Acanthocephalus minor Yamaguti, 1935

Nagasawa, Kazuya & Nitta, Masato, 2015, Rediscovery of a Fish Acanthocephalan, Acanthocephalus minor (Echinorhynchida: Echinorhynchidae), in the Lake Biwa Basin, Central Japan, with a Review of the Fish Acanthocephalan Fauna of the Basin, Species Diversity 20, pp. 73-81 : 75-76

Acanthocephalus minor Yamaguti, 1935 ( Fig. 1 )

Acanthocephalus minor Yamaguti, 1935: 253–254 , figs 3–5 (type locality: Toyama Prefecture, Japan); Fukui and Morisita 1936: 761; Yamaguti 1939: 322–323; Petrochenko 1956: 311, fig. 126; Fukui 1962: 128; Yamada 1962: 277, fig. H; Yamaguti 1963: 47; Fukui 1965: 468, figs 1–3; Golvan 1969: 276, fig. 213; Awakura 1972: 1–12, figs 1–8; Fukui 1973: 275, fig. 14-5; Uchida 1979: 149, fig. 6; Awakura 1980: 207; Nagasawa et al. 1982: 229–231; Awakura et al. 1984: 10; Amin 1985: 42; Nagasawa et al. 1987: 35; Awakura 1989: 608; Golvan 1994: 137; Shimazu 1999: 23–24; Araki 1999: 159; Araki 2003: 149; Amin 2013: 285; Yokoyama and Nagasawa 2014: 84.

Acanthocephalus echigoensis: Anonymous 1972: 146–148 ; Awakura 1973: 12–14, unnumbered fig.

Description. General. Trunk subcylindrical, unarmed, slightly tapering posteriorly. Body wall thick. Proboscis cylindrical, armed with 12–14 (usually 13) longitudinal rows of 5–7 (usually 5 or 6) hooks in both sexes. Second or third hook from distal end largest and basal hook smallest in each row. Hook blade longer than root. Neck short. Proboscis receptacle double-walled, with cerebral ganglion near its posterior end. Lemnisci saccular or elongate, longer than proboscis receptacle. Genital pore terminal.

Male (based on four specimens). Trunk 2.45–3.03 (2.72) long, 0.79–0.91 (0.84) wide. Proboscis 0.35–0.45 (0.39) long, 0.21–0.23 (0.21) wide. Blade of largest (i.e. second or third) hook in each row 65–74 (70) µm long; blade of smallest (i.e. basal) hook in each row 38–51 (47) µm long. Neck 0.15– 0.21 (0.18) long (n = 2), 0.37–0.41 (0.39) wide at junction with trunk (n = 2). Proboscis receptacle 0.34–0.40 (0.38) long, 0.19–0.23 (0.21) wide. Lemnisci 0.26–0.48 (0.36) long, 0.6–0.18 (0.11) wide. Testes oval, slightly oblique. Anterior testis 0.34–0.35 (0.34) long, 0.22–0.28 (0.25) wide; posterior testis 0.30–0.34 (0.33) long, 0.20–0.27 (0.22) wide. Cement glands pyriform, close together, 0.10–0.18 (0.12) wide in diameter, immediately behind posterior testis. Saefftigen’s pouch bulb-shaped, 0.35–0.43 (0.39) long, 0.20 (0.20) wide (n = 2).

Female (based on four specimens). Trunk 2.05–3.35 (2.62) long, 0.88–1.18 (1.03) wide. Proboscis 0.40–0.50 (0.47) long, 0.19–0.29 (0.24) wide. Blade of largest (i.e. second or third) hook in each row 85–92 (87) µm long; blade of smallest (i.e. basal) hook in each row 43–75 (60) µm long. Neck 0.17–0.23 (0.19) long, 0.38–0.53 (0.44) wide at junction with trunk. Proboscis receptacle 0.30–0.54 (0.42) long, 0.19–0.26 (0.24) wide. Lemnisci 0.23–0.48 (0.36) long, 0.8– 0.15 (0.10) wide. Reproductive system 0.50–0.73 (0.61) long, 21.6–25.5% (23.6%) of trunk length. Mature eggs with polar prolongations of middle shell 96–104 (99) µm long, 17–20 (18) µm wide when measured through body wall.

Host. Dark sleeper Odontobutis obscura ( Perciformes : Odontobutidae ).

Locality. Irrigation canal near Ado River at Adogawacho-Kawashima, Takashima city, Shiga Prefecture, Honshu, Japan.

Site of infection. Rectum.

Occurrence in fish. Ten individuals of A. minor were found in one specimen of O. obscura (64.3 mm SL). The eight stained specimens comprised four individuals of each sex.

Remarks. The morphology of the specimens collected in this study almost fully corresponds to the original description of A. minor by Yamaguti (1935) and the subsequent brief descriptions of the species by Fukui and Morisita (1936), Yamaguti (1939), and Awakura (1972), except that the number (5–7) of hooks per longitudinal row is fewer than in previous reports (7–9: Yamaguti 1935; 6–7: Fukui and Morisita 1936; 6–8: Yamaguti 1939; 7–9: Awakura 1972).

Acanthocephalus minor was originally described based on specimens from Amur catfish Silurus asotus Linnaeus, 1758 (as Parasilurus asotus ) in Toyama Prefecture, central Honshu, Japan ( Yamaguti 1935). Subsequently, this acanthocephalan has been recorded from various freshwater fish in central Honshu and Hokkaido, Japan ( Fukui and Morisita 1936; Yamaguti 1939; Awakura 1972; Nagasawa et al. 1982). There is no record of this species from other countries. Petrochenko (1956: 293) doubted the generic assignment of the species, but subsequent authors (e.g., Yamaguti 1963; Golvan 1969, 1994; Amin 1985, 2013) have continued to place it in Acanthocephalus Koelreuther, 1771 . Awakura (1973) used Acanthocephalus echigoensi s Fujita, 1920 as the scientific name of acanthocephalans that had heavily infected fish at a trout hatchery in Hokkaido (see also Anonymous 1972), but this was definitely wrong because the same author ( Awakura 1972) had already identified the parasite as A. minor based on its morphological characteristics.

In 1936, A. minor was found in Odontobutis obscura in “Lake Biwa” ( Fukui and Morisita 1936; see the Discussion section herein concerning this locality), but this acanthocephalan has not been reported from the lake or associated waters by any subsequent researcher (e.g., Fukui and Morisita 1937; Yamaguti 1939; Amin et al. 2007). The finding of A. minor in this study confirmed its occurrence in the Lake Biwa basin.

All of the present individuals of A. minor were found in the rectum of the fish examined. This site specificity of A. minor within the host’s intestine was previously noticed by Nagasawa et al. (1982).

Figures

Fig. 1. Acanthocephalus minor Yamaguti, 1935, NSMT–As 4293, from rectum of Odontobutis obscura (Temminck and Schlegel, 1845). A, Entire body of male; B, proboscis of female. Scale bars: A, 1mm; B, 100µm.