#ZooKeys ZooKeys 1193: 125-144 (2024) DOI: 10.3897/zookeys.1193.116538 Research Article Two new species and new host and distribution records of Gnathia Leach, 1814 (Crustacea, Ilsopoda, Gnathiidae) from Western Australia and the Great Barrier Reef, Australia Yuzo Ota'™®, Anja Erasmus”®, Alexandra S. Grutter®®, Nico J. Smit?© 1 San‘in Kaigan Geopark Museum of the Earth and Sea, 1794-4, Makidani, lwami-cho, lwami-gun, Tottori 681-0001, Japan 2 Water Research Group, Unit for Environmental Sciences and Management, North-West University, Private Bag X6001, Potchefstroom, 2520, South Africa 3 School of the Environment, The University of Queensland, St. Lucia, Queensland 4072, Australia Corresponding author: Yuzo Ota (yota164@gmail.com) OPEN Qaccess Academic editor: Tammy Horton Received: 29 November 2023 Accepted: 29 January 2024 Published: 5 March 2024 ZooBank: https://zoobank. org/672951D5-E7D7-4D81-AB9C- 419B811D5B97 Citation: Ota Y, Erasmus A, Grutter AS, Smit NJ (2024) Two new species and new host and distribution records of Gnathia Leach, 1814 (Crustacea, lsopoda, Gnathiidae) from Western Australia and the Great Barrier Reef, Australia. ZooKeys 1193: 125-144. https://doi.org/10.3897/ zookeys.1193.116538 Copyright: © Yuzo Ota et al. This is an open access article distributed under terms of the Creative Commons Attribution License (Attribution 4.0 International - CC BY 4.0). Abstract Gnathia antennacrassa sp. nov. from seagrass beds off Rottnest Island, Western Austra- lia is the first record of any gnathiid from the entirety of Western Australia; the male can be distinguished from congeners by the stout peduncular articles of the antenna. Gnath- ia taurus sp. nov. is described from two adult specimens reared from praniza larvae found infecting elasmobranch fishes at Heron Island, southern Great Barrier Reef; the males can be distinguished from all congeners by the dorsally strongly elongate mandi- bles and smoothly rounded mediofrontal process on the anterior part of cephalosome. Gnathia aff. maculosa Ota & Hirose, 2009 is recorded from Australia, together with fur- ther records of G. trimaculata Coetzee, Smit, Grutter & Davies, 2009 and G. grandilaris Coetzee, Smit, Grutter & Davies, 2008, all from elasmobranch fishes. Key words: Coral reefs, elasmobranchs, Heron Island, Lizard Island, marine fish para- sites, Rottnest Island, taxonomy Introduction The isopod family Gnathiidae Leach, 1814, exhibits a biphasic lifecycle character- ised by morphological differentiation among its larvae (juveniles) as well as be- tween adult males and adult females. Gnathiid larvae are temporary ectoparasites of marine teleosts and elasmobranchs. In contrast, the adult stage is non-para- sitic and reproduces in benthic substrates (Smit and Davies 2004; Tanaka 2007). The Gnathiidae includes 12 genera and approximately 240 species world- wide (Boyko et al. 2023). The Australia gnathiids are represented by seven gen- era and 60 species, which have almost exclusively been described from the eastern coasts of Australia from South Australia to Queensland. The majority of species in Australia were collected from benthic substrata (Haswell 1884; Beddard 1886; Hale 1924; Monod 1926; Cals, 1973; Seed 1979; Holdich and Harrison 1980; Cohen and Poore 1994; Svavarsson and Bruce 2012, 2019), while several studies have described species from adult specimens reared from juveniles collected from host fishes (Coetzee et al. 2008, 2009; Ferreira et al. 2009, 2010; Farquharson et al. 2012). 125 Yuzo Ota et al.: New species and a new record of gnathiid isopods from Australia Two new species are here described, Gnathia antennacrassa sp. nov. from Rottnest Island, southern Western Australia and Gnathia taurus sp. nov. from Heron Island, southern Great Barrier Reef (GBR). Additionally, we report Gnathia aff. maculosa Ota & Hirose, 2009, a new record for Australia and provide new host and distribution records of two other Gnathia species from the GBR. Materials and methods Larval isopod samples from the GBR were collected from five elasmobranch species as part of parasitological research on elasmobranchs conducted during the 1990s (Great Barrier Reef Marine Park Permit no. G96/543). These samples were reared to adults in vials containing seawater. Some of these lar- vae moulted into adult males suitable for species description. Adult male specimens were preserved in 70% ethanol, and total length mea- sured between the tip of the mandibles and posterior margin of pleotelson. Additionally, their body length was measured between the anterior margins of the frontal processes and posterior margin of pleotelson. Specimens were cleaned using a fine hair of saturated polyester resin and dissected with sharp- ened tungsten needles. The appendages were removed from the body and then mounted in CMCP-10 high-viscosity medium (Polyscience, Warrington, PA, USA). Observations were conducted using a phase-contrast light microscope, and drawings were made using a camera lucida. For scanning electron microscope (SEM) examination, one specimen was dehydrated in 99% ethanol for a day and air-dried. The dried specimen was mounted on brass SEM stubs using double-sided conductive tape, followed by sputter-coating with platinum, and then photographed using a Hitachi SU3900 SEM. New descriptions were prepared in DEscriptive Language for TAxonomy (DELTA; Dallwitz 2018) using a modified Gnathia character set (Erasmus et al. 2023). Descriptive terminology follows Smit and Davies (2004) for setal classi- fication and Cohen and Poore (1994) for the male morphology. Most literature in Gnathiidae treats the fourth article of antennule as the first article of the fla- gellum, but this article has penicillate seta characteristic of the peduncles, so it is treated as the fourth article of the peduncles. The scientific names used in the host fish records follow FishBase (Froese and Pauly 2023). Abbreviations: BL—body length; GBR—Great Barrier Reef, Queensland; QM— Queensland Museum, Brisbane; SEM—scanning electron microscope; TL—total body length; WAM—Western Australian Museum, Perth. Taxonomy Suborder Cymothoida Wagele, 1989 Superfamily Cymothooidea Leach, 1814 Family Gnathiidae Leach, 1814 Gnathia Leach, 1814 Type species. Gnathia termitoides Leach, 1814 (= Cancer maxillaris Montagu, 1804); by monotypy (Cohen and Poore 1994); type locality: south coast of Dev- on, Cornwall Peninsula, south west England. ZooKeys 1193: 125-144 (2024), DOI: 10.3897/zookeys.1193.116538 126 Yuzo Ota et al.: New species and a new record of gnathiid isopods from Australia Gnathia antennacrassa sp. nov. https://zoobank.org/DB30A1C7-2DCE-458B-920C-56AD365203EE Figs 1-3 Diagnosis. Anterior part of body (cephalosome and pereonites 1-4) not dense- ly 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 x shorter than its anterior width; lateral side of pleotelson sinuate; maxi- mum width of peduncle article of antenna 3.2 x maximum width of flagellar article; article 1 of pylopod with two areolae; appendix masculina of pleopod 2 0.8 x 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 * 13 (2.4 mm TL, 1.9 mm BL, dis- sected); 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, 11573256); Description. Body (Figs 1, 2A) 2.7 x as long as greatest width, widest at pere- onite 2; dorsal surfaces smooth, sparsely setose. Cephalosome (Fig. 1A-C) rectangular, 0.7 x 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 de- veloped, 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 frontolat- eral process absent. Supraocular lobe not pronounced; accessory supraocular lobe not pronounced. Eyes present, round, 0.3 x 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; an- teriorly 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 lat- eral lobes, dorsal sulcus wide. Pereonite 6 with strongly developed lobi latera- les, 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 medi- ally. Pleotelson (Fig. 2D) 0.8 x as long as anterior width, not covered in pecti- nate 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 x as long as antenna; peduncle article 2 0.6 x as long as article 1; article 3 2.1 x as long as article 2, 2.9 x as long as wide; flagellum as long article 3; ZooKeys 1193: 125-144 (2024), DOI: 10.3897/zookeys.1193.116538 127 Yuzo Ota et al.: New species and a new record of gnathiid isopods from Australia Figure 1. Photograph of fixed Gnathia antennacrassa sp. nov. (holotype, WAM C-79675). Scale bar: 1 mm. 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 pedun- cular and seven flagellar articles; maximum width of peduncle article 3.2 x the maximum width of flagellar articles; peduncle article 3 1.7 x as long as wide, 0.7 x as long as article 2, with one penicillate seta, and nine simple setae; article 4 as long as article 3, 1.5 x as long as wide, and with 20 simple setae; flagellum 1.2 x as long as article 4, with seven articles, terminating with two simple setae. Mandible (Figs 1, 2A—-C) 0.7 x as long as cephalosome; triangular, weakly mesially curved; apex 20% total length; mandibular seta present. Carina pres- ent, 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 later- al 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 x 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 x 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 x as long as greatest width, superior margin with three simple setae, inferior ZooKeys 1193: 125-144 (2024), DOI: 10.3897/zookeys.1193.116538 128 Yuzo Ota et al.: New species and a new record of gnathiid isopods from Australia -1 mm (B) cares. hana, — a am re I 0.1 mm (E, F) 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. ZooKeys 1193: 125-144 (2024), DOI: 10.3897/zookeys.1193.116538 129 Yuzo Ota et al.: New species and a new record of gnathiid isopods from Australia 0.1 mm (B) q Y} i HY), D oe \ a 1 | ‘| 0.1 mm (C) Pe (e) .1mm 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. ZooKeys 1193: 125-144 (2024), DOI: 10.3897/zookeys.1193.116538 130 Yuzo Ota et al.: New species and a new record of gnathiid isopods from Australia margin with two simple setae; ischium as long as basis, 2.2 x as long as wide, superior margin with five simple setae; merus 0.4 x as long as ischium, 1.1 x as long as wide, superior margin with four simple setae, and bulbous protrusion, inferior margin with three simple setae; carpus 1.1 x as long as ischium, 0.9 x as long as wide, superior margin with three setae, inferior margin with five setae; propodus 1.6 x as long as ischium, 2.6 x 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 x 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 x as long as basal width. Pleopod 2 (Fig. 3E). Exopod 2.2 x as long as wide, distally broadly round- ed, with nine plumose setae; endopod 1.8 x as long as wide, distally narrowly rounded, with seven plumose setae; appendix masculina present, with parallel margins, 0.8 x as long as endopod, distally narrowly rounded; peduncle 0.5 x 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 x as long as greatest width, apex broadly rounded, extending beyond apex of the pleotelson, dorsally with five penicillate setae; lateral and proximomesial mar- gin with seven plumose and three simple setae. Uropodal exopod not extend- ing to end of endopod, 3.8 x 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, antennacrassa, is derived from Latin, mean- ing “stout antenna’. Remarks. Among the other Gnathia species worldwide, G. i/lepida 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 x of that of the flagellar articles (Monod 1926). Gnathia vellosa Miller, 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 x that of flagellar articles; and three areolae are present on article 1 of the pylopod (Miller 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. ZooKeys 1193: 125-144 (2024), DOI: 10.3897/zookeys.1193.116538 131 Yuzo Ota et al.: New species and a new record of gnathiid isopods from Australia 2009, 2010; Farquharson et al. 2012; Svavarsson and Bruce 2012, 2019) re- mains almost undocumented. Cohen and Poore (1994) mention that G. mulier- aria Hale, 1924 occurred from Victoria and south Australia to Western Austra- lia. 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. Gnathia taurus sp. nov. https://zoobank.org/450C54D2-99D1-495C-9E66-DA81A9CA3D9F Figs 4-6 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 su- perior 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 x 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 be- yond 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. lan D. Whittington leg. (QM W29819). Paratype. 13 (9.4mm 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 x 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 x as long as wide, lateral margins sub-parallel, posterior margin concave; dorsal surface conspicuous granules anteriorly; dorsal sulcus narrow, shallow, short; translucent region absent; para- ocular 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 se- tae. 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 x 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 gran- ules on anterior parts of pereonites 2—4. Pereonite 1 not fused dorsally with ceph- alosome; dorsolateral margins not obscured by cephalosome. Pereonite 2 wider than pereonite 1. Pereonite 4 with anterior constriction, median groove absent. ZooKeys 1193: 125-144 (2024), DOI: 10.3897/zookeys.1193.116538 132 Yuzo Ota et al.: New species and a new record of gnathiid isopods from Australia Figure 4. Photograph of fixed Gnathia taurus sp. nov. (paratype, QM W29820). Scale bar: 1mm. Areae laterales present on pereonite 5, dorsal sulcus wide. Pereonite 6 with strong- ly 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. Ple- otelson (Fig. 5D) 0.8 x 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 x shorter than antenna; peduncle article 2 1.1 x as long as article 1; article 3 2.2 x as long as article 2, 4.4 x 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 sim- ple setae. Antenna (Fig. 5F) composed of four peduncular and seven flagellar articles; peduncle article 3 2.8 x as long as wide, 2.5 x as long as article 2, with two penicillate setae, and seven simple setae; article 4 1.3 x as long as article 3, 4.3 x as long as wide, with five penicillate setae, and 18 simple setae; flagellum 0.8 x as long as article 4, terminating with four simple setae. ZooKeys 1193: 125-144 (2024), DOI: 10.3897/zookeys.1193.116538 133 Yuzo Ota et al.: New species and a new record of gnathiid isopods from Australia