Gnathia taurus, Ota & Erasmus & Grutter & Smit, 2024

Ota, Yuzo, Erasmus, Anja, Grutter, Alexandra S. & Smit, Nico J., 2024, Two new species and new host and distribution records of Gnathia Leach, 1814 (Crustacea, Isopoda, Gnathiidae) from Western Australia and the Great Barrier Reef, Australia, ZooKeys 1193, pp. 125-144 : 125

Gnathia taurus sp. nov.

Figs 4 , 5 , 6

Diagnosis.

Large body length more than 8.0 mm; long setae covering most part of dorsal body (cephalosome, pereonites 1-7, and mid-dorsal and lateral parts of pleonites 1-5); frontal margin with rounded mediofrontal process and two superior frontolateral processes; paraocular ornamentation composed of several tubercles and setae; pereonite 1 reaching lateral margins of cephalon, epimera of pleonites 1-5 not prominent; pleotelson 1.3 × longer than its anterior width, eight or nine long setae present on lateral side of pleotelson; mandible almost vertically elongated; article 1 of pylopod with one areolae; appendix masculina of pleopod 2 extending half-length of the endopod; endopod of uropodal rami extends beyond apex of pleotelson; exopod of uropodal rami not extends apex of pleotelson.

Material examined.

Holotype. Australia • 1♂ (9.6 mm TL, 8.2 mm BL, dissected); reared from a juvenile collected from a species of Rhynchobatus (TL 129 cm, female), Heron Island , Great Barrier Reef (23°26'32.9"S, 151°54' 53.8"E), 7 October 1998. Ian D. Whittington leg. (QM W29819) . Paratype. 1♂ (9.4 mm TL, 8.2 mm BL); same data as holotype (QM W29820) .

Type locality.

Heron Island, Great Barrier Reef, Australia (23°26'32.9"S, 151°54'53.8"E).

Description.

Body (Figs 4 , 5A ) 2.6 × as long as greatest width, widest at pereonite 5; dorsal surfaces with tubercules or granules, densely setose. Cephalosome (Figs 4 , 5A-C ) rectangular, 0.7 × as long as wide, lateral margins sub-parallel, posterior margin concave; dorsal surface conspicuous granules anteriorly; dorsal sulcus narrow, shallow, short; translucent region absent; paraocular ornamentation weakly developed and with several tubercles and setae; posterior median tubercle present; lateral tubercles with several long setae. Frontolateral processes present. Frontal margin (Fig. 5B ) straight and medially concave, median point with process. External scissura present, wide, shallow. Mediofrontal process present, weak, rounded, without ventral notch, without setae. Superior frontolateral process present, single, strong, rounded, with four or five long simple setae. Inferior frontolateral process absent. Supraocular lobe not pronounced; accessory supraocular lobe not pronounced. Eyes present, round, 0.2 × as long as cephalosome length, contiguous with head surface, ommatidia not arranged in rows, eye colour dark brown.

Pereon (Figs 4 , 5A ) lateral margins ovate, with many setae; with sparse fine granules on anterior parts of pereonites 2-4. Pereonite 1 not fused dorsally with cephalosome; dorsolateral margins not obscured by cephalosome. Pereonite 2 wider than pereonite 1. Pereonite 4 with anterior constriction, median groove absent. Areae laterales present on pereonite 5, dorsal sulcus wide. Pereonite 6 with strongly developed lobi laterales, lobuii absent. Pereonite 7 short, narrow, and overlapping pleonite 1. Pleon epimera not dorsally visible on all pleonites. Pleonites (Figs 4 , 5A ) lateral margins with 5-7 pairs of simple setae, with 6-9 simple setae medially. Pleotelson (Fig. 5D ) 0.8 × as long as anterior width, covered in pectinate scales; lateral margins smooth, anterolateral margins strongly concave, with 1-3 submarginal setae; posterolateral margin weakly convex, with eight or nine pairs of submarginal setae; mid-dorsal surface with one pair of sub-median setae, apex with two setae.

Antennula (Fig. 5E ) composed of four peduncular and four flagellar articles, 0.6 × shorter than antenna; peduncle article 2 1.1 × as long as article 1; article 3 2.2 × as long as article 2, 4.4 × as long as wide; flagellar article 3 with one aesthetasc seta, and one simple seta; article 4 with one aesthetasc seta; article 5 with one penicillate seta, terminating with one aesthetasc seta and three simple setae. Antenna (Fig. 5F ) composed of four peduncular and seven flagellar articles; peduncle article 3 2.8 × as long as wide, 2.5 × as long as article 2, with two penicillate setae, and seven simple setae; article 4 1.3 × as long as article 3, 4.3 × as long as wide, with five penicillate setae, and 18 simple setae; flagellum 0.8 × as long as article 4, terminating with four simple setae.

Mandible (Fig. 5B, C ) 0.4 × the head length; strongly curved dorsally; apex positions before dentate blade (but it positions after dentate blade in paratype of Fig. 4 ), 23% of total length; mandibular seta present. Carina absent. Incisor dentate, distal denticulation absent. Blade present, dentate, straight, proximally convex, dentate for 28% of margin. Pseudoblade, internal lobe and dorsal lobe absent; basal neck long; erisma present; lamina dentata absent.

Maxilliped (Fig. 6A ). Article 1 lateral margin with continuous marginal scale-setae; article 2 lateral margin with six plumose setae; article 3 lateral margin with seven plumose setae; article 4 lateral margin with six plumose setae; article 5 with nine plumose setae, and six simple setae; endite extending to distal margin of article 2.

Pylopod (Fig. 6B ). Article 1 1.9 × as long as wide; with one areola; without distolateral lobe; posterior and lateral margins forming rounded curve; lateral margin with 59 plumose setae; mesial margin with continuous fringe setae; distal margin with five simple setae; article 2 1.3 × as long as wide, with 11 simple setae; article 3 minute and semicircular without setae.

Pereopod 2 (Fig. 6C ) covered in pectinate scales on inferior margins of ischium, merus carpus, and propodus; basis 2.2 × as long as greatest width, superior margin with 19 simple setae, inferior margin with 21 simple setae; ischium 4.5 × as long as basis, 4.5 × as long as wide, superior margin with 21 simple setae, inferior margin with 11 simple setae; merus 0.3 × as long as ischium, 1.1 × as long as wide, superior margin with eight simple setae and bulbous protrusion, inferior margin with eight simple setae; carpus 1.1 × as long as ischium, 1.9 × as long as wide, superior margin with eight simple setae, inferior margin with four simple setae; propodus 1.1 × as long as ischium, 2.5 × as long as wide, superior margin with three simple setae, superior margin with one penicillate seta, inferior margin with four simple setae, and two denticulate compound spines; dactylus 0.5 × as long as propodus. Pereopods 3, 5, and 6 almost same proportion of each article as pereopod 2; basis of pereopod 4 slightly shorter than that of pereopod 2.

Penes (Fig. 6D ) produced, penial process 0.4 × as long as basal width.

Pleopod 2 (Fig. 6E ) exopod 1.8 × as long as wide, distally broadly rounded, with nine plumose setae; endopod 2 × as long as wide, distally broadly rounded, with seven plumose setae; appendix masculina present, with parallel margins, 0.5 × as long as endopod, distally bluntly rounded; peduncle 1.4 × as wide as long, mesial margin with two coupling setae, lateral margin with one simple seta. All pleopods similar in shape; exopods each with 7-11 plumose or simple setae; endopods each with seven or eight plumose or simple setae in total.

Uropod (Fig. 5D ). Peduncle with two dorsal setae. Uropodal endopod 2.9 × as long as greatest width, apex narrowly rounded, extending beyond apex of pleotelson, dorsally with three penicillate setae; lateral margin weakly convex, lateral margin with nine simple setae; proximomesial margin sinuate, with seven long plumose setae. Uropodal exopod not extending to end of endopod, apex narrowly rounded, not extending beyond apex of pleotelson, 3.6 × as long as greatest width; lateral margin weakly convex, with 24 simple setae; proximomesial margin weakly convex and sinuate, with five plumose setae.

Distribution.

Heron Island, Great Barrier Reef, Australia.

Habitat of adults.

Unknown.

Hosts.

A species of Rhynchobatus . The original data label identified the host as Rhynchobatus djiddensis , but the distribution range of this species is the western Indian Ocean; therefore, the host is most probably Rhynchobatus australiae Whitley, 1939 or R. palpebratus Compagno & Last, 2008, two species that do occur on the GBR ( Last et al. 2016).

Etymology.

The specific name Gnathia taurus , the second sign of the zodiac, is derived from taûros, Latin for bull, and refers to the gnathiid’s dorsally elongated mandible which resemble the horns of a bull.

Remarks.

Among Gnathia species worldwide, Gnathia grandilaris Coetzee, Smit, Grutter & Davies, 2008 is most similar to Gnathia taurus sp. nov., but differs in that its mediofrontal process is acute, the mandible is not vertically elongated, and two areolae are present on article 1 of the pylopod ( Coetzee et al. 2008).

Gnathia nubila Ota & Hirose, 2009 is also similar but the apex of the mediofrontal process is bifid and dentate, the epimera is prominent on pleonites 3-5, and two areolae are present on article 1 of the pylopod ( Ota and Hirose 2009b).

Figures

Figure 4. Photograph of fixed Gnathia taurus sp. nov. (paratype, QM W 29820). Scale bar: 1 mm.

Figure 5. Gnathia taurus sp. nov. (holotype QM W 29819) A whole body (dorsal view) B cephalosome and mandible (dorsal view) C pereonite 1, cephalosome, and mandible (right lateral view) D pleotelson (dorsal view) E right antennula F right antenna.

Figure 6. Gnathia taurus sp. nov. (holotype QM W 29819) A right maxilliped (ventral view) B right pylopod (ventral view) C right pereopod 2 (lateral view) D penes (ventral view) E. right pleopod 2.