Axine Abildgaard, 1794

Nitta, Yusuke Kamio and Masato, 2025, Redescription of Axine japonica (Monogenea: Axinidae) from New Exocoetid Hosts in Japan, Species Diversity 30 (2), pp. 165-175 : 168-171

Genus Axine Abildgaard, 1794 Axine japonica Price, 1945 ( Fig. 1 )

Axine ( Axine) cypseluri Yamaguti, 1940: 54–55 ; Yamaguti 1943a: 120 (list); Kamegai and Ichihara 1972: 103 (list; recorded under this name).

Axine cypseluri ( Meserve, 1938) : Sproston 1946: 454 (in part).

Axine japonica Price, 1945: 22 ; Price 1962: 4, figs 2, 15, 27, 39, 51; Yamaguti 1963: 255; Kurochkin 1980: 287 (list).

Redescription. Body elongate, tapering anteriorly, 2625–9450 (6428, n = 17) long including haptor, 450–1750 (839, n = 18) wide at level of germarium ( Fig. 1A ). Haptor asymmetrical, 1075–2850 (1983, n = 17) long, 150–525 (371, n = 17) wide, armed with single row of 47–77 (61, n = 17) clamps along truncate posterior extremity of body ( Fig. 1A ).

Clamps of equal structure, typical Microcotyle - type, each clamp 25–67 × 45–107 (47 × 75, n = 20). Posterior jaw and anterior jaw curved inward, dorsal arm of median spring short, ventral arm of median spring long, both arms of the median spring bifurcated at the tip. Small accessory sclerites extend inward from the proximal end of the anterior jaw ( Fig. 1A, F ).

Haptoral lappet small, embedded generally in middle of haptor, armed with one pair of anchors and one pair of crochet en fléau ( Fig. 1D, E ). Anchors and crochets en fléau situated at 298–385 (390, n = 14) from posterior end of haptor. Anchors 45–57 (51, n = 5) long, with sickle-blades and slender roots; crochets en fléau 26–50 (44, n = 5) long with small lunate blades and long slender shafts. Deeply stained small round cells present on either side of non-stained strip in the haptor, forming a broad area extending laterally.

Mouth opening anterior terminal. Pair of oral suckers circular, 40–75 × 35–67 (57 × 48, n = 20). Pharynx subspherical, lying behind oral suckers, 35–100 × 30–60 (56 × 40, n = 20). Esophagus narrow, connecting intestinal ceca, bifurcating posterior to the genital atrium. Intestinal ceca blind, extending into haptor, with numerous lateral diverticula ( Fig. 1A ).

Testes ( Fig. 1A, B ) irregular in shape, 27–45 (37, n = 14) in number, pre-ovarian, arranged in the posterior half of body and confined to intercrural field. Vas deferens ( Fig. 1A, B ) long, coming from anterior testes on left side of the body at level posterior of oötype, running forward along body midline, entering base of cirrus. Cirrus muscular, 50–57 (53, n = 4) long, when protruded, armed at its base with 10–17 (13, n = 11) small spines. Prostate cells around genital atrium observed in three of 15 stained specimens. Genital atrium ( Fig. 1A–C ) armed with 3 groups of spines on pad-like muscle bases; 8–14 (11, n = 9) spines of median group sharp, conical, in a single concentric circle; 12–24 (18, n = 12) spines of lateral groups arranged in two rows.

Germarium ( Fig. 1A, B ) J-shaped, beginning on center of body, extending to posterior of left side of body, curved anteriorly and then extended toward oviduct. Oviduct ( Fig. 1B ) extending anteriorly from distal end of germarium to anterior to body, turned posterior to body and then extend- ed toward oötype. Genito-intestinal canal ( Fig. 1B ) originating from right intestinal branch, connecting to vitelline duct. Vitelline duct ( Fig. 1B ) unpaired, extending from genito-intestinal canal to oviduct. Receptaculum seminis ( Fig. 1A, B ) irregular oval, anterior to germarium, connecting to oviduct. Oötype elongate, expanding from oviduct to level of anterior end of germarium ( Fig. 1B ). Uterus originating from oötype, running anteriorly along body midline, ventral to vas deferens, opening anterior to genital atrium ( Fig. 1B ). Vaginal pore dorsolateral, armed with horn-like spine, posterior to genital atrium. Vagina with thick muscular walls and shaped like bell of trumpet ( Fig. 1A, B ), dorsal to uterus and vas deferens, arising from vaginal pore, connecting to oviduct. Egg not observed. Vitelline follicles coextensive with intestinal branches, extending from level of posterior to vagina to haptor, fused posterior to testes and extending to median region of haptor. Vitelline duct not observed.

Representative DNA sequences. Newly obtained cox1 sequences from specimens collected off Danjo Island, Nagasaki Prefecture ( MPM Coll.-No. 25357), and Iwami Town , Tottori Prefecture ( MPM Coll.-No. 25358), were deposited in the INSDC under accession numbers LC799034 and LC799035 , respectively .

Partial 28S rDNA sequences obtained from specimens collected in Miyazu City, Kyoto Prefecture ( MPM Coll.-No. 25359), Tosashimizu City , Kochi Prefecture ( MPM Coll.-No. 25359), and Iwami Town , Tottori Prefecture ( MPM Coll.- No. 25358), have been deposited under accession numbers LC799036, LC799037 , and LC799038 , respectively .

Partial 18S rDNA sequences from specimens collected in Minami-Ise Town, Mie Prefecture ( MPM Coll.-No. 25360), and Iwami Town , Tottori Prefecture ( MPM Coll.-No. 25358), were deposited under accession numbers LC872843 and LC872844 , respectively .

Remarks. The present specimens collected from the C. poecilopterus and C. hiraii in Japanese waters show the diagnostic morphological characteristics of Axine provided by Price (1962) and agree approximately with the descriptions of A. japonica by Yamaguti (1940). The holotype and paratypes of the species, as described by Yamaguti (1940), are in a good state and possess common morphological characters described by Yamaguti (1940). However, the lengths of the oral suckers, pharynx, and genital atrium were longer than Yamaguti’s measurements. These differences in measurements are likely attributable to the larger size of our newly collected specimens compared to the type specimens, as well as the effects of specimen preparation. Additionally, developed prostate cells around the genital atrium of the type specimens were observed, but were not observed in some of our specimens. These differences may be due to intraspecific variation or fixative methods (Yamaguti used Shaudinn’s fixative: Yamaguti 1943b) because the other diagnostic morphological characteristics of our specimens agree well with the descriptions of A. japonica by Yamaguti (1940). Morphological measurements of Axine species are presented in Supplementary Table 1.

Axine japonica most closely resembles A. belones regarding the spination of the genital atrium ( Price 1962; Strelkov 1964; Châari et al. 2016; Bouguerche et al. 2023). However, A. japonica differs in its thick, muscular vaginal wall and the absence of transverse vitelloducts ( Bouguerche et al. 2023). The vaginal pore of A. japonica is armed with a horn-like spine, while the vaginal pore of A. buccina is not armed with a spine ( Kritsky and Bakenhaster 2022). The number of clamps of A. japonica (47–77) differs from A. cypseluri (41: Meserve 1938), A. depauperate (22–31: Yamaguti 1968), A. hemirhamphae (80–84: Tripathi 1959), A. inada (80–100: Ishii and Sawada 1938), and A. parini (39: Gichenok 1977). The number of testes separates A. japonica (27–45) from A. cypseluri (62: Meserve 1938), A. hyporhampi (17: Price 1962), A. ibanezi (more than 70: Tantaleán 1974), A. inada (62: Ishii and Sawada 1938), A. parini (numerous: Gichenok 1977), and A. yamagutii (52: Meserve 1938). This species differs from A. depauperate by the number of spines of the median group (8–14 vs. 26–28: Yamaguti 1968), and from A. ibanezi and A. yamagutii by the spines on the lateral groups of the genital atrium (12–24 vs. 30–34 vs. 30: Tantaleán 1974; Meserve 1938, respectively). The genital atrium of A. japonica possesses 3 pad-like muscle bases, but that of A. spilonotopteri has a circular muscle base ( Yamaguti 1968). Axine japonica is also distinguished from A. hemiramphae by the shape of the anchor (sickle-blades and slender roots vs. Y shaped) and crochets en fléau (small lunate blades and long slender shafts vs. like a dactylogyrid marginal hook) ( Unnithan 1957).

Axine triglae Van Beneden and Hess, 1863 was originally described after being taken from the gills of Chelidonichthys lucerna (Linnaeus, 1758) (as Trigla hirundo ) ( Triglidae ) by Van Beneden and Hesse (1863). Sproston (1946) indicated that A. triglae was similar to A. belones but can be distinguished from A. belones by the shape and size of the haptor. Although, the original description of A. triglae was very inadequate and without figures, Price (1962) regarded this species as a species inquirenda because of the non-beloniform host and its inadequate description by Van Beneden and Hesse (1863).

The present redescription clarifies and extends the original description of A. japonica by Yamaguti (1940), providing a more comprehensive morphological diagnosis. Notably, the morphological characteristics such as the arrangement and number of spines in the genital atrium, number of clamps, and number of testes, were clearly described and compared with those of closely related species. These comparisons reveal several distinguishing characteristics, such as fewer spines in the median and lateral groups of the genital atrium compared to A. depauperate , A. ibanezi , and A. yamagutii , and differences in the number of clamps and testes from other Axine species. Although some measurements (e.g., oral sucker, pharynx, length of genital atrium) differ from the original description, these differences may be due to natural intraspecific variation. The absence of prostate cells in some of the present specimens is also worth noting, although this may lie within the range of intraspecific variability.

Molecular data analysis

The results of the BLAST search for the sequences of cox1 gene of A. japonica from C. poecilopterus obtained in this study are shown in Table 4. The closest hits were newly collected sequences of the cox1 gene of Ondoella sajori Horikawa, Nitta, and Kamio, 2025 ( Axinidae ) from Hyporhamphus sajori (Temminck and Schlegel, 1846) . The other close hits were Microcotyle caudata Goto, 1894 , M. sebastis Goto, 1894 , M. kasago Ono, Matsumoto, Nitta, and Kamio, 2020 , and M. pacinkar Kamio and Nitta, 2023 ( Microcotylidae ).

The trimmed multiple sequence alignment length of the 28S rDNA fragments consisted of 596 base pairs including gaps. The topologies of each constructed by ML and BI analysis were almost identical, and the phylogenetic trees based on ML analysis are shown in Fig. 2 . In the phylogenetic tree based on the 28S rDNA, sequences for A. japonica and O. sajori formed a clade with strong support. The Mazocraeidea are divided into two major clades. The clade includes the Chauhaneidae , Allodiscocotylidae , Mazocraeidae , Protomicrocotylidae , Gastrocotylidae , Gotocotylidae , Thoracocotylidae . The other clade consists of Axinidae , Heteraxinidae , Microcotylidae , Heteromicrocotylidae , Diplozoidae , Discocotylidae , Octomacridae , Macrovalvitrematidae , Diclidophoridae , Plectanocotylidae , Hexostomatidae . Axinidae , Heteraxinidae , Microcotylidae and Heteromicrocotylidae constituted a monophyletic group, and Axinidae was sister to the clade consisting of the other three families.

Figures

Fig. 1.　Axine japonica Price, 1945 from Hirundichthys oxycephalus. A, Whole body (ventral view, MPM Coll.-No. 25358); B, reproductive organs (ventral view, MPM Coll.-No. 25358); C, genital atrium (ventral view, MPM Coll.-No. 25358); D, anchor (MPM Coll.-No. 25357); E, crochet en fléau (MPM Coll.-No. 25357); F, clamp (left side, ventral view, MPM Coll.-No. 25359). Scale bar: A, 1mm; B, C, 50µm; D–F, 10µm.

Fig. 2. Bayesian inference (BI) tree for the Mazocraeidea based on partial 28S rDNA data (596 bp) using two species of Hexabothriidae (Diclybothriidea) as the outgroup. The corresponding INSD accession numbers are shown. The tree includes results for BI and maximum likelihood analyses with PP/BS branch support values.