propodus 3.4 times as long as wide, with one row of setae and one row of serrate setae; propodal palm length 1.6 times propodus proximal width, 4 teeth along margin, row of serrate setae present.

Pleopod 2 length 1.9 width; lateral margins with 2–5 setae, anterior half of margins straight and tapering; apex pointed, a seta on each lobe; ventral surface sparsely setose.

Size. Males 4.0 mm; females ovig. 3.1–3.4; Bolstad and Kensley (1999) recorded maximums of male at 4.8 mm and female ovig. at 5.0 mm.

Coloration. This species has little pigmentation. The cephalon has a faint pigment band across the cephalon anterior margin extending to the eyes and partially circling the eye lateral margin. The pereonites have a few pinpoint spots of pigmentation and the pleotelson is without colour.

Remarks. Machatrium tropex is characterised by the male pereopod 1 having a spatulate carpal blade with straight margins that extend nearly the entire length of the propodal inferior margin; the propodus is elongate, 3.5 times as long as wide, and the palm has two teeth. The female pleopod 2 ( Fig 20 F) differs slightly to that figured by Bolstad and Kensley’s (2002, fig. 1F) in terms of setation.

Distribution. Madang, Papua New Guinea; at depths of 3 to 4 metres.

Figures

FIGURE 20. Hansenium tropex comb. nov. All male except (MTQ W 34002). A – G, male; A, pereopod 1; B, enlargement of dactylus, pereopod 1; C, pleopod 1; D, pleopod 2; E, pleopod 3; F, pleopod 4; G, pleopod 5; H, female pleopod 2.