A new genus and species of the family Pennellidae (Copepoda, Siphonostomatoida) infecting the Pacific viperfish Chauliodus macouni

A new genus and species of pennellid copepod, Protosarcotretes nishikawai n. g., n. sp., is described on the basis of an ovigerous female infecting a Pacific viperfish Chauliodus macouni collected from the deep-waters of Suruga Bay, Japan. The new genus exhibits the most plesiomorphic states in the first to fourth legs of pennellids, and is differentiated from two closely related pennellid genera Sarcotretes and Lernaeenicus by the morphology of the oral appendages. Two species of the genus Lernaeenicus are transferred to the new genus as Protosarcotretes multilobatus (Lewis, 1959) n. comb. and Protosarcotretes gnavus (Leigh-Sharpe, 1934) n. comb. The host specificity and life cycle of deep-sea pennellids are discussed. Sarcotretes scopeli Jungersen, 1911 and Cardiodectes bellottii (Richiardi, 1882) show low differentiated host-specificity, while P. nishikawai seems to be limited to the Stomiidae, which are rare hosts of pennellids, in contrast to the Myctophidae family. In the Pennellidae family, two patterns of the life cycle are found: with or without naupliar stages.

During a survey on the deep-water plankton of Suruga Bay, Japan, an undescribed pennellid copepod was discovered on the Pacific viperfish Chauliodus macouni, 1890, Bean (Fig. 1A, B). This animal generally resembles three pennellid genera, Sarcotretes, Lernaeenicus and Peniculus, placing it within the family Pennellidae as defined by Boxshall [4], although the first two genera appear taxonomically confused. In Sarcotretes and Lernaeenicus, the neck (see "ne" in Fig. 1) is composed of the first to fourth pedigerous somites, while in Peniculus, the fourth pedigerous somite is incorporated into the trunk [8]. According to the keys to pennellid genera provided by Kabata [23] and Boxshall & Halsey [8], a feature distinguishing these two genera is the presence (in Sarcotretes) or absence (in Lernaeenicus) of a middle constriction of the neck. However, this is not applicable to all species of the former. For example, Sarcotretes longirostris Ho, Nagasawa, & Kim, 2007 bears a slender neck without a constriction midway (see Fig. 1A in Ho et al. [17]). On the other hand, Lernaeenicus also seems to be a catch-all group when the morphological variability in the cephalosomes, abdomens and legs is considered. Some species of Lernaeenicus bear a well-developed abdomen, while in others it is highly reduced like in Sarcotretes. In many species, legs 3 and 4 are uniramous, while in L. multilobatus Lewis, 1959 they are biramous. Castro Romero [11] provided a different key to pennellid genera, and suggested that the key characteristics differentiating these two genera are the morphology of the cephalic holdfasts, proboscis and labium.
The present paper deals with the taxonomy of the undescribed pennellid copepod parasitizing the Pacific viperfish, and discusses the validity of the genera Sarcotretes and Lernaeenicus.

Materials and methods
The present specimens (a parasitic copepod attached posterior to the right eye of its host fish) were captured in Suruga Bay (35°02.3'N, 138°40.5'E) between 12:21-13:51 on September 8, 2017 in an oblique tow (0-810 m depth) of an ORI net (335 mm mesh, 1.6 m mouth diameter) during cruise SRM17-9-VPR of the T/V Hokuto (Tokai University). The specimens were photographed live before being preserved in 99.5% ethanol (see Fig. 1). The host fish was identified as Chauliodus macouni Bean, 1890 by reference to Nakabo [29].
The parasitic copepod was removed from the host tissue and then partly dissected in lactophenol with a pair of fine needles under a dissecting microscope (SZX7, Olympus Co., Ltd.). The body and appendages were examined in lactophenol and drawn with the aid of a camera lucida attached to a compound microscope (BX53, Olympus Co., Ltd.). The specimens were deposited in the Kitakyushu Museum of Natural History and Human History (KMNH). Terminology follows Huys & Boxshall [19] and Ho et al. [17]. Etymology. The new generic name is derived from proto (Greek prefixed, meaning primitive) and a closely related genus Sarcotretes, and refers to the primitive condition, especially in the segmentation and setation of legs 1-4, of the new genus. Gender masculine.

Results
Diagnosis. Body straight, without brush-like structure on abdomen. Cephalothoracic holdfast represented by pair of lateral expansions. Oral cone weakly produced anteroventrally to form proboscis. Neck comprising pedigers 2-4, first urosomite and anterior part of trunk. Trunk cylindrical; abdomen highly reduced; caudal rami present, bilobate with 2 and 4 setae, respectively. Egg string uniseriate. Total length ca. 10 mm.
Antennule indistinctly 4-segmented. Antenna 3-segmented, heavily sclerotized; second segment produced at subterminal corner into stout triangular process; third segment curved inward to form subchela with process of preceding segment, bearing minute basal seta. Mandible simple stylet-like, with no teeth distally. Maxillule unilobate, inner lobe with 2 terminal setae; outer lobe absent. Maxilla 2-segmented; first segment with no accessory process; second segment bearing 4 rows of spinular prominences on calamus.
Remarks. Once both Sarcotretes and Lernaeenicus are rigidly defined, it is evident that the establishment of a new genus for the present material is warranted. However, since many taxa belonging to these genera were poorly described in the 18th and at the beginning of the 19th centuries, the definitions below are still tentative and await a complete revision (see Raja et al. [38]).
The new genus described here shows many plesiomorphies in the oral cone and legs (see Boxshall [4]), but some states in the mandible, maxillule and maxilla can be regarded as apomorphic. Although Lernaeenicus multilobatus Lewis, 1959 parasitic on the angler-fish Gigantactis sp. (Gigantacinidae), was poorly described by Lewis [27], it can be assigned to the new genus by: (1) the holdfast composed of a pair of cephalothoracic lateral expansions, (2) the abdomen being highly reduced, and (3) leg 4 being biramous. Lernaeenicus gnavus Leigh-Sharpe, 1934 was poorly described on the basis of a single adult female with a damaged cephalothorax, in which the oral cone cannot be seen in Fig. 35 of the original description [26]. However, the morphological and ecological features suggest that it is probably assignable to the new genus we describe: (1) the abdomen is reduced; (2) the body length is about 10 mm, regardless of the damaged cephalothorax; (3) the host fish Polyipnus spinosus Günther belongs to the deep-sea family Sternoptychidae.
An evolutionary trend in reduction of segmentation of the legs is distinct in adult females of the Protosarcotretes-Sarcotretes-Lernaeenicus lineage (present study). Similar patterns can be found in the legs of parasitic copepod families such as Chondracanthidae, Pandaridae, and Hatschekiidae [23]. Generally, anterior legs are relatively conserved and show full segmentation in their rami, while posterior legs tend to have the number of segments reduced, finally leading to a vestigial condition (Table 1).
Remarks. The new species is easily distinguished from its poorly described congener, P. multilobatus (Lewis, 1959) by the morphology of the holdfast: simple in the former and ramified in the latter. It differs from P. gnavus (Leigh-Sharpe, 1934) by the relative length of the trunk to the cephalothorax and neck combined (2.2 times in P. nishikawai n. sp. vs ca. 0.7 in P. gnavus).

Discussion
Members of the parasitic family Pennellidae have successfully colonized the deep-sea [5,8,49]. Colonization of pennellids into the deep-sea seems to have occurred repeatedly, because the most basal genus Peniculus is a shallow-water taxon [4,23,47] and more derived groups are  Protosarcotretes multilobatus (Lewis, 1959) Gigantactinidae Gigantactis sp. [27] Exopenna crimmeni (Boxshall, 1986) Moriidae Antimora rostrata (Günther) [4] composed of a mixture of shallow-and deep-water taxa [4,23]. Host-parasite relationships in deep-sea taxa in the family are shown in Table 2. Sarcotretes and Protosarcotretes seem to be limited to deep waters, while only a few members of Lernaeenicus and Cardiodectes infect deepsea fish. As already pointed out by Boxshall [5] and Boxshall & Halsey [8], Sarcotretes scopeli and Cardiodectes bellottii (Richiardi, 1882) (as C. medusaeus (Wilson, 1908) exhibit low host-specificity, utilizing a wide range of fish families or genera. Sarcotretes scopeli infects eight families of fish, while C. bellottii parasitizes only Myctophidae. The Stomiidae host family utilized by P. nishikawai has only rarely been reported as a host of pennellids in contrast to the family Myctophidae. It is interesting to note that Stomiiformes is generally thought to be basal relative to the Myctophidae [45], mirroring the condition in their copepod parasites. Visual observations with a Remotely-Operated Vehicle (ROV) have clearly recorded ectoparasitism of siphonostomatoid copepods such as Lernaeopodidae and Sphyriidae on deep-sea demersal fish, but not yet for Pennellidae [37]. This may partly be due to the relatively small-size of their bodies and partly due to their low abundances in the deep-sea. The life cycle of Protosarcotretes is unknown, but can be deduced on the basis of that of other pennellids, especially Lernaeenicus sprattae (Sowerby, 1806) [1,7,10,16,20,24,36,39,40,41,44,49]. Basal pennellid groups such as Peniculus, Sarcotretes, Lernaeenicus [4,23] and Protosarcotretes may be characterized by the possession of a single host [7,20,23]. However, the number of developmental stages depends on the taxon. In Peniculus, Lernaeenicus and Peroderma two patterns with or without naupliar stages are recognized. i.e., 2 nauplii, 1 copepodid, 4 chalimi, and adult (inL. sprattae), and 1 copepodid, 4 chalimi, and adult (in Peroderma cylindricum Heller, 1868, Peniculisa shiinoi Izawa, 1965, and Peniculus minuticaudae Shiino, 1958) [2,20,21]. In P. cylindricum and P. shiinoi, only hatching stages were observed [2,21], while in L. sprattae and P. minuticaudae all post-embryonic developmental stages were fully described [20,39]. In deep-sea pennellids, the hatching stage is an infective copepodid in C. bellottii (Richiardi, 1882) [36], but is unknown in Sarcotretes and Protosarcotretes. Clarification of the life cycle would be possible if embryos developed to before the hatching stage were to be found inside the egg strings.