Off Lizard Island (14°40'54.68"S, 145°26'53.72"E), Australia.

Australia • 1♂ (6.4 mm TL, 5.2 BL); reared from a juvenile collected from a cowtail stingray Pastinachussephen (Forsskål, 1775) (TL and sex, unknown), Lizard Island, GBR (14°40'54.68"S, 145°26'53.72"E), 19 June 1998, Ian D. Whittington leg. (QM W29823). 2♂ (5.8 mm TL and 4.6 mm BL, 5.7 mm TL and 4.6 mm BL); reared from a juvenile, infested on P.sephen (TL and sex, unknown), Heron Island, GBR (23°26'32.9"S, 151°54'53.8"E), 9 July 1998, Ian D. Whittington leg. (QM W29824). 1♂ (4.2 mm TL, 3.6 mm BL, drawings); reared from a juvenile, infested on epaulette shark, Hemiscylliumocellatum (Bonnaterre, 1788), Heron Island, GBR (23°26'32.9"S, 151°54'53.8"E), 7 November 1998, Ian D. Whittington leg. (QM W29825).

This species can be identified as Gnathiatrimaculata Coetzee, Smit, Grutter, & Davies, 2009 by a frontal border with a mediofrontal process divided into two lobes which almost touch anteriorly and form a distinct key-hole shape, four or five pairs of long pappose setae present ventrally on both lobes, a mandible with seven or eight processes on the dentate blade, a cluster of setae between all processes, and an armed carina (Coetzee et al. 2009).

Ota and Hirose (2009a) reported G.trimaculata from the Ryukyu Islands, demonstrating a greater number of setae on peduncle 4 of antenna than that of the GBR specimens. In the present material, we observed that the mediofrontal process of our specimens does not almost touch and has a smooth margin (Fig. 7E). Therefore, it appears to be two frontolateral processes instead of one mediofrontal process.

This shape of mediofrontal process looks like that of G.aff.maculosa. Gnathiaaff.maculosa of GBR also has a bundle of several long setae on the ventral frontal border. Thus, these two species cannot be distinguished by the morphology of the frontal border alone. However, G.trimaculata can be distinguished from G.maculosa by pectinate scales covering the pleotelson, four pairs of long setae on the lateral margin of pleotelson, and a long pear-shaped pylopod with one areola.

This record of G.trimaculata establishes two new hosts for this widely distributed species. Ota et al. (2012) recorded G.trimaculata from several areas in the Ryukyu Islands and southern Pacific coast of Japan. They demonstrated the first and second stages of the juveniles ectoparasitised four teleost species, while the third stage ectoparasitised 25 elasmobranch species including two unidentified elasmobranch species (see Ota et al. 2012: table 3). Ota (2015: table 2) also showed G.trimaculata collected from 18 elasmobranch species including two unidentified elasmobranch species but all of them except for one were already reported by Ota et al. (2012). These host species are listed below; in GBR, our host records of Pastinachussephen and Hemiscylliumocellatum were not included the previous studies and these are new host records.

Off Lizard Island and Heron Island, Great Barrier Reef, Australia. The Ryukyu Islands and southern Pacific coast of Japan.

Unknown.

Four elasmobranch species from GBR: Carcharinusmelanopterus (Quoy & Gaimard, 1824), Carcharinusamblyrhynchos (Bleeker, 1856), Pastinachussephen (Forsskål, 1775), and epaulette shark Hemiscylliumocellatum (Bonnaterre, 1788). Three teleost species from Japan: Enneapterygiusetheostomus (Jordan & Snyder, 1902), Enneapterygiusmiyakensis Fricke, 1987, Springerichthysbapturus (Jordan & Snyder, 1902), 24 elasmobranch species and two unidentified species from Japan: Urolophusaurantiacus Müller & Henle, 1841, Gymnurajaponica (Temminck & Schlegel, 1850), Rhynchobatusdjiddensis (Forsskål, 1775), Neotrygonorientalis Last, White & Séret, 2016 [Neotrygonkuhlii Müller & Henle, 1841 in Ota et al. 2012 and Ota 2015], Taeniurameyeni Müller & Henle, 1841, Dasyatisizuensis Nishida & Nakaya, 1988, Hemitrygonakajei (Müller & Henle, 1841) [Dasyatisakajei (Müller & Henle, 1841) in Ota et al. 2012 and Ota 2015], Himanturaundulata (Bleeker, 1852), Himantura spp., Aetomylaeusvespertilio (Bleeker, 1852), Aetobatusocellatus (Kuhl, 1823) [Aetobatusnarinari (Euphrasen, 1790) in Ota et al 2012 and Ota 2015], Aetobatusflagellum (Bloch & Schneider, 1801), Rhinopterajavanica Müller & Henle, 1841, Mobulamobular (Bonnaterre, 1788) [Mobulajapanica (Müller & Henle, 1841) in Ota et al. 2012 and Ota 2015], Mobulathurstoni (Lloyd, 1908) [Mobuladiabolus (Shaw, 1804) in Ota et al. 2012 and Ota 2015], Mobulatarapacana (Philippi, 1892), Nebriusferrugineus (Lesson, 1831), Rhincodontypus Smith, 1828, Stegostomafasciatum (Hermann, 1783), Sphyrnalewini (Griffith & Smith, 1834), Triaenodonobesus (Rüppell, 1837), Negaprionacutidens (Rüppell, 1837), Galeocerdocuvier (Péron & Lesueur, 1822), Carcharhinusalbimarginatus (Rüppell, 1837), Carcharhinuslimbatus (Müller & Henle, 1839), and Carcharhinus spp.

Gill chambers, interbranchial septa, gill filaments, and the floor of oral cavities. Rarely nostrils, body surface near the gill slits, or claspers of elasmobranchs. Fins and skin of teleosts.