Microcotylidae Taschenberg, 1879

Family Microcotylidae Taschenberg, 1879 View in CoL Subfamily Microcotylinae Taschenberg, 1879 Genus Microcotyle Van Beneden and Hesse, 1863 Microcotyle elegans Goto, 1894

( Figs 1–2 View Fig View Fig )

Microcotyle elegans Goto, 1894: 188–189 View in CoL , pl. I, fig. 4, pl. V, fig. 2; MacCallum and MacCallum 1913: 230–231 (list); Meserve 1938: 51 (list); Yamaguti 1943a: 20 (list); Yamaguti 1943b: 119 (list); Sproston 1946: 431 (list); Tripathi 1954: 241 (list); Yamaguti 1963: 242 (list); Unnithan 1971: 387 (list); Mamaev 1986: 200 (list); Mamaev 1989: 34 (list).

Redescription. Body ( Fig. 1 View Fig ) elongated, 2225–5600 (3989, n = 20) long including haptor, 275–925 (570, n = 22) wide at level of germarium. Haptor wedge-shaped, subsymmetrical, with 48–95 (68, n = 14) clamps, arranged in 2 subequal lateral rows. Clamps ( Fig. 2C View Fig ) of equal structure, each clamp 50–90 × 75–135 (75 × 116, n = 14) in diameter. Clamps of Microcotyle - type, formed by two jaws. Ventral arm of median spring long, thin, ends distally in an inverted T, with short branches. Lateral sclerites of ventral jaw approaching midline distally. Dorsal arm of median spring inverted Y- shaped. Posterolateral sclerite and anterolateral sclerites curved toward inside.

Pair of buccal suckers ( Fig. 1 View Fig ) septate, elliptical, 65– 120 × 70–125 (89 × 90, n = 21). Pharynx ( Fig. 1 View Fig ) globular, immediately posterior to buccal sucker, sometimes overlapping buccal sucker, 70–95 × 55–105 (84 × 73, n = 20). Esophagus ( Fig. 1 View Fig ) short, without lateral diverticula. Intestinal bifurcation ( Fig. 1 View Fig ) behind genital atrium. Intestinal ceca ( Fig. 1 View Fig ) blind, extending to haptor, with numerous lateral diverticula, not united posteriorly.

Testes ( Figs 1 View Fig , 2A View Fig ) with irregular shape, 16–28 (22, n =21) in number, post-ovarian, intercaecal, in posterior half of body. Vas deferens ( Fig. 2A View Fig ) long, narrow, coming from anterior of testes, ventral to germarium, extending anteriorly, dorsal to uterus along its dorsal side, entering base of genital atrium. Genital atrium ( Figs 1 View Fig , 2B View Fig ) mushroom-shaped, 190–400 × 150–300 (289 × 218, n = 21), located posterior to pharynx, with lateral expansions, surrounded by radiate muscle fibers. Genital atrium comprises anterior atrium proper and a posterior pocket. Genital atrium proper bearing numerous small spines, 161–210 (189, n = 8) in number. Edge of genital atrium opening and its inner walls armed with numerous conical spines, 5–7 (6, n = 22) in length. Spines more dense in central anterior region, less dense in lateral pockets; no spines present in center of posterior pockets; lateral margin with row of longer spines, 7–10 (9, n = 10) long in posterior pockets.

Germarium complex ( Figs 1 View Fig , 2A View Fig ) begins at level of anteriormost testes, continuing anteriorly in midline, reflexing approximatively at level of confluence of vitelline ducts, reflexing again toward anterior extremity, forming an equally thin anterior curve and reflexing a last time posteriorly before ending as oviduct, 350–800 (556, n = 12) long, 270–750 (382, n = 12) wide. Oviduct ( Figs 1 View Fig , 2A View Fig ) wide, twisted, dorsal to receptaculum seminis, arising from distal end of germarium, extending towards anterior of testes, connected to receptaculum seminis and vitelline duct. Genito-intestinal canal ( Figs 1 View Fig , 2A View Fig ) originates from right intestinal cecum and connecting to receptaculum seminis. Receptaculum seminis ( Figs 1 View Fig , 2A View Fig ) tubular shaped, extending from genito-intestinal canal to oviduct and vitelline duct. Vitelline duct ( Figs 1 View Fig , 2A View Fig ) Y-shaped, ventral, connecting to anterosinistral side of receptaculum seminis, bifurcating to either side near the germarium.

Oötype ( Figs 1 View Fig , 2A View Fig ) extending from ovovitelline duct. Mehlis’ gland not observed. Uterus ( Figs 1 View Fig , 2A View Fig ) originating from oötype, extending anteriorly along body midline, ventral to vas deferens, to the opening of the genital atrium. Vaginal pore ( Fig. 2A View Fig ) unarmed, ventral in midbody, posterior to genital atrium. Vaginal duct ( Fig. 2A View Fig ) dorsal to uterus and vas deferens, arising from vaginal pore, bifurcating posterior to vaginal pore, connecting to vitelline duct. Eggs ( Fig. 2D View Fig ) fusiform 180–375 × 55–155 (305 × 120, n = 3) excluding filament, with filaments at both ends broken. Vitelline follicles ( Fig. 1 View Fig ) coextensive with intestinal branches, extending from behind genital atrium to posterior end of body, fused posterior to testes.

Material examined. Twenty-two monogenean specimens from off Tatsugahama, Arita City ( MPM Coll.-No. 25326), and one specimen off Minami-Ise , Mie Prefecture ( MPM Coll.-No. 25327) .

Localities. The Seto Inland Sea off Tatsugahama, Arita City, Wakayama Prefecture, and Pacific Ocean off Minami-Ise, Mie Prefecture in Japan .

Host. Scombrops boops (Houttuyn, 1782) .

Site of infection. Gill filaments.

Representative DNA sequences. Newly obtained cox1 sequences (918 bp and 928 bp) from specimens collected in Arita City, Wakayama Prefecture ( MPM Coll.-No. 25326) and Minami-Ise Town , Mie Prefecture ( MPM Coll.- No. 25327) were deposited under DDBJ accession number LC815758 and LC865998 , respectively . The genetic variation of cox1 between the two specimens was 1.23% (11/895 bp) with synonymous codon substitutions at the third codon position. A sequence for the partial 28S rDNA obtained from a specimen collected in Minami-Ise Town ( MPM Coll.-No. 25327) have been deposited in the DDBJ under LC865999 View Materials .

Molecular data analysis. The trimmed multiple sequence alignment of cox1 fragments consisted of 361 positions. Sequences of two microcotylids, Bivagina pagrosomi ( Murray, 1931) and Polylabris halichoeres Wang and Zhang, 1998 , were used as outgroups ( Fig. 3 View Fig ). The topologies of each phylogenetic tree constructed by ML and BI analysis were almost identical, and the phylogenetic trees based on ML analysis are shown in Fig. 3 View Fig .

Sequences for seven species of Microcotyle formed a weakly supported clade. This clade separates into two groups: one consists of M. elegans and a clade including M. algeriensis Ayadi, Gey, Justine, and Tazerouti, 2017 and M. merche Víllora-Montero, Pérez-del-Olmo, Valmaseda-Angulo, Raga, and Montero, 2023 ; the other is comprised of the lineage including M. sebastis Goto, 1894 and M. pacinkar Kamio and Nitta, 2023 and one consisting of M. caudata Goto, 1894 and M. kasago Ono, Matsumoto, Nitta, and Kamio, 2020 ( Fig. 3 View Fig ). Microcotyle elegans formed a sister lineage with the clade containing M. algeriensis and M. merche , but this relationship was not supported by either analysis.

Remarks. The present specimens collected from S. boops in the Seto Inland Sea and Pacific Ocean conform to the descriptions and illustrations of M. elegans by Goto (1894). However, the number of the clamps is slightly larger than Goto’s description [ 48–95 in this study vs. about 50 in Goto (1894)].

Additionally, the distended receptaculum seminis between the genito-intestinal canal and the oötype was illustrated in the original description ( Goto 1894), but was not observed in our specimens. Goto (1894) may have misidentified the wide, twisted oviduct on the dorsal side as distended seminal vesicle. Furthermore, our observations provided additional details about the genital atrium: it is mushroom-shaped and surrounded by radial muscle fibers. Although Goto (1894) noted the presence of conical spines in the genital atrium, the shape of the genital atrium and surrounding structures were not described in his original account.

Microcotyle elegans View in CoL most closely resembles M. sebastis View in CoL and M. caudata View in CoL regarding shape of the haptor and the number of clamps ( Goto 1894; Yamaguti 1934, 1938, 1958; Bonham and Guberlet 1937; J. Zhang 2007; Ono et al. 2020). However, M. elegans View in CoL differs from M. sebastis View in CoL in the length of the genital spines (5–7 µm vs. 12–17 µm) (see Goto 1894; Yamaguti 1934; Bonham and Guberlet 1937). Microcotyle elegans View in CoL can be differentiated from M. caudata View in CoL by the shape of the genital atrium [mushroom-shaped (anterior atrium and posterior pocket) vs. kidney-shaped (only anterior atrium)] ( Goto 1894; Yamaguti 1934, 1938; Ono et al. 2020). Ishii and Sawada (1938a) suggested that M. mouwoi Ishii and Sawada, 1937 most closely resembles M. elegans View in CoL . However, M. elegans View in CoL is distinguished from M. mouwoi by the length of the genital spines (5–7 µm vs. 19–33µm) and the extension of the vitelline follicles (extending from behind genital atrium to posterior end of body vs. extending from behind genital atrium to posterior end of haptor) ( Ishii and Sawada 1938a).

Microcotyle elegans View in CoL can also be distinguished from M. fusiformis Goto, 1894 , M. tanago Yamaguti, 1940 , M. gimpo Yamaguti, 1958 View in CoL , and M. kasago , originally described from the Seto Inland Sea ( Goto 1894; Yamaguti 1940, 1958; Ono et al. 2020). Microcotyle elegans View in CoL has more testes (16–28) than M. fusiformis (15) and M. kasago (10–14), and differs from M. kasago in the features around the genital atrium (radiate muscle fibers vs. nothing). Additionally, the genital atrium of M. elegans View in CoL has no lateral recesses or hemispherical protuberance, but that of M. tanago possesses paired lateral recesses surrounded by short radial muscles, and M. gimpo View in CoL has a hemispherical protuberance at the posterior end.

Microcotyle ditrematis Yamaguti, 1940 View in CoL , M. aigoi Ishii and Sawada, 1937 , and M. toba Ishii and Sawada, 1937 have been reported from Ise Bay, Japan, where the newly collected specimens in this study were obtained. ( Ishii and Sawada 1937, 1938a; Yamaguti 1940). Microcotyle elegans View in CoL and M. ditrematis View in CoL have differences in the arrangement of the spines in the genital atrium (spines more dense in central anterior region, less dense in lateral pockets, but no spines present in center of posterior pockets vs. entirely covered in spines) ( Yamaguti 1940). Microcotyle elegans View in CoL is distinguished from M. aigoi by the number of testes (16–28 vs. about 30) and length of genital spines (5–7 µm vs. 9–14 µm) and from M. toba by the number of clamps (48–95 vs. 23 on each side) and length of genital spines (5–7 µm vs. 10–14 µm) ( Ishii and Sawada 1937, 1938a). Morphological measurements of Microcotyle species are presented in Table 3.

As a result of taxonomical re-evaluation of Microcotyle View in CoL , 34 species were regarded as species inquirendae ( Table 1).