Two New Genera and Species of the Parasitic Copepod Family Chondracanthidae Milne Edwards, 1840 (Copepoda: Cyclopoida) from Deep-Sea Fishes Off Suruga Bay, Japan

Purpose The present paper describes two new genera and species of the parasitic copepod family Chondracanthidae Milne Edwards, 1840 based on specimens collected from two species of deep-sea fishes at a depth of 212 m off Suruga Bay, Japan. Avatar nishidai gen. et sp. nov. is described from the host fish Chaunax abei Le Danois, 1978 (Chaunacidae). Kokeshioides surugaensis gen. et sp. nov. is described from the host fish Setarches longimanus (Alcock, 1894) (Setarchidae). Methods Fresh specimens of chondracanthids were collected from the buccal cavity of two species of deep-sea fishes (fish hosts were frozen), Chaunax abei Le Danois, 1978 (Lophiiformes: Chaunacidae) and Setarches longimanus (Alcock, 1894) (Perciformes: Setarchidae), caught at a depth of 212 m in Suruga Bay, Japan (34° 37′48.87″ N, 138° 43′2.958″ E). Both the species are described and illustrated based on ovigerous females. Results The genus Avatar gen. nov. can readily be distinguished from all other chondracanthid genera by the following combination of features: cephalothorax slightly wider than long with anterior pair of large and posterior pair of small lateral lobes, and two pairs of ventro-lateral processes; the very posteriormost part of the first pedigerous somite contributes to the neck; cylindrical trunk with two pairs of blunt proximal fusiform processes; antennule with small knob terminally; antenna bearing distal endopodal segment; labrum protruding ventrally; two pairs of biramous legs each with 2-segmented rami. Kokeshioides gen. nov. has the following combinations of features that distinguish it from other chondracanthid genera: body flattened, without lateral processes; cephalothorax much wider than long, with paired anterolateral and posterolateral lobes, folded ventrally; the very posteriormost part of the first pedigerous somite contributes to the neck; mandible elongate; legs unique, heavily sclerotized, represented by two pairs of acutely pointed processes. Conclusion With the addition of two new genera presently reported, the family Chondracanthidae currently includes 52 valid genera. Among the described genera Avatar gen. nov. seems to be very primitive, while Kokeshioides gen. nov. is highly advanced. The deduced evolutionary history of chondracanthid genera is also discussed.

The Chondracanthidae is one of the most speciose copepod families, that utilize fishes as hosts accommodating nearly 192 species in 50 valid genera [2].Among them, 27 genera are monotypic and only two genera have more than ten species, Acanthochondria Oakley, 1930 with 54 valid species, and Chondracanthus Delaroche, 1811 with 41 valid species [2][3][4][5][6].Molecular analyses are absolutely needed to confirm the validity of monotypic genera.
In the presently reported study, we describe two new genera and species collected from two species of deep-sea fishes in Suruga Bay, Japan.

Materials and Methods
Specimens of chondracanthids were collected from the buccal cavity of two species of deep-sea fishes (fish hosts were frozen), Chaunax abei Le Danois, 1978 (Lophiiformes: Chaunacidae) and Setarches longimanus (Alcock, 1894) (Perciformes: Setarchidae), caught at a depth of 212 m in Suruga Bay, Japan (34°37′48.87″N, 138°43′2.958″E).Methods for preservation, dissection, mounting, and drawing of appendages were according to the techniques described in Aneesh et al. [18][19][20][21].The specimens were microphotographed using Olympus microscopes (Olympus SZX7 and Olympus Bx50, Olympus Co., Ltd.) and image-capturing software (DP2-SAL, Olympus Co., Ltd).Total body length was measured (without egg sacs) from the anterior margin of the cephalothorax to the distal end of the caudal rami.Drawings were digital-inked using Adobe Illustrator and a WACOM CTL-472/K0-c drawing pad.Morphological terminology follows Huys and Boxshall [22].The taxonomy and nomenclature of host fishes were adopted from Catalogue of Fishes [23] and FishBase [24].The type material is deposited in the National Museum of Nature and Science, Tsukuba, Japan.

Order Cyclopoida Burmeister, 1834 Family Chondracanthidae Milne Edwards, 1840
Genus Avatar gen.nov.Diagnosis based on the adult female (bold = key features).Body small, flattened.Cephalothorax and first pedigerous somite fused forming cephalothorax.Cephalothorax, slightly wider than long with anterior pair of large and posterior pair of small lobes, and two pairs of ventro-lateral processes; latter processes located at level of mouthparts.The very posterior most part of the first Etymology: The generic name is derived from a worldfamous epic science fiction film, James Cameron's "Avatar", in which the dragon-like aerial predator "Mountain Banshee" with two pairs of wings reminds us of the present new taxon with two pairs of lateral processes on the trunk.Gender feminine.
Remarks: Avatar can readily be separated from all other known chondracanthid genera by the following combination of features: (1) cephalothorax, slightly wider than long with anterior pair of large and posterior pair of small lobes, and two pairs of lateral processes; (2) posteriormost part of the first pedigerous somite contributes to the neck; (3) cylindrical trunk with two pairs of blunt, proximally fusiform processes; (4) antennule with small terminal knob; (5) antenna distal endopodal segment represented by the atrophied tip, slender segment with three apical elements; (6) labrum protruding ventrally; (7) two pairs of biramous legs, each with 2-segmented rami.
Two pairs of biramous legs (Fig. 5C); protopod unarmed; exopod and endopod 2-segmented, armed with setae.Other legs absent.Leg 1 (Fig. 4A), located near maxilliped; first exopodal segment with an outer seta; second segment with nine setae along free margin; first endopodal segment Etymology: The specific name of the new species, 'nishidai', is dedicated to Mr. Yusuke Nishida (Hiroshima University) who found this enigmatic chondracanthid in the Suruga Bay, Japan.It is a noun in the genitive case.
Diagnosis based on the adult female (bold = key features).Body small, flattened.Cephalothorax fused with first pedigerous somite forming cephalothorax.Cephalothorax much wider than long, with lobate anterior and posterior corners, folded ventrally.Posteriormost part of the first pedigerous somite contributes to the neck.Third and fourth pedigerous somites fused forming shieldlike trunk, without lateral processes; trunk anteriorly narrower at transition to neck.Antennule fleshy, swollen, with many spinules.Antenna uncinate, without accessory process.Mandible elongate, falcate and bilaterally denticulate.Maxillule armed with one element.Maxilla 2-segmented; basis curved, slender, with attenuate process bearing row of teeth.Maxilliped 3-segmented, first segment robust, unarmed; second segment with a patch of minute spinules on inner edge; distal segment, small, ending in curved claw-like structure with small, subterminal hooklet on inner surface.Legs 1 and 2, represented by pairs of acutely pointed processes.Other legs absent.Genital double-somite, highly reduced with paired genital apertures on ventral surface.Caudal rami bearing 1 long and 3 short setae.Egg sac unknown.
Etymology: The generic name is derived from a Japanese traditional wooden toy called "Kokeshi" and the Latin suffix -oides meaning "like".Gender masculine.
The presence of two pairs of highly modified spiniform legs makes Kokeshioides gen.nov.unique among all known chondracanthid genera.The genera Humphreysia and Immanthe show some similarity to Kokeshioides gen.nov., especially in general body shape and the absence of trunk processes (see Table 1).In addition to the key diagnostic   Host: Known only from Setarches longimanus (Alcock, 1894) (Setarchidae).Distribution: Known only from the type locality.Etymology: The specific name is derived from the type locality, Suruga Bay, Japan.It is in the nominative singular, gender masculine.

Discussion
The family Chondracanthidae was thoroughly revised by Ho [25], who excluded 12 old genera and provided a key to the 30 valid genera recognized at that time.Twelve genera were added subsequently by various authors which Ho [26] included in his revised key.Three genera were subsequently synonymised [27].After the revision of Ho [26] eight new genera have been established, the latest addition of which being the discovery of Ttetaloia Uyeno and Nagasawa, 2012 from Japan and the three previously excluded genera (viz.Immanthe Leigh-Sharpe, 1934, Lernaeosolea Wilson, 1944, andPharodes Wilson, 1935) were restored, based on unique combinations of generic characters [2,6,26,27].With the addition of the two new genera in the presently reported study, Avatar gen.nov.and Kokeshioides gen.nov., the family Chondracanthidae currently includes 52 valid genera.
Many chondracanthid genera possess usually paired ventrolateral processes on the 3 or 4 pedigerous somites; for instance, members of the genus Chondracanthus have well-developed trunk processes, and their number may vary in different species.Our observation of Chondracanthus kabatai Aneesh, Helna, and Kumar, 2020 reveals that these ventrolateral processes contain oviducts, providing space for the maturing oocytes, and thereby enhancing the fecundity.The majority of chondracanthid genera possess an uncinate, strongly prehensile antenna; few others have a bifurcate, trifurcate or clavate (non-uncinate type) antenna and some relatively primitive chondracanthid genera such as Avatar gen.nov., Humphreysia, Immanthe, Juanettia etc., possess the distal endopodal segment, represented by the atrophied tip of the antenna as a slender segment with 2-6 apical elements [28,29].The structure seems to have been lost during evolution.For instance, it is totally absent in highly advanced genera such as Acanthochondria, Chondracanthus, Kokeshioides gen.nov.etc.In contrast, the morphology of the mouth appendages seems to be relatively constant within the family, suggesting that most species share a similar feeding mode [21,25,26].
Various evolutionary trends can be observed in the development of legs, especially legs 1 and 2 in the family.Legs are highly modified or rudimentary in females chondracanthids and vary in different genera: only one biramous 2-segmented leg is present in primitive chondracanthids such as Juanettia, Auchenochondria Dojiri and Perkins, 1979 and  [25,26]).The highly modified, spiniform legs in Kokeshioides gen.nov.can be considered the most advanced state in the Chondracanthidae.The segmentation of the legs is clear in the relatively primitive genera such as Cryptochondria Izawa, 1971, Juanettia and Rhynchochondria Ho, 1967.Between these most advanced and primitive conditions, a wide variety of intermediate conditions can be seen: the segmentation of the rami of the legs is lost in some species of Protochondria Ho, 1970, reduction in size of the rami of legs in Brachiochondrites Markevich, 1940, Blias, Chondracanthodes Wilson, 1932 or just in the size of the endopod in Humphreysia (see Kabata [30]).Along with the size reduction, the number and size of setae are also reduced in Pseudacanthocanthopsis Yamaguti and Yamasu, 1959 and some species of Protochondracanthus Kirtisinghe, 1950, and seta are totally absent in many advanced genera in Blias, Chondracanthus, Chondracanthodes, Pseudacanthocanthopsis, Pseudoblias Heegaard, 1962, Acanthochondria, and Acanthochondrites Oakley, 1930.The legs will be either totally absent or highly modified, represented by setae or spiniform processes during the course of evolution (in Kokeshioides gen.nov.) (see Kabata [30]).Throughout the evolution of the family, legs might have been reduced into functionless elements or modified new functional grasping organs.

Conclusions
New material collected from two different species of deepsea fishes of Suruga Bay, Japan were found to be different from all other known chondracanthid genera, and based on the clear morphological features we described two new monotypic genera.Accordingly, we described Avatar nishidai gen.et sp.nov.from Chaunax abei Le Danois, 1978 (Chaunacidae) and Kokeshioides surugaensis gen.et sp.nov.from Setarches longimanus (Alcock, 1894) (Setarchidae).By the description of two new genera in the presently reported study, the family Chondracanthidae currently includes 52 valid genera.Among the described genera Avatar gen.nov.seems to be very primitive, while Kokeshioides gen.nov. is highly advanced.The deduced evolutionary history of chondracanthid genera is also discussed in the present paper.

Table 1
Character differences between the closely related chondracanthid genera,

pedigerous somite contributes to the neck. Third and fourth pedigerous somites fused into cylindrical trunk, bearing two pairs of blunt proximal fusiform processes. Antennule with large, fleshy, basal and terminal portion with few small elements. Antenna uncinate, distal endop- odal segment represented by atrophied tip of antenna, a slender segment with five elements. Labrum protruding ventrally.
Mandible falcate and bilaterally denticulated.Maxillule armed with two elements.Maxilla 2-segmented, terminal segment curved, slender, and with attenuated process bearing row of teeth.Maxilliped 3-segmented, armature of the typical chondracanthid type.

Two pairs of biramous legs, each with 2-segmented rami. Other legs absent. Genitoabdomen
with paired genital apertures on ventral surface.Caudal rami small, bearing one long and three short elements.Egg sac multiseriate.