A new species of Peniculus (Copepoda: Siphonostomatoida) parasitizing mesopelagic myctophid fish: first discovery of colonization of the genus in deep water

Peniculus hokutoae n. sp. is described on the basis of an ovigerous adult female parasitizing the caudal fin of the myctophid fish Symbolophorus evermanni (Gilbert, 1905), collected from Suruga Bay, Japan. This is the first record of parasitism by this genus on mesopelagic myctophid fish. The new species is easily distinguished from other congeners in: (1) the presence of a conical process anterior to the rostrum; (2) the secondary elongation of the first pedigerous somite; (3) the incorporation of the third and fourth pedigerous somites into the trunk; (4) the unilobate maxillule bearing two unequal apical setae; (5) the lack of any processes on the first segment of the maxilla. Four morphological patterns of the cephalothorax, neck and anterior parts of the trunk can be found in the genus. We infer that initial colonization of a mesopelagic myctophid fish as host is likely to have occurred when the diurnally-migrating myctophid host was feeding in near-surface waters at night and was exposed to infective stages of Peniculus.

During deep-sea zooplankton surveys in Suruga Bay, middle Japan by Tokai University, an undescribed species of the pennellid genus Peniculus infesting the caudal fin of a myctophid fish (Fig. 1A) was found, in addition to the recent discovery of a new pennellid genus and species Protosarcotretes nishikawai Ohtsuka, Lindsay & Izawa, 2018 [Japanese name: houraieso-no-namida (new), Japanese, meaning a tear drop of Pacific viper fish] parasitic on Pacific viper fish [20]. The present paper provides a taxonomic description of the

Materials and methods
A parasitic copepod specimen attached to the caudal fin of its host fish was collected in Suruga Bay (35 03 0 20 00 N, 138 41 0 00 00 E), Japan between 11:26 and 13:12 on July 12, 2018 in an oblique tow (0-917.9 m depth) of the ORI net (335 lm mesh, 1.6 m mouth diameter) [22] during cruise SRM-18-7-1 of the R/V Hokuto (Tokai University). The copepod specimen was photographed live before being preserved in 99.5% ethanol (see Figs. 1A and 1B). The only host fish in the plankton sample was identified as Symbolophorus evermanni (Gilbert, 1905) by reference to Nakabo [18]. The copepod was observed in lactophenol using a Humes & Gooding's [11] slide, and illustrated with the aid of a drawing tube attached to an Olympus microscope. Copepod terminology follows Huys & Boxshall [12]. The type specimen of the parasitic copepod and the host fish are deposited at the National Museum of Nature and Science, Tsukuba, Japan (NSMT).  [4] regarded the incorporation of one or more of the posterior pedigerous somites into the trunk as a key character in defining Peniculus, it was not mentioned in Castro-Romero's [7] diagnosis. After comparisons among related congeners (cf. Fig. 3), this relatively stable feature should be added to the generic diagnosis: pediger 4 or pedigers 3 and 4 incorporated into trunk.
The presence or absence of rami of legs is not mentioned in previous definitions of the genus, basically because the rami of the legs are typically missing in mature adult females. However, our observations of the new species described below have revealed remnants or scars of rami on the basis of the legs. This indicates that legs 1 and 2 are biramous, while legs 3 and 4 are uniramous. This configuration of the legs resembles that of other pennellid genera such as Sarcotretes and Lernaeenicus that retain the rami on the legs in the adult female (see Table 1 in [20]). Coloration. The specimen was still alive on collection, although the host was dead. The body coloration is based on this live specimen. The cephalothorax was translucent except for the naupliar eye (Figs. 1C and 1D). The trunk was entirely tinged with reddish brown with numerous pigmented droplets dorsally (Fig. 1B). Paired ovaries were yellowish in color (Fig. 1B). Eggs were light orange in color (Figs. 1A and 1B).
Remarks. The taxonomy of the genus Peniculus and related genera (Metapeniculus, Propeniculus Castro-Romero, 2014, Pseudopeniculus Castro-Romero, 2014) was revised by Castro-Romero [7] and Castro-Romero et al. [9]. The morphological features of the new species essentially fall within the diagnosis of Peniculus as redefined by Castro-Romero [7], although the structures of the maxillule, maxilla and legs differ slightly from those of other congeners.
The present new species is easily distinguished from other congeners by: (1) the secondary elongation of the first pedigerous somite (Fig. 3); (2) the third and fourth pedigerous somites are included in the trunk (Fig. 3); (3) the frontal expansion anterior to the rostrum; (4) unilobed maxillule bearing two terminal setae of unequal lengths; (5) the lack of a process on the first segment of the maxilla.
In the type specimen of the new species, some rami of legs still remain and the possibility of accidental detachment of the other rami during handling and observation of the specimen for this description cannot be ruled out.
Etymology. The new species is named after the research vessel Hokuto, of Tokai University.
Castro-Romero [7] [7] nine species and P. quadratus are listed as valid in the genus. However, the description of P. communis by Leigh-Sharpe [15] is inadequate by modern standards and the exact identity cannot now be confirmed. The absence of any sign of the buccal cone in P. communis indicates a possible similarity to Propeniculus, but in this species the protopods of legs 1-4 can be clearly seen in Leigh-Sharpe [15], while in Propeniculus no legs were observed. Peniculus clavatus was redescribed by Leigh-Sharpe and Perkins [16] and appears to differ rather remarkably from other congeners, in the presence of a special frontal organ and antennules (see Fig. 3 in [16]). After consideration of the position and shape of this ''frontal organ'', we infer that this structure is probably the grasping antennae which were misinterpreted. The morphology of the antennae and maxillules also differs greatly from that of other congeners. Given these problems, we believe that these two species should be treated as incertae sedis. As pointed out by many taxonomists, a systematic revision of Peniculus based on modern taxonomic techniques is needed.
Based on the seven well-described species, four morphological patterns of the cephalothorax, neck and anterior parts of the trunk can be found (Fig. 3). In P. minuticaudae, the cephalothorax and neck are not elongate, and only the fourth pedigerous somite is incorporated into the trunk. In P. hokutoae n. sp., the part of the cephalothorax representing the first pedigerous somite is secondarily elongated, and both the third and fourth pedigerous somites are involved in the trunk. In the other five species, elongation of the third pedigerous somite is found. In P. fistula, the third and fourth pedigerous somites are both incorporated into the trunk, while in the other four species, only the fourth is involved.

Host and distribution
The known hosts and geographical distributions of Peniculus species are summarized in Table 1. Surprisingly,  P. fistula (including P. cf. fistula sensu Castro-Romero et al. [9]) infests a wide range of hosts (two superorders, six orders, 20 families, 42 species) occurring across the world. Castro-Romero et al. [9] reported low genetic diversity of the populations of P. cf. fistula parasitic on nine species of hosts occurring off the coast of Chile, irrespective of some intraspecific morphological variation. In contrast, other species exhibit relatively limited host-specificity in a restricted geographical area. For example, the hosts of P. minuticaudae and P. truncatus seem to be restricted to East Asian fish belonging to the Monacanthidae and Sebastidae, respectively. One exceptional case in P. minuticaudae was parasitism on brown-banded butterflyfish Rao modesta, but this seems to have occurred in the artificial conditions of an aquarium tank [21]. It is most likely that the host-usage of Peniculus species other than P. hokutoae n. sp. is restricted to coastal or epipelagic fish. In contrast, the host of P. hokutoae n. sp. is a mesopelagic myctophid Symbolophorus evermanni [18]. This is the first record of the occurrence of any Peniculus on a member of the family Myctophidae. This host family is frequently utilized by deep-sea pennellids including species of Sarcotretes, Lernaeenicus and Cardiodectes [20], but never before by a Peniculus. This parasitism could be explained by the following evolutionary scenario. Myctophids are mainly distributed in meso-and bathypelagic zones, and many species exhibit diel vertical migration [18,26,27]. Symbolophorus evermanni is one of the dominant myctophid fish that occur nocturnally in surface waters of the Kuroshio region in the northwestern Pacific (Watanabe and Kawaguchi [26]). The myctophid is distributed at depths of 600-1150 m during daytime and upper 125 m at night [18,26,27] and can be ecologically classified as a migrant [27]. Exposure to infective stages of the Peniculus could have occurred nocturnally while the hosts were in the surface waters. Once on the new host, the copepod would eventually have adapted to its deep-sea myctophid host.