Gnathia acrorudus Svavarsson & Bruce 2019, sp. nov.

Gnathia acrorudus sp. nov.

(Figs 5–8)

Material examined. Holotype. ♂ (2.6 mm), Heron Island, ‘ Blue Pools’, 23.4354°S 151.9224°E, in coral rock on boulder zone, 17 January 1979, coll. N.L. Bruce (MTQ W34138).

Paratypes. 5 ♂ (2.6 [dissected + 3 microslides], 2.5 [dissected + 1 microslide], 2.4, 2.2 and 1.7 mm), Heron Island, ‘ Blue Pools’, 23.4354°S, 151.9224°E, 17 January 1979, in coral rock, coll. N.L. Bruce (MTQ W34139 [dissections], W34140). 2 ♂ (2.1, 2.6 mm), Wistari Reef, 23.4847°S, 151.9104°E, 4 December 1979, southwestern corner, dead coral, 11 m, coll. N.L. Bruce (MTQ W10696). ♂ (2.3 mm), Heron Island, 14 January 1979, outer reef in dead coral, coll. N.L. Bruce (MTQ W10697). ♂ (2.2 mm), Heron Island, 23°27’S, 151°55’E, inner edge, ‘ Blue Pools’, 17 January 1979, coll. N.L. Bruce (MTQ W30560). ♂ (2.2 mm), Wistari Reef, northern side, 23°27.257’S, 151°52.840’E, 23 November 2009, small rubble under large coral slabs, 2.5 m, CReefs stn HI09-084E, coll. N.L. Bruce and K. Schnabel (MTQ W31666). 2 ♂ (2.3, 2.7 mm), Heron Island, 5 January 1979, mid reef flat, in dead coral, coll. N.L. Bruce (MTQ W10715). ♂ (2.5 mm), Heron Island, 5 January 1979, just off beach rock in dead coral, coll. N.L. Bruce (MTQ W10716). ♂ (2.7 mm), Heron Island, 8 January 1979, mid reef flat, in dead coral, coll. N.L. Bruce (MTQ W10717). 2 ♂ (2.3, 2.8 mm), Heron Island, 5 January 1979, mid reef flat, in dead coral, coll. N.L. Bruce (MTQ W10714).

Description. Body (Fig. 5A) 2.7 times as long as greatest width, widest at pereonite 5; dorsal surfaces polished, sparsely setose. Cephalosome (Fig. 5 B–E) quadrate, 0.6 times as long as wide, lateral margins parallel; dorsal surface smooth; dorsal sulcus wide, shallow, extended; translucent region present; elliptical; paraocular ornamentation absent, posteromedian tubercle absent. Frontolateral processes present. Frontal margin straight, median point with process. External scissura present, wide, deep. Mediofrontal process present, weak, rounded or acute, without ventral notch, without fine setae. Supraocular lobe pronounced, wide; accessory supraocular lobe not pronounced. Superior frontolateral process present, single, strong, conical, without setae. Inferior frontolateral process absent. Mesioventral margin straight. Eyes present, round, 0.3 times as long as cephalosome length, contiguous with head surface, ommatidia arranged in rows, eye colour yellow.

Pereon (Fig. 5A) lateral margins subparallel, with few setae; anteriorly smooth. Pereonite 1 not fused dorsally with cephalosome; dorsolateral margins not obscured by cephalosome; Pereonite 2 wider than pereonite 1; areae laterales present on pereonite 5. Pereonite 6 without lobi laterales; lobuii absent.

Antennula (Fig. 6A) peduncle article 2 0.8 times as long as article 1, article 3 1.9 times as long as article 2, 4.7 times as long as wide; flagellum 1.2 times as long as article 3, with 5 articles. Antenna (Fig. 6B) peduncle article 4 3.7 times as long as wide, 1.8 times as long as article 3, with 2 penicillate setae, and 7 simple setae; article 5 1.4 times as long as article 4, 3.7 times as long as wide, inferior margin with 2 penicillate setae, with 11 simple setae; flagellum 1.4 times as long as article 5, with 7 articles.

Mandible (Fig. 5 B–E) 0.5 times as long as width of cephalosome, mandible rectangular, weakly curved distally; mandible apex 20 % total length; mandibular seta present. Carina present, smooth along entire length. Incisor knoblike, distal denticulation absent. Blade present, dentate, with distinct angle, dentate along 100 % of margin. Pseudoblade absent; internal lobe absent; dorsal lobe absent; basal neck short; erisma present; lamina dentata absent.

Maxilliped (Fig. 6C) 5-articled. Maxilliped article 1 lateral margin with continuous marginal scale-setae; article 2 lateral margin with 3 plumose setae; article 3 lateral margin with 8 plumose setae; article 4 lateral margin with 5 plumose setae; article 5 with 7 plumose setae; endite extending to distal margin of article 2; without coupling setae. Pylopod (Fig. 6D) article 1 1.6 times as long as wide, without distolateral lobe; posterior and lateral margins forming rounded curve; lateral margin with 29 large PMS; mesial margin with continuous scale-setae, distal margin with 3 simple setae; article 2 (Fig. 6E, F) 0.7 times as long as wide; article 3 minute, with 1 setae.

Pereopods 2–6 without long plumose setae; propodus distal RS slightly longer than proximal RS; lateral and inferior margins with weak tubercles, pereopod 2 with tubercles on basis to ischium. Pereopod 2 (Fig. 7A) basis 2.6 times as long as greatest width, superior margin with 3 setae, inferior margin with 4 setae; ischium 0.6 times as long as basis, 2.1 times as long as wide, superior margin with 3 setae, inferior margin with 5 setae; merus 0.5 times as long as ischium, 1.3 times as long as wide, superior margin with 3 setae, inferior margin with 4 setae; carpus 0.7 times as long as ischium, 2.1 times as long as wide, superior margin with 1 seta, inferior margin with 5 setae (one biserrate); propodus (Fig. 7B) 0.9 times as long as ischium, 3.3 times as long as wide, superior margin 4 setae (one papillose), inferior margin without short setae, 2 simple setae, and 2 RS; dactylus (Fig. 7B) 0.5 times as long as propodus. Pereopods 3 and 4 similar to pereopod 2. Pereopod 5 similar to pereopod 6. Pereopod 6 (Fig. 7C) with weak tubercles on basis and ischium, basis 2.4 times as long as greatest width, superior margin with 3 setae, and 1 penicillate seta, inferior margin with 1 seta; ischium 0.7 times as long as basis, 2.3 times as long as greatest width, superior margin with 5 setae, inferior margin with 3 setae; merus 0.6 times as long as ischium, 1.6 times as long as wide, superior margin with 4 setae (two biserrate), inferior margin with 5 setae, without dense patch of scale-setae; carpus 0.5 times as long as ischium, 2.4 times as long as wide, superior margin with 1 seta, inferior margin with 4 setae (one biserrate); propodus (Fig. 7D) 0.8 times as long as ischium, 4.5 times as long as wide, superior margin with 3 setae (one papillose), inferior margin with 3 setae, and 2 RS; dactylus (Fig. 7D) 0.5 times as long as propodus. Penes low tubercles.

Pleotelson (Fig. 8C) 1.0 times as long as anterior width, lateral margins smooth, anterolateral margins weakly concave, posterolateral margin distally concave; mid-dorsal surface with 2 sub-median setae, anterolateral margin with 2 submarginal setae, posterolateral margin with 2 submarginal setae, apex with 2 setae.

Pleopod 2 (Fig. 8A) exopod 1.9 as long as wide, with 9 PMS; endopod 2.1 as long as wide, with 8 PMS. Pleopod 2 appendix masculina absent.

Uropod (Fig. 8B, C) rami extending beyond pleotelson, apices broadly rounded. Peduncle with 1 dorsal seta. Endopod 2.0 times as long as greatest width, dorsally with 6 sensory setae; lateral margin proximally convex, lateral margin with 2 simple setae; mesial margin weakly convex, with 6 long plumose setae. Exopod not extending to end of endopod, 3.8 times as long as greatest width; lateral margin weakly convex, with 6 simple setae; proximomesial margin straight, mesiodistal margin with 4 long PMS.

Remarks. Gnathia acrorudus sp. nov. bears some resemblance to G. latidens Beddard, 1886 (see Monod 1926) from Flinders Passage, north-eastern Australia, such as in the shape of the cephalosome, the mid-frontal margin and the superior frontolateral processes. It differs, however, from G. latidens in having weak mediofrontal process (strong, actute and triangular in shape in G. latidens) (Monod 1926). It has also certain similarities to G. falcipenis Holdich & Harrison, 1980 from Magnetic and Lizard Islands (Holdich & Harrison 1980; Svavarsson & Bruce 2012). It differs from the latter in lacking an internal lobe on the mandible (internal lobe present in G. falcipenis) and in having only a small penis (penis ½ length of pleon in G. falcipenis) (Holdich & Harrison 1980).

There is considerable variation in the shape of the frontal margin between individuals of Gnathia acrorudus sp. nov. The frontal margin varies from being only slightly pointed, sometimes with small median process (as in the holotype, Fig. 5A) to being a clearly rounded, pointed process (e.g. illustrated paratype, Fig. 5B). The eyes are yellow, but prolonged preservation time may have influenced the colour.

Etymology. The name refers to the coral family Acroporidae Verrill, 1901, which provides abundant coral rubble on coral reefs; rudus (Latin) means crushed stone and alludes to the coral rubble; noun in apposition.

Distribution. Known only from the type locality, Heron Island, southern Great Barrier Reef; intertidal to 11 m.