Orientomysis japonica Marukawa, 1928

Fukuoka, Kouki & Murano, Masaaki, 2005, A revision of East Asian Acanthomysis (Crustacea: Mysida: Mysidae) and redefinition of Orientomysis, with description of a new species, Journal of Natural History 39 (9), pp. 657-708 : 671-675

Orientomysis japonica Marukawa, 1928

( Figures 3 , 4 )

Orientomysis japonica Marukawa 1928: 5–6 , Figures 23–29.

Acanthomysis japonica: Banner 1948: 86 (key); Gordan 1957: 337 (list); Mauchline and Murano 1977: 44 (list); Müller 1993: 193 (list).

Acanthomysis sp. Ii 1964: 507–509 (part), Figure 131B, D, G, J, K.

Acanthomysis nakazatoi Ii 1964: 519–521 , Figure 135 (type locality: Iwai, near Tateyama, Chiba, central Japan); Mauchline and Murano 1977: 45 (list); Müller 1993: 195 (list). Acanthomysis japonica?: Ii 1964: 522–524 , Figure 136.

Type locality

Sagami and Suruga Bays, central Japan.

Material examined

Syntype of A. nakazatoi : 29 males (9.5–13.5 mm) and 40 females (8.6–13.2 mm), Iwai , near Tateyama, Chiba, central Japan, 1 May 1940, Ii’s coll. no. 283 .

Others. Two males (7.2 and 9.5 mm) and two females (damaged), Todoro, Miyazaki , western Japan, 31 October 1937, Ii’s coll. no. 299, described as ‘‘ A. japonica ?’’ by Ii (1964). Eighteen males (8.1–14.2 mm), 23 immature males (6.0–8.0 mm), 20 females (8.0– 12.4 mm) and 35 immature females (5.2–8.0 mm), Uchiura Bay , Chiba, central Japan, 29 April 1975, NSMT-Cr 15560. Five males (11.2–12.5 mm) and three females (13.8–14.4 mm), Akita, northern Japan, shrimp net, 27 July 1976, NSMT-Cr 15561. One male (11.0 mm) and two females (10.2 and 10.6 mm), Tateyama Bay , Chiba, central Japan, 19 June 1978, NSMT-Cr 15562. One male (9.7 mm), two immature males (5.6 and 8.0 mm), one female (13.2 mm) and five immature females (6.5–8.5 mm), Ohno Bay , Iwate, northern Japan, 4 m, 31 July 1989, NSMT-Cr 15563. One male (7.8 mm) and two females (8.9 and 9.0 mm), Ohno Bay , Iwate, northern Japan, 4 m, sledge net, 11 September 1990, coll. H. Yamada, NSMT-Cr 15564. Ten males (9.8–15.3 mm) and five females (8.7–14.0 mm), Ohno Bay , Iwate, northern Japan, 4 m, sledge net, 26 April 1991, coll. H. Yamada, NSMT-Cr 15565. Eight males (8.3–13.2 mm), five females (8.5– 11.4 mm) and one immature female (6.2 mm), Ohno Bay , Iwate, northern Japan, 10 m, sledge net, 5 August 1991, coll. H. Yamada, NSMT-Cr 15566. Three males (14.4– 16.0 mm) and four females (13.7–15.7 mm), Ohno Bay , Iwate, northern Japan, 4 m, sledge net, 12 June 1992, coll. H. Yamada, NSMT-Cr 15567. One male (8.2 mm) and one female (9.3 mm), Yugahama, Yamagata, northern Japan, 10 August 1996, coll. K. Kimoto, NSMT-Cr 15568. One female (13.9 mm), Kashima, Ibaraki, central Japan, sledge net, 4 December 1996, coll. K. Kimoto, NSMT-Cr 15569. One female (17.0 mm), Kashima, Ibaraki, central Japan, sledge net, 16 January 1997, coll. K. Kimoto, NSMT-Cr 15570. Nine males (8.0– 11.7 mm) and 18 females (7.0– 12.4 mm), between mouth of Jusan-ko and Ajigasawa, Japan Sea side of Aomori, northern Japan, 5–10 m, trawl net, date unknown, 1984, coll. H. Ikeuchi, NSMT-Cr 15571. Nineteen males (11.4–13.8 mm) and 23 females (10.8–16.8 mm), Tottori, western Japan, date unknown, NSMT-Cr 15572 .

Remarks

Location of type specimens is unknown.

Orientomysis japonica was established by Marukawa (1928) on the basis of specimens collected from Sagami and Suruga Bays, central Japan. Ii (1964) identified specimens obtained from coastal waters near Nobeoka, Miyazaki, western Japan, and near Shimoda, Shizuoka, central Japan, as ‘‘ Acanthomysis japonica ?’’ with some doubts, because the second to fourth abdominal somites of his specimens were observed to have one dorsal fold each while all the abdominal somites of O. japonica were illustrated to be smooth. Ii (1964) thought that Marukawa (1928) may have overlooked these folds.

On the other hand, Ii (1964) established Acanthomysis nakazatoi , which is a similar species to O. japonica , based on specimens collected from the coast of Iwai, near Tateyama, Chiba, central Japan. He mentioned differences between A. nakazatoi and O. japonica as follows: (1) in A. nakazatoi the apical margin of the telson bears small spines between outer and inner pairs of large spines, whereas in O. japonica there are no small spines; (2) the distal part of the lateral margin of the telson in A. nakazatoi is armed with three to five small spines between larger spines instead of one or two small spines in O. japonica ; (3) in A. nakazatoi each of the anterior four abdominal somites is furnished with one or two dorsal folds, whereas all the abdominal somites of O. japonica lack dorsal folds; and (4) the uropodal endopod is armed in the ventral statocyst region with three spines in A. nakazatoi compared to four spines in O. japonica . Ii (1964) also noted a variation in the spination on the apical margin of the telson of A. nakazatoi : the apical margin, in most cases, bears four equally long stout spines and two small spines in each of three intervals between these long spines, but the number of small spines varied from zero to two.

In the present specimens, the number of small spines between inner and outer pairs of large spines on the telson apex and between lateral large spines of the telson varies from zero to four and two to six, respectively ( Figures 3 , 4 ; Table IV), and the number of spines in the statocyst region of the uropodal endopod varies from two to five. The spine arrangement on the telson and uropod varies with individuals and these specimens cannot be divided into species groups.

As for the abdomen, specimens from Pacific coasts, except those of Miyazaki, are always furnished with two dorsal folds on the first somite and with a single dorsal fold on the second to fourth somites (Table IV) . Four specimens from Miyazaki have no folds on the first somite, except for one specimen which has two dorsal folds (Table IV) . However, specimens from the coasts of the Japan Sea are considerably variable in this respect, although the majority are without folds (Table IV) . In some specimens the first to fourth somites have an indistinct groove instead of folds.

As a result, it is concluded that the differences between A. nakazatoi and O. japonica mentioned by Ii (1964) are not markedly distinct and fall within the realm of intraspecific variation. Consequently, A. nakazatoi is synonymized with O. japonica .

The telson is somewhat different morphologically between specimens from Pacific coasts and those from Japan Sea coasts. Telsons of Japan Sea specimens are slightly more slender compared with those of Pacific specimens: 2.2–2.7 times as long as basal breadth in the Japan Sea specimens against 2.0–2.6 times as long as basal breadth in Pacific specimens ( Figure 3 ; Table IV). The distal end of the telson is truncate in Pacific specimens ( Figures 3B–D , 4A–F ), whereas in the Japan Sea specimens it is rather rounded and narrower and the outer pair of apical long spines arises from a more proximal position than the inner pair ( Figures 3E , 4G–J ).

Ii (1964) reported six males and four female specimens of Acanthomysis sp. collected from the mouth of Mogami River, Yamagata, northern Japan. According to Ii (1964), these specimens, except one female which was identified with Acanthomysis robusta by Murano (1984), were different from A. nakazatoi in the rostrum, with a bluntly pointed apex, and in the telson, with lateral spines being more slender than those of A. nakazatoi . Reexamination of the type specimens of A. nakazatoi indicated that the apex of the rostrum was not so sharply pointed, as illustrated by Ii (1964), in most specimens. The lateral spines of the telson are more slender in the Japan Sea specimens than in Pacific specimens in the present study. Under the circumstances, Ii’s Acanthomysis sp. is identified as O. japonica .

Distribution

This species is known only from coastal waters of Japan: Japan Sea coasts from Aomori to Yamaguchi ( Ii 1964; Hirota et al. 1989; present study) and Pacific coasts from Iwate to Miyazaki ( Marukawa 1928; Ii 1964; present study).

This species inhabits coastal waters shallower than 10 m depth ( Ii 1964; present study).

Figures

Figure 3. Orientomysis japonica Marukawa, 1928. (A) First to sixth abdominal somites, lateral; (B–E) telson, dorsal. (A) Male (11.0 mm), Tateyama Bay, Pacific, NSMT-Cr 15562; (B) male (11.0 mm), Tateyama Bay, NSMT-Cr 15562; (C) male (14.2 mm), Uchiura Bay, Pacific, NSMT-Cr 15560; (D) female (17.0 mm), Kashima, Pacific, NSMT-Cr 15570; (E) male (10.6 mm), Japan Sea side of Aomori, NSMT-Cr 15571.

Figure 4. Orientomysis japonica Marukawa, 1928. Distal end of telson, dorsal. (A) Female (13.9 mm), Kashima, Pacific, NSMT-Cr 15569; (B) male (10.2 mm), Uchiura Bay, Pacific, NSMT-Cr 15560; (C) female (broken), Uchiura Bay, Pacific, NSMT-Cr 15560; (D) male (11.8 mm), type of Acanthomysis nakazatoi, Iwai, Pacific; (E) male (11.7 mm), type of A. nakazatoi, Iwai, Pacific; (F) female (10.2 mm), Tateyama Bay, Pacific, NSMT-Cr 15562; (G) female (11.5 mm), Japan Sea side of Aomori, NSMT-Cr 15571; (H) female (11.4 mm), Japan Sea side of Aomori, NSMT-Cr 15571; (I) female (13.0 mm), Tottori, Japan Sea, NSMT-Cr 15572; (J) female (15.8 mm), Tottori, Japan Sea, NSMT-Cr 15572.