﻿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

﻿Abstract Gnathiaantennacrassasp. nov. from seagrass beds off Rottnest Island, Western Australia 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. Gnathiataurussp. 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 mandibles and smoothly rounded mediofrontal process on the anterior part of cephalosome. Gnathiaaff.maculosa Ota & Hirose, 2009 is recorded from Australia, together with further records of G.trimaculata Coetzee, Smit, Grutter & Davies, 2009 and G.grandilaris Coetzee, Smit, Grutter & Davies, 2008, all from elasmobranch fishes.


Introduction
The isopod family Gnathiidae Leach, 1814, exhibits a biphasic lifecycle characterised by morphological differentiation among its larvae (juveniles) as well as between adult males and adult females.Gnathiid larvae are temporary ectoparasites of marine teleosts and elasmobranchs.In contrast, the adult stage is non-parasitic and reproduces in benthic substrates (Smit and Davies 2004;Tanaka 2007).
The Gnathiidae includes 12 genera and approximately 240 species worldwide (Boyko et al. 2023).The Australia gnathiids are represented by seven genera 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 andBruce 2012, 2019), while several studies have described species from adult specimens reared from juveniles collected from host fishes (Coetzee et al. 2008(Coetzee et al. , 2009;;Ferreira et al. 2009Ferreira et al. , 2010;;Farquharson et al. 2012) 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 larvae moulted into adult males suitable for species description.
Adult male specimens were preserved in 70% ethanol, and total length measured 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 sharpened 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 classification 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 flagellum, 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).
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.
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.
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, 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. 2008Coetzee et al. , 2009;;Ferreira et al. 2009Ferreira et al. , 2010;;Farquharson et al. 2012;Svavarsson andBruce 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.
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.
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.
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.
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, 1939or R. palpebratus Compagno & Last, 2008, two species that do occur on the GBR (Last et al. 2016).
Etymology.The specific name 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).
Remarks.The male morphologies of these GBR specimens show the deep and narrow dorsal sulcus on the cephalosome, the narrow body (Fig. 7A, B), and the almost semicircular pylopod article 1 with three areolae (Fig. 7C).These characters can be identified as Gnathia maculosa Ota & Hirose, 2009.However, this species was originally described from the Ryukyu Islands, southwestern Japan (Ota and Hirose 2009a) and our records are a great distance from this island group.The apices of the frontolateral processes on anterior margin of heads of the present specimens are smooth, while those of original description are serrate (Ota and Hirose 2009a).The number of setae on the pleotelson of the present species is two pairs (Fig. 7D), while that of the original description is three pairs (Ota and Hirose 2009a).Thus, these GBR specimens are identified as G. aff.maculosa.
The GBR specimens of G. aff.maculosa have a bundle of several long setae on the ventral frontal border (Fig. 8).Ota and Hirose (2009a) did not show the ventral frontal border but the Japanese specimens G. maculosa also have a bundle of several long setae (YO pers.obs.).
Remarks.This species can be identified as Gnathia trimaculata 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 keyhole 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.Gnathia aff.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 Pastinachus sephen and Hemiscyllium ocellatum were not included the previous studies and these are new host records.
Distribution.Off Lizard Island and Heron Island, Great Barrier Reef, Australia.The Ryukyu Islands and southern Pacific coast of Japan.