Elthusa winstoni sp. n. (Isopoda, Cymothoidae), a new fish parasitic isopod from Hawaii

Abstract The new cymothoid species, Elthusa winstoni sp. n., a branchial parasite of fishes from the family Acanthuridae Bonaparte, 1835 in Hawaii, is described and figured. The female adults can be distinguished by the strongly vaulted body and compacted body shape; rostrum with a small median point; short antennae which are close together (only 6 articles in both antennula and antenna); short and wide uropods extending to half the length of the pleotelson; short dactyli on pereopod 7; and large recurved robust setae on the maxilla. This is the first record of an Elthusa Schioedte & Meinert, 1884 species from the Hawaiian Islands and only the fifth cymothoid described from this region.


Introduction
The cymothoid genus Elthusa Schioedte & Meinert, 1884was first established in 1884, with E. emarginata (Bleeker, 1857 the only known species at the time. Over a century later, Bruce (1990) revised the genus, reclassifying 21 species from Livoneca Leach, 1818 into Elthusa, and adding two new species. Since then, another four species have been described from New Caledonia and Mexico (Trilles and Justine 2004, 2006, Rocha-Ramírez et al. 2005. In their review of cymothoids, Smit et al. (2014) stated that there were 28 known Elthusa species. Since then, an Elthusa homonym has been corrected (Hadfield et al. 2016a), making a current total of 29 recognised species within the genus.
Elthusa has a global distribution (Rocha-Ramírez et al. 2005), being absent only from polar waters. While moderately well-known from the Caribbean, Australia and central Indo-Pacific, the vast central Pacific region has been scarcely studied, with recent records only from New Caledonia (Trilles and Justine 2004, 2006. In Hawaii, Bunkley-Williams and Williams Jr. (1996) listed the isopods known from Hawaiian fish hosts, recording only four cymothoid species, namely: Anilocra gigantea (Herklots, 1870); Creniola breviceps (Schioedte & Meinert, 1881); Cymothoa recta Dana, 1853; and Ichthyoxenus puhi (Bowman, 1960). This study is the first record of a gill-attaching Elthusa species in Hawaii, with both the male and female being described for the new species.

Methods
During a research visit to the Smithsonian National Museum of Natural History in Washington DC (USA) in November 2014, an unnamed specimen originally collected in 1959 from Acanthurus nigroris Valenciennes, 1835, from Oahu, Hawaii, was observed. Recently, new material of the same species was collected from the gill rakers of a kole tang, Ctenochaetus strigosus (Bennett, 1828), speared by a SCUBA diver at approximately 20 m depth on 28 September 2015 in marine waters adjacent to the island of Niihau, Hawaii (22.00296, -160.11894). The host fish and parasites were stored frozen at-sea and initially examined approximately one month later at the Hawaii Institute of Marine Biology, Kaneohe, Hawaii. Isopods were subsequently removed from the gill chambers of the host fish and preserved in 70% ethanol. All specimens were processed following the techniques recorded in Hadfield et al. (2010Hadfield et al. ( , 2013. The female designated as the holotype (and male paratype) were minimally dissected in order to conserve the specimens. The species descriptions were prepared in DELTA (Descriptive Language for Taxonomy) using a general Cymothoidae character set (see Hadfield et al. 2016b). Isopod classification follows Brandt and Poore (2003) and host nomenclature follows that of FishBase (Froese and Pauly 2016) and Catalog of Fishes (Eschmeyer 2016).
Abbreviations. AMNH -American Museum of Natural History, New York, USA; USNM -National Museum of Natural History, Smithsonian Institution, Washington, D.C., USA; TL -total length; W -width. Remarks. Diagnostic characters for Elthusa include a weakly vaulted body; pleonite 1 as wide as or slightly narrower than pleonite 2; posterior margin of cephalon not trilobed; wide pleon; antennula shorter than antenna, bases not in contact (varying from close together to wide apart); and pleopods all simple, lamellar. A revised diagnosis of the genus was provided by Bruce (1990) as well as Trilles and Randall (2011).
Elthusa can be regarded as one of the most morphologically varied cymothoid genera, with many Elthusa species still requiring detailed study and redescription. The antennula position (being close together or far apart), shape of the cephalon anterior margin, as well as the width of the pleon and pleonite 5 are some of the varying characters noted within species of this genus (Bruce 1990). This variation can cause confusion in genus identifications and may lead to incorrect species placement if not taken into account with all of the other genus characters. Description. Holotype female. Length 17.5 mm, width 11.5 mm. Body compact, weakly twisted, 1.7 times as long as greatest width, wider anteriorly, dorsal surfaces rugose and strongly arched longitudinally, widest at pereonite 3, most narrow at pereonite 1, lateral margins slightly convex. Cephalon 0.7 times longer than wide, visible from dorsal view, square and deeply immersed in pereonite 1. Frontal margin forming rounded rostrum with small median point. Eyes oval with distinct margins, one eye 0.1 times width of cephalon; 0.25 times length of cephalon. Pereonite 1 with slight indentations, anterior border straight, anterolateral angle with large broad projections, extend to anterior margin of eyes. Posterior margins of pereonites not smooth, with irregular nodules in certain areas. Coxae 2-3 wide, with posteroventral angles rounded; 4-7 rounded, not extending past pereonite margin. Pereonites 1-3 increasing in length and width; 4-7 decreasing in length and width; becoming more progressively rounded posteriorly. Pleon with pleonite 1 largely concealed by pereonite 7 and same width as other pleonites, partially visible in dorsal view; pleonites posterior margin not smooth. Distal ends of pleonite 2 partially overlapped by pereonite 7; posterolateral angles of pleonite 2 narrowly rounded. Pleonites 3-5 similar in form to pleonite 2. Pleotelson 0.6 times as long as anterior width, dorsal surface slightly depressed, lateral margins convex, posterior margin rounded.
Antennula approximately the same length as antenna, bases narrowly separated, consisting of 6 articles, extending to anterior margin of eye. Antenna consisting of 6 articles, extending to middle of the eye. Mandibular molar process ending in an acute incisor, with a single simple seta. Maxillula simple with 4 terminal robust setae. Maxilla mesial lobe partially fused to lateral lobe; lateral lobe with 3 recurved robust setae; mesial lobe with 1 large recurved robust seta. Maxilliped weakly segmented, with lamellar oostegite lobe, article 3 with 3 recurved robust setae.
Pereopod 1 basis 1.5 times as long as greatest width; ischium 0.7 times as long as basis; merus proximal margin with bulbous protrusion; carpus with straight proximal margin; propodus as long as wide; dactylus slender, 0.9 times as long as propodus, 1.8 times as long as basal width. Pereopod 7 same length as other pereopods, basis 1.8 times as long as greatest width; ischium 0.7 times as long as basis, without protrusions; merus proximal margin with slight bulbous protrusion, distal margin produced, 0.7 times as long as wide, 0.4 times as long as ischium; carpus 0.9 times as long as wide, 0.3 times as long as ischium, without bulbous protrusion; propodus 1.7 times as long as wide, 0.5 times as long as ischium; dactylus slender, 0.7 times as long as propodus, 2.4 times as long as basal width.
Uropod half the length of pleotelson, peduncle 0.8 times longer than rami, peduncle lateral margin without setae. Endopod wide, apically rounded, 1.6 times as long as greatest width, lateral margin weakly convex, mesial margin weakly convex, terminating without setae. Exopod extending to end of endopod, 2.2 times as long as greatest   width, apically rounded, lateral margin straight, mesial margin straight, terminating without setae.
Male. Length 8 mm, width 3.5 mm. Male similar to female but much smaller. Body rectangular, body 2.1 times as long as wide. Cephalon with scattered chromatophores. Pereonite 1 anterolateral margin broad with scattered chromatophores. Antennula bases separated, consisting of 8 articles, extending to posterior margin of eye. Antenna consisting of 8 articles, extending to posterior margin of cephalon. Mandibular molar process ending in an acute incisor. Maxillula simple with 4 terminal robust setae. Maxilla with 2 recurved robust setae. Maxilliped weakly segmented, with lamellar oostegite lobe, article 3 with 3 recurved robust setae. Penes opening flush with surface of sternite 7, tubercules separate, penial process 0.75 times as long as basal width. Pleopod 2 appendix masculina with parallel margins, 0.8 times as long as endopod, distally narrowly rounded. Uropods same length as pleotelson.
Etymology. Named after one of the collectors of the type specimen, Morgan Winston, who collected the type specimens while diving adjacent to the island of Niihau, Hawaii.
Distribution. Known only from Hawaii.
Remarks. Elthusa winstoni sp. n. can be distinguished from all congeners by the irregular, compact body shape; short antennae; short and wide uropods only extending to middle of the pleotelson; and a strongly vaulted body.
This species conforms with many of the Elthusa characters in having a wide pleon with pleonite 1 as wide a pleonite 2; the cephalon posterior margin is straight; the antennula bases are not in contact; and all of the pleopods are simple and lamellar. Elthusa winstoni sp. n. differs from other Elthusa species in the asymmetrical, strongly vaulted body and short antennae not extending past the cephalon (only 6 articles for both antennula and antenna in female, 8 in male). Furthermore, the mandibular palp is slender with only one seta, large recurved robust setae on maxilla, short dactyli on pereopod 7, and a very compact body (1.2 -1.6 times as long as wide). There are no similar cymothoid species from Hawaii.
Many of the Elthusa hosts remain unknown; however, both Acanthurus nigroris and Ctenochaetus strigosus appear to be new hosts for Elthusa isopods. Creniola breviceps has previously been located on Ctenochaetus strigosus in Hawaii, but Acanthurus nigroris is a new host record for cymothoid isopods.

Acknowledgments
The financial assistance of the National Research Foundation (NRF) of South Africa towards this research is hereby acknowledged (NRF project IFR2011040100022, NJ Smit, PI and SFP12091012541, KA Hadfield, PI). Opinions expressed and conclusions arrived at, are those of the authors and are not necessarily to be attributed to the NRF. Hawaiian specimens were collected under permits PMNM-2012-0450A1, PMNM-2014-031, PMNM-2015-020, PMNM-2015-030, PMNM-2015-029, and PMNM-2015. Many thanks to the collectors, Erik Franklin and Morgan Winston, for the new material of this species; as well as to Niel Bruce for comments on the manuscript. Further thanks to the Claude Leon Foundation of South Africa for their financial support of KA Hadfield for part of this project and to Karen Osborn and Karen Reed from the Smithsonian National Museum of Natural History for access to the museum material. This is contribution number 169 from the NWU-Water Research Group.