A new parasitic copepod (Copepoda; Cyclopoida; Chondracanthidae) from two pomacentrid fishes caught on the Great Barrier Reef, Queensland, Australia

A new species of the copepod genus Pseudacanthocanthopsis Yamaguti & Yamasu, 1959 (family Chondracanthidae) is described based on material of both sexes collected from two pomacentrid host fishes caught off Lizard Island, Queensland. The type host is Neopomacentrus azysron (Bleeker) and the additional host is N. cyanomos (Bleeker). The new species is distinguishable from all congeners by the form of the antennule of the female, which is dorso-ventrally flattened and extends out anteriorly to the front of the cephalothorax margin.


Introduction
The copepod family Chondracanthidae comprises 191 valid species currently classified in 50 genera (Walter & Boxshall, 2021). Adult female chondracanthids are highly transformed parasites that live on marine fish hosts. The males are reduced in size and have been traditionally been referred to as dwarf males (Østergaard et al., 2005). Male chondracanthids are typically found attached to specialized secretory organs located in the genital region of the females from which they obtain nutrients (Østergaard & Boxshall, 2004).

Materials and methods
The material was collected by Alexandra S. Grutter as part of a field experiment testing the effect of cleaner fish Labroides dimidiatus Valenciennes presence/ removal on ectoparasites (Grutter, 1996). Fish were collected by scuba divers using barrier nets and hand nets, and immediately placed in quick-sealing plastic bags to retain the parasites. Fish died when placed in an ice slurry during transport to the laboratory. The fish and the contents of the bag were fixed in 10% formalin in seawater. Parasites were found by examining the body of the fish, oral and branchial cavities, inside of operculum, and detached pectoral and pelvic fins and gills spread out on a petri dish, all scanned under a microscope at a magnification of 35x. Collected parasites were transferred to individual vials containing 10% formalin in seawater. Fish standard length (SL) and total length (TL) were measured.
Three females of Pseudacanthocanthopsis were collected from the gills of Neopomacentrus azysron (Bleeker, 1877) caught between October 18 and 26, 1993. A further three females of Pseudacanthocanthopsis were collected from the gills of Neopomacentrus cyanomos (Bleeker, 1856) caught between October 18 and 19, 1993. Specimens were cleared in lactic acid and observed whole on a Leitz dissecting microscope. Dissected appendages were examined on an Olympus BH2 microscope using differential interference contrast. Drawings were made using a drawing tube and measurements were made using a stage micrometer. Morphological terminology conforms to Huys & Boxshall (1991). The structure referred to in older chondracanthid literature as the ''accessory antennule'' was shown to be the atrophied (and laterally displaced) terminal segment of the antenna (Ho, 1984). Names of hosts follow FishBase (Froese & Pauly, 2021). Type material and voucher specimens are deposited in the collections of the Queensland Museum and in the Natural History Museum, London.

Description of Female
Adult female body transformed, consisting of head, trunk and genitoabdomen (Figs. 1, 2A, 3A); total body length 868 lm (excluding antennules and caudal setae). Head comprising fused cephalosome and first pedigerous somite; slightly wider than long (347 x 362 lm); with median cuticular thickening along dorsal midline extending posteriorly from frontal margin to beyond middle of head (Fig. 3A). Proximal segment of antennules flattened and extending anteriorly to form bipartite frontal plate, conspicuous in dorsal view. Head without obvious dorsal or lateral processes. Paired processes associated with antennae present; processes and antennae can together be displaced anteriorly ( Fig. 1) or posteriorly ( Fig. 2A) in preserved material. Small posterolateral rounded lobes present ventrally either side of labrum. Trunk about 1.5 times wider than long (370 x 574 lm), subrectangular, lacking processes. Genitoabdomen (Fig. 2B) small, comprising genital complex bearing paired genital openings dorsolaterally, and unsegmented abdomen. Genital complex about 2.2 times wider than abdomen; ornamented with paired setules on ventral surface. Abdomen about as wide as long, fused to genital complex, bearing paired caudal rami on posterior margin. Caudal rami each armed with large fused apical seta plus 1 inner distal seta, 1 dorsal seta and 2 lateral setae. Antennule (Fig. 2C, D) 2-segmented, comprising proximal segment forming enlarged dorso-ventrally flattened plate and cylindrical distal segment. Proximal segment expanded anteriorly, supplied with extrinsic muscles entering via lumen at base of limb. Three stubby elements (modified setae?) present on ventral surface of proximal segment ( Fig. 2A). Distal segment (Fig. 2D) displaced posteriorly, armed with 5 setae apically.
Legs 1 (Fig. 3C) and 2 ( Fig. 3D) located on walls of groove separating head and trunk; difficult to observe. Both leg pairs biramous but very reduced, each comprising unsegmented protopodal part, unsegmented, lobate endopod and indistinctly 2-segmented exopod. Endopod with single apical seta in both legs. Exopod armed with outer element on proximal segment and 1 lateral plus 3 apical elements on distal segment in both legs.
Etymology. The new species honours its discoverer Alexandra S. Grutter for her important contributions to our knowledge of the ectoparasites of Great Barrier Reef fishes and the impact of cleaner fish on these communities.

Remarks
The genus Pseudacanthocanthopsis was established by Yamaguti & Yamasu (1959) (Izawa, 1975). Two additional species were subsequently reported from Japan. The first was P. secunda Yamaguti & Yamasu, 1960 from Apogon lineatus Temminck & Schlegel. This species was described under the name Pseudacanthopsis secunda: however, this generic name has never been proposed and is obviously an error since Yamaguti and Yamasu (1960)   (Temminck & Schlegel) caught off Hiroshima Prefecture, Japan, and the sparid Pagrus major (Temminck & Schlegel) from the Seto Inland Sea, Japan. They also extended its known distribution range on the type host, A. lineatus, to include the East China Sea off Japan and Korea. The third Japanese species was P. bicornutus (Shiino, 1960) from a cepolid host, Owstonia totomiensis Tanaka. Ho & Kim (1995) redescribed P. bicornutus based on material from a second host, the pomacentrid Chromis notatus (Temminck & Schlegel), caught in the Sea of Japan. The fourth species in the genus is P. rohdei which was based on material collected from two hosts belonging to the family Pomacentridae, Dascyllus reticulatus (Richardson) and Pomacentrus chrysurus (Cuvier) [as P. rhodonotus Bleeker] in Australian waters (Ho & Dojiri, 1976).
The new species can be readily distinguished from all four of its congeners by the form of the antennule in females, which is dorso-ventrally flattened and extends out anteriorly to the front of the cephalothorax. Together the antennules are visible as a bifid plate in dorsal view (Fig. 3A). In contrast, in the three Japanese species, P. apogonis, P. secunda and P. bicornutus, the antennules of the female are cylindrical and more-or-less directed laterally. In P. rohdei the antennules are modified, with the large, fleshy proximal segment forming a curved structure with a ventromedial lobe and a large protrusion at its posterodistal corner (Ho & Dojiri, 1976). However it is fleshy and cylindrical rather than dorso-ventrally flattened. The unique form of the antennule of the new species is observable in undissected females and supports the establishment of the new species.
The morphology of the new species is most similar to that of P. rohdei and P. bicornutus; all three species have very similar gross morphology in the female and the appendages share numerous character states. For example, all three species have a reduced number of setae on the atrophied tip of the antenna, an accessory claw on the maxillary basis, a spinulose lobe on the basis of the maxilliped, subequal legs 1 and 2, and a reduced number of setae on the endopod of both legs in the adult female; and lack a strong claw on the atrophied tip of the antenna and have a reduced number of setae on the endopod of leg 2 in the adult male. In addition to the antennules, the new species differs from P. rohdei and P. bicornutus by having a large fleshy process near the base of each antenna, one seta (vs. none) on the atrophied tip of the antenna, no tooth on the maxillary basis (vs. one small tooth -see Fig. 5D in Ho & Dojiri (1976) for P. rohdei and Fig. 6A in Ho & Kim (1995) for P. bicornutus), four setae (vs. five setae) on the distal exopodal segment of leg 1, one seta (vs. two setae) on leg 1 endopod and four setae (vs. 3 or 5 setae) on the distal exopodal segment of leg 2 in the female; and by having one seta (vs. 3 or 5 setae) on the atrophied tip of the antenna, and no setae (vs. 2 or 4 setae) on the endopod of legs 1 and 2 in the male.

Discussion
The form of the female antennules in the new species is so unusual that investigation of the musculature was necessary in order to confirm the interpretation. Two extrinsic muscles pass into the antennule from the head (Fig. 2C). The larger, more anteriorly located muscle has a tendinous origin inside the head and passes into the proximal segment of the antennule and on towards a broad insertion on the lateral wall of the segment. The smaller muscle also originates inside the head and passes into the limb, inserting directly onto the lateral wall of the segment posterior to the anterior muscle. The presence of musculature indicates that the flattened structure is derived from paired limbs and the presence of the defined, setose, distal segment, albeit displaced posteriorly, confirms that this structure comprises the proximal part of the antennule. These muscles may represent an opposing pair and appear to function to adduct and abduct the flattened proximal segment relative to the head, possibly moving the anterior expansions in to meet at the midline and to separate them. Ho & Dojiri (1976) noted that the ''pygmy'' male of P. rohdei attached to one of a pair of small processes found at the junction of the trunk and the genital complex of the adult female. No such processes were observed in the new species but it is clear that the structures reported by Ho & Dojiri (1976) represent the paired nuptial organs. Østergaard & Boxshall (2004) interpreted these specialized structures as secretory organs which provide nutrient secretion that sustains the attached male. available to me for study. ASG would like to thank Mark A. Johnson for assistance in the fieldwork, Bryony Dixon and Libby Liggins for conducting some of the parasite surveys, and Olivia Cenne for data compilation and producing the photographs. ASG is grateful for the support of the staff of the Lizard Island Research Station. ASG and the author acknowledge the Dingaal people, the traditional owners and custodians of the land and country upon which this research was carried out and pay respects to elders past and present of this nation. ASG's research was funded by the Australian Museum in the form of a Post Graduate Award and a Lizard Island Doctoral fellowship and by James Cook University and The University of Queensland.

Author contributions GAB is sole author
Funding The Natural History Museum, London allowed the author access to laboratory facilities as a Scientific Associate.
Data availability Type material is stored in the Queensland Museum and Natural History Museum, London and is available for study.

Declarations
Conflict of interest The author has no competing interests.
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