Tethygonium monocuspis, Just & Wilson, 2021

Just, Jean & Wilson, George D. F., 2021, Redescriptions and new species in the ‘ Austrosignum-Munnogonium’ complex sensu Just & Wilson (2007), mainly from the Southern Hemisphere (Crustacea Isopoda: Paramunnidae), Zootaxa 4952 (3), pp. 401-447 : 441-444

Tethygonium monocuspis sp. nov.

( Figs 26–27 )

Type fixation. Holotype, ♀, AM P60463.—Here designated.

Type locality. Victoria, south-east Australia.

Etymology. The species is named for the single lateral spine on pereionite 2.

Material examined. Holotype. ♀, 0.9 mm, Australia, Victoria, east of Seal Islands , 38 o 59.1’S, 148 o 31.6’E, collected with benthic sled, stn SS05/94/60, AM P60463.

Paratypes. Australia, New South Wales, off Nowra , 34 o 59.52’S, 151 o 5.94’E, 204 m, coarse shell, WHOI epibenthic sled, GCB Poore et al., CSIRO RV Franklin, 14 July 1986, stn SLOPE 1, NMV J20116 , (1 late ♂ manca, pereiopod VII and pleopod I not fully developed) . Australia, New South Wales, off Nowra , 34 o 59.52’S, 151 o 5.94’E, 204 m, coarse shell, WHOI epibenthic sled, Poore, GCB et al., CSIRO RV Franklin, 14 July 1986, stn SLOPE 1, NMV J20117 , (1 ♀) .

Description (female). Body width 0.50 length, widest at pereionite 2. Head length 0.53 width; length posterior to eyestalks 1.2 anterior length. Frontal margin convex with weak rounded mid projection, with blunt angular lateral margins adjacent to antennae. Eyestalks slightly longer than broad, reaching to about middle of pereionite 1 anterior margin, apex rounded, long axis angling forward at approximately 30°, (ocelli not observed).

Pereionites 1 and 3–7 lateral margin rounded, lateral margin of pereionite 2 angular with forward projecting spine; coxae visible in dorsal view on pereionites 1 and 5–7 (1–7 in ♂). Pereionite 1 sagittal length 2.5 pereionite midline length.

Pleon length 1.2 width. Pleonite 1 width equal to distance between uropods, length 0.2 width. Pleotelson laterally convex, lacking inflection between lateral and proximal margins, with 9–10 small denticles per side; posterior projection, set apart from lateral margins by concavity at level of uropods, forming 70° angle with broadly rounded setiferous apex.

Antennula article 1 reaching much beyond eye lobes and pereionites1, article 2 about 0.75 length of 1, 1 and 2 of equal width, tubular; articles 5 and 6 elongate, of subequal length, each as long as combined 3 and 4. Antenna article 3 tubular to slightly widening to medio-apical projection, width approximately 0.35 length, article 4 with distolateral spine, article 5 0.85 length of 6; flagellum with 6 successively shorter articles.

Pereiopod I basis smooth, length 3.5 width; ischium smooth; merus with a few simple setae; carpus oval, posterior margin with 3 subequal, forward curved robust setae; propodus narrowing distally to insertion of dactylus; dactylus with strong posterior claw, half length of unguis, and marginal spine proximal to posterior claw. Pereiopod II basis with single anterior spine.

Female operculum width 0.73 length; distal part tapering with weakly concave distolateral margins, forming 70° angle, apex broadly rounded.

Uropods dorsal and adjacent to lateral margin of pleotelson; small protopod visible.

Size. Largest female, 0.85 mm.

Distribution. Eastern Australia, New South Wales to Victoria.

Remarks. The immature male is generally like the female, except all coxae are visible in dorsal view. Tethygonium monocuspis sp. nov. and its only Australian congener T. quadricuspis Just and Wilson, 2007 (Bass Strait, Australia) are the only species in the genus with lateral pereionite spines. They differ primarily in that the latter has lateral spines on pereionites 1–4 rather than pereionite 2 only, obsolete eyestalks, pereiopod I basis with a single and ischium with 2 stout spine.

Figures

FIGURE 26. Tethygonium monocuspis sp. nov. Holotype, ♀, except M, paratype, late ♂ manca. a, antenna; au, antennula; cvv, head ventral view; p1–2, pereiopods I and II; up, uropods. Scale for habitus (both), 0.5 mm.

FIGURE 27. Tethygonium monocuspis sp. nov. Holotype, ♀, except M, paratype, late ♂ manca. dv, dorsal view; md l, left mandible; op, operculum; pl1, pleopod 1; plt, pleotelson (oblique view in M); vv, ventral view.