Gnathia formosa, 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 : 53-57

Gnathia formosa sp. nov.

( Figs 19–21 )

Material examined. Holotype . ♂ (4.8 mm), Wistari Reef, 23.4500°S, 151.9053°E, 26 December 1980, western end of channel, from sponge, c. 20 m, coll. N.L. Bruce (MTQ W10679).

Paratypes. 4♂ (4.4–4.5 mm), Heron Island , 23°28.389’S, 151°57.835’E, 8 December 1979, ‘ Canyons’, dead coral rock, 7 m, coll. N.L. Bruce ( MTQ W10677; 1 ♂ UI 2017.2 ) .

Description. Body ( Fig. 19A ) 3.3 times as long as greatest width, widest at pereonite 3; dorsal surfaces polished, sparsely setose. Cephalosome ( Fig. 19 B–E) quadrate, 0.7 times as long as wide, lateral margins sub-parallel; dorsal surface with sparse granules; dorsal sulcus wide, deep, extended; translucent region absent; paraocular ornamentation weakly developed, posteromedian tubercle present. Frontolateral processes present. Frontal margin straight, median point with process. External scissura present, narrow, shallow. Mediofrontal process present, strong, bifid, with fine setae. Supraocular lobe pronounced, long; accessory supraocular lobe not pronounced. Superior frontolateral process present; single, strong, acute, with few simple setae. Inferior frontolateral process present, acute, without setae. Mesioventral margin concave. Eyes present, elongate, 0.3 times as long as cephalosome length, contiguous with head surface, ommatidia arranged in rows, eye colour yellow.

Pereon ( Fig. 19A ) lateral margins subparallel, with few setae; anteriorly smooth. Pereonite 1 not fused dorsally with cephalosome; dorsolateral margins fully obscured by cephalosome. Pereonite 2 wider than pereonite 1; areae laterales present on pereonite 5; Pereonite 6 without lobi laterales; lobuii weak, conical.

Antennula ( Fig. 20A, B ) peduncle article 2 0.8 times as long as article 1, article 3 2.1 times as long as article 2, 4.5 times as long as wide; flagellum 0.9 times as long as article 3, with 4 articles. Antenna ( Fig. 20C ) peduncle article 4 3.8 times as long as wide, 2.8 times as long as article 3, with 2 penicillate setae, and 12 simple setae; article 5 1.2 times as long as article 4, 5.2 times as long as wide, inferior margin with 4 penicillate setae, with 23 simple setae; flagellum 1.4 times as long as article 5, with 7 articles.

Mandibl e ( Fig. 19 B–E) 0.7 times as long as width of cephalosome, mandible triangular, strongly curved, evenly curved; mandibular seta present. Carina present, finely dentate, along distal half. Incisor knob-like, distal denticulation absent. Blade present, smooth, weakly convex proximally. Pseudoblade absent; internal lobe present, rounded, small, smooth; dorsal lobe absent; basal neck short; erisma present; lamina dentata absent.

Maxilliped ( Fig. 20D ) 5-articled. Maxilliped article 1 lateral margin with continuous marginal scale-setae; article 2 lateral margin with 7 plumose setae; article 3 lateral margin with 7 plumose setae; article 4 lateral margin with 5 plumose setae; article 5 with 8 plumose setae; endite extending to mid-margin of article 3; without coupling setae. Pylopod ( Fig. 20E ) article 1 1.9 times as long as wide, without distolateral lobe, with distomesial lobe; posterior and lateral margins forming rounded curve; lateral margin with 15 large PMS; mesial margin with scale-setae on distal part only, distal margin with 10 simple setae; article 2 ( Fig. 20F ) 1.3 times as long as wide; article 3 ( Fig. 20F ) minute, without 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 carpus. Pereopod 2 ( Fig. 21A ) basis superior margin with 1 seta, inferior margin with 4 setae; ischium 2.5 times as long as wide, superior margin 7 setae (one long and finely serrated distally), inferior margin with 14 setae; merus 0.5 times as long as ischium, 1.6 times as long as wide, superior margin with 4 setae (one long and finely serrated distally), inferior margin with 11 setae; carpus 0.5 times as long as ischium, 3.3 times as long as wide, superior margin with 3 setae, inferior margin with 10 setae (one biserrate); propodus ( Fig. 21B ) 0.7 times as long as ischium, 5 times as long as wide, superior margin with 6 setae (one penicillate), inferior margin with 10 short setae, and 2 RS; dactylus ( Fig. 21B ) 0.3 times as long as propodus. Pereopods 3 and 4 similar to pereopod 2. Pereopod 5 similar to pereopod 6. Pereopod 6 ( Fig. 21C ) with tubercles on carpus, basis 2.5 times as long as greatest width, superior margin with 6 setae, and 1 penicillate seta, inferior margin with 6 setae; ischium 0.8 times as long as basis, 2.7 times as long as greatest width, superior margin with 9 setae, inferior margin with 12 setae; merus 0.6 times as long as ischium, 2.3 times as long as wide, superior margin with 7 setae, inferior margin with 4 setae, with dense patch of scale-setae; carpus 0.6 times as long as ischium, 2.7 times as long as wide, superior margin with 7 setae (two biserrate), inferior margin with 4 setae. Penes opening flush with surface of sternite 7.

Pleotelson ( Fig. 21E ) 1.3 times as long as anterior width, lateral margins serrate, anterolateral margins concave, posterolateral margin straight; mid-dorsal surface with 2 sub-median setae, anterolateral margin with 3 submarginal setae, posterolateral margin with 2 submarginal setae, apex with 2 setae.

Pleopod 1 exopod 1.6 times as long as wide, distally broadly rounded, with ~ 8 PMS; endopod 1.9 times as long as wide, distally broadly rounded; peduncle 1.2 times as wide as long, mesial margin with 2 coupling setae. Pleopod 2 ( Fig. 21D ) exopod 1.4 times as long as wide, with 9 PMS; endopod 1.8 as long as wide. Pleopod 2 appendix masculina absent.

Uropod ( Fig. 21E ) rami extending beyond pleotelson, apices narrowly rounded. Endopod 2.3 times as long as greatest width, dorsally with 4 sensory setae; lateral margin weakly convex, lateral margin with 3 simple setae; mesial margin strongly convex, with 7 long plumose setae. Exopod not extending to end of endopod, 3.4 times as long as greatest width; lateral margin weakly convex, with 12 simple setae; proximomesial margin straight, distally convex, mesiodistal margin with 5 long PMS.

Remarks. Gnathia formosa sp. nov. is most similar to Gnathia phallonajopsis ( Monod, 1925) from the Mediterranean and Gnathia maxillaris (Montagu, 1804) from the North Atlantic Ocean. It differs from both species in the shape of the superior frontolateral processes, being single, acute and strong (bifid superior frontolateral process in G. phallonajopsis ; short and somewhat bifid in G. maxillaris ) ( Monod 1925, 1926).

The eyes are yellow, but prolonged preservation time may have influenced the colour.

Etymology. The epithet is derived from Latin word formosus, meaning beautiful, which expresses well the looks of the species.

Distribution. Heron Island and Wistari Reef, southern Great Barrier Reef; at depths of 7– 20 m.

Figures

FIGURE 19. Gnathia formosa sp. nov., A, ♂, holotype, 4.8 mm (MTQ W10679); B–E, ♂, paratype, 4.5 mm (MTQ W10677). A, habitus. B, cephalosome, dorsal view. C, cephalosome, dorso-lateral view. D, cephalosome, lateral view. E, cephalosome, ventral view. Scale for A = 1.0 mm.

FIGURE 20. Gnathia formosa sp. nov., ♂, paratype, 4.5 mm (MTQ W10677). A, antennula. B, distal end of antennula. C, antenna. D, maxilliped. E, pylopod. F, second and third article of pylopod.

FIGURE 21. Gnathia formosa sp. nov., ♂, paratype, 4.5 mm (MTQ W10677). A, pereopod 2. B, propodus and dactylus of pereopod 2. C, pereopod 6. D, pleopod 2. E, pleotelson and uropod.