Gnathia antennacrassa, 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 antennacrassa sp. nov.

Figs 1 , 2 , 3

Diagnosis.

Anterior part of body (cephalosome and pereonites 1-4) not densely covered by tubercles; frontal margin with serrated triangular mediofrontal process and two superior frontolateral processes; paraocular ornamentation not developed; pereonite 1 not reaching lateral margins of cephalon; pereonites 4-6 with two lateral lobes; epimera of pleonites 1-5 not prominent; pleotelson 0.8 × shorter than its anterior width; lateral side of pleotelson sinuate; maximum width of peduncle article of antenna 3.2 × maximum width of flagellar article; article 1 of pylopod with two areolae; appendix masculina of pleopod 2 0.8 × as long as endopod; endopod of uropodal rami extend beyond apex of pleotelson; exopod of uropodal rami almost apex of pleotelson.

Material examined.

Holotype. Australia • 1♂ (2.4 mm TL, 1.9 mm BL, dissected); sandy substrata of seagrass Amphibolis griffithii (J.M. Black) Hartog, 1970 patch bed surrounded by the seagrass Posidonia sinuosa Cambridge & Kuo, 1979 bed, 5 m depth, Thomson Bay, Rottnest Island , Western Australia (32°00'S, 115°32.5'E), 18 January 1996, Hiroshi Mukai leg. (WAM C-79675).

Type locality.

Thomson Bay, Rottnest Island, Western Australia (32°00'S, 115°32.5'E).

Description.

Body (Figs 1 , 2A ) 2.7 × as long as greatest width, widest at pereonite 2; dorsal surfaces smooth, sparsely setose. Cephalosome (Fig. 1A-C ) rectangular, 0.7 × as long as wide, lateral margins parallel, posterior margin concave; dorsal surface tubercles around eyes; dorsal sulcus wide, shallow, short; translucent region present, elliptical; paraocular ornamentation not developed, posterior median tubercle absent. Frontolateral processes present. Frontal margin (Fig. 1B ) straight, median point with process. External scissura present, wide, shallow. Mediofrontal process present, strong, serrate triangular, without ventral notch and fine setae. Superior frontolateral process present, single, strong, conical, with three pairs of long simple setae. Inferior frontolateral process absent. Supraocular lobe not pronounced; accessory supraocular lobe not pronounced. Eyes present, round, 0.3 × as long as cephalosome length, contiguous with head surface, ommatidia arranged in rows, eye colour faded.

Pereon (Figs 1 , 2A ) lateral margins narrowing posteriorly, with few setae; anteriorly smooth. Pereonite 1 not fused dorsally with cephalosome. Pereonite 2 wider than pereonite 1. Pereonite 4 with anterior constriction, median groove present. Areae laterales present on pereonite 4 and pereonite 5, with two lateral lobes, dorsal sulcus wide. Pereonite 6 with strongly developed lobi laterales, lobuii weak, conical. Pereonite 7 short, narrow, and overlapping pleonite 1. Pleon epimera not dorsally visible on pleonites. Pleonites (Figs 1 , 2A ) lateral margins with one pair of simple setae, with two pairs of simple setae medially. Pleotelson (Fig. 2D ) 0.8 × as long as anterior width, not covered in pectinate scales; lateral margins smooth, anterolateral margins concave, without submarginal seta; posterolateral margin concave, with one pair of submarginal setae, mid-dorsal surface with a pair of sub-median setae, apex with two setae.

Antennula (Fig. 2E ) composed of four peduncular and three flagellar articles, 0.8 × as long as antenna; peduncle article 2 0.6 × as long as article 1; article 3 2.1 × as long as article 2, 2.9 × as long as wide; flagellum as long article 3; article 3 with one aesthetasc and one simple seta; article 4 terminating with one aesthetasc and four simple setae. Antenna (Fig. 2F ) composed of four peduncular and seven flagellar articles; maximum width of peduncle article 3.2 × the maximum width of flagellar articles; peduncle article 3 1.7 × as long as wide, 0.7 × as long as article 2, with one penicillate seta, and nine simple setae; article 4 as long as article 3, 1.5 × as long as wide, and with 20 simple setae; flagellum 1.2 × as long as article 4, with seven articles, terminating with two simple setae.

Mandible (Figs 1 , 2A-C ) 0.7 × as long as cephalosome; triangular, weakly mesially curved; apex 20% total length; mandibular seta present. Carina present, smooth along proximal half. Incisor elevated, standing clear of surface. Blade present, dentate, weakly convex, straight, dentate along 58% of margin. Pseudoblade, internal lobe, and dorsal lobe absent; basal neck short; erisma present; lamina dentata not visible in dorsal view.

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

Pylopod (Fig. 3B ). Article 1 2.0 × as long as wide; with two distinct areolae; without distolateral lobe; posterior and lateral margins forming rounded curve; lateral margin with 24 plumose setae; mesial margin with continuous fringe setae; distal margin with five simple setae; article 2 1.2 × as long as wide, with five simple setae; article 3 semicircular with two short setae.

Pereopod 2 (Fig. 3C ) sparsely covered with short simple setae on basis and ischium, inferior margins with prominent tubercles on basis to carpus; basis 1.3 × as long as greatest width, superior margin with three simple setae, inferior margin with two simple setae; ischium as long as basis, 2.2 × as long as wide, superior margin with five simple setae; merus 0.4 × as long as ischium, 1.1 × as long as wide, superior margin with four simple setae, and bulbous protrusion, inferior margin with three simple setae; carpus 1.1 × as long as ischium, 0.9 × as long as wide, superior margin with three setae, inferior margin with five setae; propodus 1.6 × as long as ischium, 2.6 × as long as wide, superior margin with two simple setae and one penicillate seta, inferior margin with three pectinate scales, and two robust setae; dactylus 0.4 × as long as propodus. Pereopods 3 and 5 similar proportions of each article as pereopod 2. Pereopod 4 longer than pereopod 2, basis, ischium, and merus slightly longer than those of pereopod 2; propodus somewhat rounded. Pereopod 6 slightly shorter than pereopod 2, basis shorter than that of pereopod 2, distal margin of merus rounded.

Penes (Fig. 3D ) with two small papillae, 0.5 × as long as basal width.

Pleopod 2 (Fig. 3E ). Exopod 2.2 × as long as wide, distally broadly rounded, with nine plumose setae; endopod 1.8 × as long as wide, distally narrowly rounded, with seven plumose setae; appendix masculina present, with parallel margins, 0.8 × as long as endopod, distally narrowly rounded; peduncle 0.5 × as wide as long, mesial margin with two coupling setae, lateral margin with one simple seta. All pleopods similar in shape; exopods each with eight or nine plumose setae; endopods each with 7-9 plumose setae.

Uropod (Fig. 1D ). Peduncle without dorsal setae. Uropodal endopod 1.6 × as long as greatest width, apex broadly rounded, extending beyond apex of the pleotelson, dorsally with five penicillate setae; lateral and proximomesial margin with seven plumose and three simple setae. Uropodal exopod not extending to end of endopod, 3.8 × as long as greatest width, apex broadly rounded, reaching almost apex of pleotelson; lateral and proximomesial margin with four plumose and seven simple setae.

Distribution.

Known only from the type locality.

Habitat of adults.

Sandy substrata of seagrass; 5 m depth.

Hosts.

Unknown.

Etymology.

The specific name, Gnathia antennacrassa , is derived from Latin, meaning "stout antenna".

Remarks.

Among the other Gnathia species worldwide, G. illepida Monod, 1923 is similar to G. antennacrassa sp. nov., but differs in that the tubercles densely cover the anterior part of the body (cephalosome and pereonites 1-4), the paraocular ornamentation is developed with several distinct tubercles and setae, and the maximum width of peduncle articles of the antenna is 2.4 × of that of the flagellar articles ( Monod 1926).

Gnathia vellosa Müller, 1988 is also similar, but differ in that tubercles and long setae densely cover the anterior part of the cephalosome and pereonites 2, 3, and anterior part of pereonite 4; the maximum width of peduncle articles of antenna is 2.4 × that of flagellar articles; and three areolae are present on article 1 of the pylopod ( Müller 1988).

Gnathia luxata Kensley, Schotte & Poore, 2009 differs from our new species as it has three processes on the frontal border but the mesial lobe is present on the mandible and, similarly to G. vellosa , it has three areolae present on article 1 of the pylopod ( Kensley et al. 2009).

The gnathiid fauna of Western Australia, in contrast to the eastern Australian coast (see Cohen and Poore 1994; Coetzee et al. 2008, 2009; Ferreira et al. 2009, 2010; Farquharson et al. 2012; Svavarsson and Bruce 2012, 2019) remains almost undocumented. Cohen and Poore (1994) mention that G. mulieraria Hale, 1924 occurred from Victoria and south Australia to Western Australia. However, the original description mentioned G. mulieraria only from South Australia and there is no evidence or reference to its distribution as referred to in Cohen and Poore (1994). Therefore, G. antennacrassa represents the first recorded species of Gnathiidae from Western Australia.

Figures

Figure 1. Photograph of fixed Gnathia antennacrassa sp. nov. (holotype, WAM C- 79675). Scale bar: 1 mm.

Figure 2. Gnathia antennacrassa sp. nov. (holotype, WAM C- 79675) A whole body (dorsal view) B cephalosome and mandible (dorsal view) C pereonite 1, cephalosome, and mandible (left lateral view) D pleotelson (dorsal view) E left antennula F left antenna.

Figure 3. Gnathia antennacrassa sp. nov. (holotype, WAM C- 79675) A left maxilliped (ventral view) B left pylopod (ventral view) C left pereopod 2 (lateral view) D penes (ventral view) E left pleopod 2.