Gnathia glaucostega, Svavarsson & Bruce, 2019

Svavarsson, Jörundur & Bruce, Niel L., 2019, New gnathiid isopod crustaceans (Cymothoida) from Heron Island and Wistari Reef, southern Great Barrier Reef, Zootaxa 4609 (1), pp. 31-67 : 57-61

Gnathia glaucostega sp. nov.

(Figs 22–25)

Material examined. Holotype . ♂ (5.3 mm), Heron Island–Wistari Reef channel, 23.4520°S, 151.9078°E, 30 November 1979, in dead coral, 20 m, coll. N.L. Bruce (MTQ W10695).

Description. Body (Fig. 22A) 2.7 times as long as greatest width, widest at pereonite 5; dorsal surfaces punctate, densely setose. Cephalosome (Fig. 22B–E) semi-pentagonal, 0.9 times as long as wide, lateral margins narrowing posteriorly; dorsal surface with numerous granules; dorsal sulcus narrow, deep, extended; translucent region present; elliptical; paraocular ornamentation well developed, posteromedian tubercle present. Frontolateral processes present. Frontal margin produced, triangular, median point with process. External scissura present, narrow, shallow. Mediofrontal process present, strong, acute, without ventral notch, without fine setae. Supraocular lobe pronounced, wide; accessory supraocular lobe not pronounced. Superior frontolateral process present, single, strong, unequally apically bifid, with few long simple setae. Inferior frontolateral process absent. Mesioventral margin straight. Eyes present, elongate, 0.2 times as long as cephalosome length, contiguous with head surface, ommatidia arranged in rows, eye colour pale brown.

Pereon (Fig. 22A) lateral margins subparallel, with many setae; anteriorly with numerous fine granules. Pereonite 1 not fused dorsally with cephalosome; dorsolateral margins not obscured by cephalosome; Pereonite 2 as wide as pereonite 1; areae laterales present on pereonite 5; Pereonite 6 without lobi laterales; lobuii absent.

Antennula ( Fig. 23A ) article 3 2.5 times as long as article 2, 5.1 times as long as wide; flagellum 1.0 times as long as article 3, with 5 articles. Antenna ( Fig. 23B ) peduncle article 4 3.1 times as long as wide, 1.9 times as long as article 3, with 2 penicillate setae, and 6 simple setae; article 5 1.3 times as long as article 4, 4.2 times as long as wide, inferior margin with 4 penicillate setae, with 8 simple setae; flagellum 1.2 times as long as article 5, with 6 articles.

Mandibl e (Fig. 22B–E) 0.6 times as long as width of cephalosome, mandible triangular, strongly curved distally; mandible apex 10 % total length; mandibular seta present. Carina present, finely dentate along proximal half. Incisor knob-like, distal denticulation absent. Blade present, dentate, with distinct angle, convex, dentate along 60 % of distal margin. Pseudoblade absent; dorsal lobe absent; basal neck short; erisma present; lamina dentata visible in dorsal view.

Maxilliped ( Fig. 23C ) 5-articled. Maxilliped article 1 lateral margin with continuous marginal scale-setae; article 2 lateral margin with 5 plumose setae; article 3 lateral margin with 7 plumose setae; article 4 lateral margin with 5 plumose setae; article 5 with 5 plumose setae; endite extending to distal margin of article 2; without coupling setae. Pylopod ( Fig. 23D ) article 1 1.5 times as long as wide, without distolateral lobe; posterior and lateral margins forming rounded curve; lateral margin with 36 large PMS; mesial margin with continuous scale-setae, distal margin with 4 simple setae; article 2 ( Fig. 23E ) 1.1 times as long as wide, with 5 setae; article 3 ( Fig. 23E ) minute, with 6 scale-like setae.

Pereopods 2–6 with long simple setae; propodus distal RS slightly longer than proximal RS; lateral and inferior margins with weak tubercles, pereopod 2 with tubercles on basis to carpus. Pereopod 2 ( Fig. 24A ) basis 2.5 times as long as greatest width, superior margin with 14 setae, inferior margin with 7 setae; ischium 0.8 times as long as basis, 2.6 times as long as wide, superior margin with 5 setae, inferior margin with 5 setae; merus 0.4 times as long as ischium, 1.3 times as long as wide, superior margin with 3 setae, inferior margin with 8 setae; carpus ( Fig. 24B ) 0.6 times as long as ischium, 2.2 times as long as wide, superior margin with 1 seta, inferior margin with 6 setae (one biserrate); propodus ( Fig. 24B ) 0.8 times as long as ischium, 3.8 times as long as wide, superior margin 4 setae (one penicillate), inferior margin with 1 simple setae and 2 RS; dactylus ( Fig. 24B ) 0.5 times as long as propodus. Pereopods 3 and 4 similar to pereopod 2. Pereopod 5 similar to pereopod 6. Pereopod 6 ( Fig. 24C ) with tubercles on basis to carpus, basis 2.6 times as long as greatest width, superior margin with 5 setae, and 1 penicillate seta, inferior margin with 8 setae; ischium 0.8 times as long as basis, 2.3 times as long as greatest width, superior margin with 8 setae, inferior margin with 6 setae; merus 0.5 times as long as ischium, 1.3 times as long as wide, superior margin with 5 setae, inferior margin with 7 setae, with dense patch of scale-setae; carpus 0.5 times as long as ischium, 2.1 times as long as wide, superior margin with 2 setae, inferior margin with 6 setae, with dense patch of scale-setae; propodus ( Fig. 24D ) 0.7 times as long as ischium, 3.8 times as long as wide, superior margin with 4 setae (one penicillate), inferior margin with 12 simple setae, and 2 RS; dactylus ( Fig. 24D ) 0.5 times as long as propodus.

Pleotelson (Fig. 25C) 1.3 times as long as anterior width, lateral margins finely serrate, anterolateral margins concave, posterolateral margin straight; mid-dorsal surface with 2 sub-median setae, anterolateral margin with 4 submarginal setae, posterolateral margin with 2 submarginal setae, apex with 2 setae.

Pleopod 2 (Fig. 25A) exopod 2 times as long as wide, with 9 PMS; endopod 2.5 times as long as wide, with 7 PMS. Pleopod 2 appendix masculina present, curved, with parallel margins, 0.6 times as long as endopod, distally bluntly rounded.

Uropod (Fig. 25B, C) rami extending beyond pleotelson, apices broadly rounded. Peduncle with 1 dorsal seta. Endopod 2.8 times as long as greatest width, dorsally with 4 sensory setae; lateral margin proximally straight, lateral margin with 5 simple setae; mesial margin strongly convex distally, with 4 long plumose setae. Exopod not extend-

ing to end of endopod, 4.5 times as long as greatest width; lateral margin straight, with 7 simple setae; proximomesial margin straight, distally convex, mesiodistal margin with 4 long PMS.

Remarks. Gnathia glaucostega sp. nov. is most closely related to Gnathia excavata Ota, 2012 from Southwestern Japan, but differs in having the mediofrontal margin evenly rounded distally (bifid apex in G. excavata ), in the shape of the frontolateral processes (rounded in G. excavata ; slightly bifid in Gnathia glaucostega ), in having

Figures

FIGURE 23. Gnathia glaucostega sp. nov., ♂, paratype, 3.8 mm (MTQ W24860). A, antennula. B, antenna. C, maxilliped. D, pylopod. E, second and third article of pylopod.

FIGURE 24. Gnathia glaucostega sp. nov., ♂, paratype, 3.8 mm (MTQ W24860). A, pereopod 2. B, carpus, propodus and dactylus of pereopod 2. C, pereopod 6. D, propodus and dactylus of pereopod 6.