Pigmentation patterns are useful for species identification of third-stage larvae of gnathiids (Crustacea: Isopoda) parasitising coastal elasmobranchs in southern Japan

Abstract

Previous studies from southern Japan reported larval stages of eight gnathiid isopod species parasitising coastal elasmobranchs. Since gnathiid larvae of these different species closely resembled each other, it was necessary to obtain specimens of free-living adult males for identification to the species level. This was achieved by allowing larvae of the final stage to moult into adult males. From these males, specimens of a species new to science were discovered and described here as Gnathia rufescens n. sp. The main differentiating characteristics of G. rufescens n. sp. are: (i) the apex of pleotelson is oval shaped; (ii) the dorsal sulcus is wide in the posterior part; and (iii) the article 3 of the pylopod is not reduced in the male. Additionally, this paper summarises the specific pigmentation patterns of third-stage larvae of the new species and eight previously described species. Furthermore, host records and host use by the gnathiids were summarised based on data from 158 hosts and over 4,500 gnathiid samples; these are discussed with a focus on host-specificity of the nine gnathiid species studied.

Description of a Small-headed Gnathiid Isopod (Crustacea), <i>Gnathia capitellum</i> sp. nov., from Coastal Japan

“…Most of the species (10) have been reported from elasmobranch fishes. Larvae of these species (4-13 mm; Nunomura and Honma 2004;Coetzee et al 2008Coetzee et al , 2009Ota and Hirose 2009a, b;Ota 2011Ota , 2014Ota , 2015 are much larger than third praniza larvae (=finale stage larvae) of the present new species (2.5-3.1 mm). In the re- maining two species, G. camuripenis Tanaka, 2004 is distinguished from G. capitellum sp.…”

“…Of 24 Gnathia species known from Japanese coastal and adjacent waters, larval morphologies have been described in 12 species (Nunomura and Honma 2004;Tanaka 2004;Ota et al 2007;Coetzee et al 2008Coetzee et al , 2009Ota and Hirose 2009a, b;Ota 2011Ota , 2014Ota , 2015. Most of the species (10) have been reported from elasmobranch fishes.…”

“…Currently, Gnathiidae includes 12 genera and approximately 200 species from around the world (Boyko et al 2008 onwards). In waters adjacent to Japan, six genera and 38 species have been recorded (Ota 2014(Ota , 2015. In this study, larvae of gnathiids ectoparasitizing host fishes were reared in the laboratory until maturation and adult specimens of the species were also sampled by dredge.…”

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

“…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.…”

“… Ota and Hirose (2009a) recorded the two host elasmobranchs of G. maculosa and Ota (2015 : table 2) summarised gnathiids ectoparasitising on elasmobranchs in the Ryukyu Islands and documented G. maculosa obtained from 11 elasmobranch species including two host species previously recorded by Ota and Hirose (2009a) : Rhynchobatus djiddensis (Forsskål, 1775), Neotrygon orientalis Last, White & Séret, 2016 [ Neotrygon kuhlii Müller & Henle, 1841 in Ota 2015 ], Taeniura meyeni Müller & Henle, 1841, Himantura undulata (Bleeker, 1852), Himantura sp., Aetomylaeus vespertilio (Bleeker, 1852), Aetobatus ocellatus (Kuhl, 1823) [ Aetobatus narinari (Euphrasen, 1790) in Ota 2015 ], Rhinoptera javanica Müller & Henle, 1841, Nebrius ferrugineus (Lesson, 1831), Triaenodon obesus (Rüppell, 1837), and Negaprion acutidens (Rüppell, 1837).…”