Gnathia capricornica, 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 : 42-48

Gnathia capricornica sp. nov.

( Figs 10–13 )

Material examined. Holotype . ♂ (1.9 mm), Wistari Reef, southwest corner, 23.4847°S 151.9104°E, 2 December 1979, in dead coral, 11 m, coll. N.L. Bruce (MTQ W10699).

Paratypes. ♂ (1.8 mm), dissected, same data as holotype, + 2 microslides) ( MTQ W34142) . ♂ (1.9 mm), Lizard Island , off Coconut Beach, reef front, 14.68441°S, 145.47197°E, 17 February 2009, small rubble on sand between bommies, 4.5 m, CReefs stn LI09-09C, coll. N.L. Bruce & M. Błażewicz ( MTQ W31054) .

Description. Body ( Fig. 10A ) 2.8 times as long as greatest width, widest at pereonites 2 and 3; dorsal surfaces punctate, sparsely setose. Cephalosome ( Fig. 10 B–E) quadrate, 0.7 times as long as wide, lateral margins convex; dorsal surface with numerous granules; dorsal sulcus narrow, deep, short; translucent region absent; paraocular ornamentation absent, posteromedian tubercle present. Frontolateral processes present. Frontal margin straight, median point even. External scissura present, wide, shallow. Mediofrontal process absent. Supraocular lobe not pronounced; accessory supraocular lobe not pronounced. Superior frontolateral process present, single, strong, rounded, with 1 long simple seta. Inferior frontolateral process absent. Mesioventral margin straight. Eyes present, round, 0.2 times as long as cephalosome length, bulbous, standing out from head surface, ommatidia arranged in rows, eye colour dark brown.

Pereon ( Fig. 10A ) lateral margins subparallel, with few setae. 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 absent.

Antennula ( Fig. 11A ) peduncle article 2 0.8 times as long as article 1, article 3 2.1 times as long as article 2, 3.9 times as long as wide. Antenna ( Fig. 11B ) peduncle article 4 2.0 times as long as wide, 2.0 times as long as article 3, with 4 penicillate setae, and 3 simple setae; article 5 1.4 times as long as article 4, 2.9 times as long as wide, distally with 3 penicillate setae, with 11 simple setae.

Mandibl e ( Fig. 10 B–E) 0.7 times as long as width of cephalosome, mandible triangular, weakly curved, evenly; mandible apex 17 % total length; mandibular seta present. Carina present, finely crenulated along mid-part. Incisor elevated, standing clear of surface, distal denticulation absent. Blade present, dentate, unevenly convex, proximally convex, dentate along 88 % of distal margin. Pseudoblade absent; internal lobe absent; dorsal lobe absent; basal neck short; erisma present; lamina dentata absent.

Maxilliped ( Fig. 11C ) 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 5 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. 11D ) article 1 1.8 times as long as wide, with distolateral lobe; posterior and lateral margins forming rounded curve; lateral margin with 22 large PMS; mesial margin with scale-setae on proximal part; distal margin with 4 simple setae; article 2 ( Fig. 11E ) 2.1 times as long as wide; article 3 ( Fig. 11E ) minute, with 2 setae.

Pereopods 2–6 with long simple setae; propodus distal RS as long as proximal RS; lateral and inferior margins with weak tubercles, pereopod 2 with tubercles on merus and carpus. Pereopod 2 ( Fig. 12A ) basis 2.2 times as long as greatest width, superior margin with 5 setae, inferior margin with 4 setae; ischium 0.7 times as long as basis, 1.9 times as long as wide, superior margin with 1 long seta, inferior margin with 6 setae; merus 0.5 times as long as ischium, 1.3 times as long as wide, superior margin with 1 short and 1 long seta, inferior margin with 3 setae; carpus 0.6 times as long as ischium, 1.7 times as long as wide, superior margin without setae, inferior margin with 3 setae (one biserrate); propodus ( Fig. 12B ) 0.8 times as long as ischium, 3.0 times as long as wide, superior margin with 4 setae (1 penicillate), inferior margin with 2 RS; dactylus ( Fig. 12B ) 0.6 times as long as propodus. Pereopods 3 and 4 similar to pereopod 2. Pereopod 5 similar to pereopod 6. Pereopod 6 ( Fig. 12C ) with tubercles on merus and carpus, basis 2.1 times as long as greatest width, superior margin with 2 setae, and 1 penicillate seta, inferior margin with 6 setae; ischium 0.8 times as long as basis, 1.9 times as long as greatest width, superior margin with 3 setae, inferior margin with 5 setae; merus 0.6 times as long as ischium, 1.7 times as long as wide, superior margin with 3 setae, inferior margin with 5 setae, without dense patch of scale-setae; carpus 0.5 times as long as ischium, 2 times as long as wide, superior margin with 2 setae, inferior margin with 3 setae; propodus ( Fig. 12D ) 0.9 times as long as ischium, 0.3 times as long as wide, superior margin with 3 setae (one penicillate), inferior margin with 2 simple setae, and 2 RS; dactylus ( Fig. 12D ) 0.4 times as long as propodus.

Pleopod 2 ( Fig. 13A ) exopod 2.5 times as long as wide, with 9 PMS; endopod 2.6 as long as wide, with 8 PMS. Pleopod 2 appendix masculina absent.

Uropod ( Fig. 13B ) rami extending beyond pleotelson, apices broadly rounded. Peduncle with 1 dorsal seta. Endopod 2.7 times as long as greatest width, dorsally with 7 sensory setae; mesial margin weakly convex, with 6 long plumose setae. Exopod not extending to end of endopod, 4 times as long as greatest width; lateral margin straight, with 6 simple setae; proximomesial margin straight, mesiodistal margin with 4 long PMS.

Remarks. Gnathia capricornica sp. nov. shows some resemblance to G. meticola Holdich & Harrison, 1980 from North-Eastern Australia, such as in having the mid-frontal margin straight. Gnathia capricornica sp. nov. differs, however, from G. meticola in the shape of the mandible (smaller, not as pronounced incisor in G. meticola ) and in having the superior frontolateral processes more distant from each other than seen in G. meticola . The frontolateral processes of G. meticola were, however, not accurately illustrated by Holdich & Harrison (1980), and are wide and low.

The dorsal surface of the pleotelson of the holotype is obscured by an epizoitic foraminiferan. The pleotelson of the dissected male was destroyed during dissection. The male from Lizard Island is preserved intact to allow further comparison in the future.

Etymology. Heron Island belongs to the Capricorn group, southern Great Barrier Reef. The name refers to the region.

Distribution. Great Barrier Reef at Heron Island and Lizard Island; at depths of 4.5 to 11 m.

Figures

FIGURE 10. Gnathia capricornica sp. nov., A, ♂, holotype, 1.9 mm (MTQ W10699); B–E, ♂ paratype, 1.8 mm (MTQ W34142). A, habitus. B, cephalosome, dorsal view. C, cephalosome, dorso-lateral view. D, cephalosome, lateral view. E, cephalosome, ventral view. Scale for A = 0.5 mm.

FIGURE 11. Gnathia capricornica sp. nov., ♂, paratype, 1.8 mm (MTQ W34142). A, antennula (distal articles missing). B, antenna (setae on distal articles missing). C, maxilliped. D, pylopod. E, outline of second and third article of pylopod.

FIGURE 12. Gnathia capricornica sp. nov., ♂, paratype, 1.8 mm (MTQ W34142). A, pereopod 2. B, propodus and dactylus of pereopod 2. C, pereopod 6. D, propodus and dactylus of pereopod 6.

FIGURE 13. Gnathia capricornica sp. nov., ♂, paratype, 1.8 mm (MTQ W34142).A, pleopod 2. B, uropod.