(Figs 11, 12)

Type material. Holotype female, 2.5 mm, AM P38468, Blue Lagoon bommie, Lizard Island (14°40’S 145°28’E), light trap 1 m below surface in 12–15 m depth, M. Milicich & J.K. Lowry, 7 February 1987 (QLD 2014). Paratypes: 1 male, 2.0 mm, AM P77845, same locality; 35 females 2.0– 3.2 mm, AM P77846 same locality.

Type locality. Blue Lagoon, Lizard Island, Queensland, Australia.

Additional material examined. Many males (more than 200), AM P38467 same locality as types.

Etymology. Named in reference to its diminutive size.

Description. Based on holotype female.

Head. Rostrum constricted; eyes present (red pigmented in preserved specimens). Antenna 1 peduncle article 2 with ventral setae situated proximally. Antenna 2 peduncle article 4 facial robust setae not in a single row, dorsal setae present; article 5 midfacial robust setae present as a single set of 2; flagellum multiarticulate. Mandible palp article 2 without outer setae; molar simple, form D; right molar with 8 spines, without disjunct spines; right incisor with 3 teeth, right lacinia mobilis simple, right raker row of 5–6 teeth. Maxilla 1 inner plate with 4 setae; outer plate without an especially thickened robust seta. Maxilliped inner plate with 1 large thin-thick apical robust seta; outer plates, inner margin with well developed setae.

Pereon. Coxae, long ventral setae present on coxae 1–3 but absent from coxa 4, main ventral setae of coxae 1–4 = 6–7–8–0. Coxa 1 subrectangular. Gnathopods similar. Gnathopod 1 palms oblique. Pereopods 3– 4, carpus lacking posteroproximal robust setae, propodus slender setae absent. Pereopod 5 basis of broad form, not tapering, slightly widened distally, without facial ridge. Pereopod 6 basis with 1 facial ridge. Pereopod 7 basis with few or no long ventral setae, with 1 facial ridge.

Pleon. Epimeron 1 anteroventral margin with or without setae, posteroventral face with 0–2 long setae, not set vertically. Epimeron 2, with 5–9 facial setae, not crowded anteriorly. Epimeron 3 with fully facial setae arranged in oblique row, posterior setae absent, with 2–4 ventral setae. Uropod 1, peduncle basofacial setae present, without major displaced robust setae, with 3–4 apicolateral spines; inner ramus of uropod 1 with 1 dorsal spine, without subapical accessory robust seta; outer rami not continuously spinose to apex. Uropods 1–2 rami with well developed apical nails. Uropod 3 outer ramus article 2 with 2 apical setae.

Habitat. Collected in light traps 1 m below surface in water 12–15m deep.

Remarks. This new species of Birubius can be distinguished from all other species of the genus by the following combination of traits; constricted rostrum; coxa 1 not expanded distally (sub rectangular); pereopods 3–4 carpus without posteroproximal robust setae; pereopod 5 basis is slightly widened distally and the arrangement of facial setae on epimeron 3. The males of the species collected are very similar in appearance to the females, except that they are slightly smaller in size, possess the usual sexual dimorphic characters and have more setae on epimeron 1.

Distribution. Australia. Queensland: Lizard Island (current study).