A revision of East Asian Acanthomysis (Crustacea: Mysida: Mysidae) and redefinition of Orientomysis, with description of a new species

Twenty species of Acanthomysis s. l., distributed in Japanese, Korean and Chinese coastal waters, are revised. These species are distinguished from Acanthomysis s. str. by the following characters: the carpopropodus of the endopod of the third to eighth thoracic limbs divided into three to eight subsegments; the exopod of the fourth male pleopod with two long subequal terminal setae; the telson long linguiform or triangular without basal dilated portion and armed with spines throughout margins. On the basis of these characters, the genus Orientomysis, which was revived by Holmquist (1981b) and comprised O. japonica and O. mitsukurii, is redefined and recognized to contain the following 18 valid species: O. aokii, O. aspera, O. crassispinosa, O. fujinagai, O. hwanhaiensis, O. koreana, O. leptura, O. meridionalis, O. okayamaensis, O. pseudomitsukurii, O. robusta, O. rotundicauda, O. sagamiensis, O. serrata, O. sheni, O. tamurai, O. tenella, and O. tenuicauda. Acanthomysis longicauda and A. nakazatoi are synonymized with O. rotundicauda and O. japonica, respectively. A new species, O. arenaria, is described from Japan.


Introduction
established Metamysis for the accommodation of M. mitsukurii and M. sagamiensis from Japan. This genus was a junior homonym of Metamysis Sars, 1895, so Derzhavin (1913) introduced a new replacement name for it, Orientomysis. Subsequently, Orientomysis was placed in synonymy with Neomysis Czerniavsky, 1882, by reason of the similarity in the fourth male pleopod (Zimmer 1915).

Revision of East Asian Acanthomysis
Antennule. Peduncle of male more robust than that of female, with developed appendix masculina on distal segment.
Antenna. Scale lanceolate with rounded apex, setose on all margins, with subapical suture; sympod with spiniform process at outer distal angle.
Labrum. Anterior margin with acute process.
Mandible. Second segment of palp slightly expanded in middle.
Maxillule. Outer lobe with 11-15 robust spines on distal margin and three slender setae on ventral surface, with hump on middle of outer margin.
Maxilla. Distal segment of endopod longer than broad, with or without tiny spines among long setae on outer margin (Table I).
Thoracic limbs. Endopod of first pair short and robust, with expanded inner lobe on preischium, ischium and merus; endopod of second pair robust; endopods of third to eighth pairs long, slender, with carpopropodus three-to eight-subsegmented (Table I); exopods with flagellum eight-segmented in first and eighth pairs and nine-segmented in second to seventh pairs; basal plate of exopods with several spinules on outer distal angle.
Penis. Oval in lateral view, armed with one to four long plumose setae on distal half of anterior margin, four to eight inwardly curved setae on distal margin, and two to eight short and three to seven long setae on posterior margin.
Marsupium. Seventh and eighth thoracic limbs with developed oostegites; oostegite of seventh limb with baling lobe.
First to third and fifth pleopods of male and all pleopods of female. Reduced to uniramous unsegmented lobe, gradually increasing in length from first to fifth; pseudobranchial lobe poorly developed.
Fourth pleopod of male. Biramous; endopod reduced to unsegmented lobe; exopod long, not extending beyond posterior end of last abdominal somite, two-segmented; proximal segment long, armed with one short seta at outer distal corner in all species and with one long plumose seta at inner distal corner (except O. aokii and O. aspera, having one short plumose seta, and O. fujinagai and O. leptura, having no seta); distal segment short, one-seventh to one-fourth of length of proximal segment (Table I), armed with one or two short setae on corners and two long, barbed, spiniform setae on terminal end; latter setae subequal in length (except O. leptura and O. tenuicauda), not forming slender, naked, spiniform termination (Table I).
Uropod. Endopod armed with one to six spines on ventral surface near statocyst region (except O. crassispinosa, having no spine) (Table I).
Telson. Elongate triangular or linguiform, with or without spines on dorsal surface near base (Table I); lateral margin armed with numerous spines along entire length (except O. sagamiensis and O. tamurai, in which unarmed part is inserted) (Table I), on distal twothirds to half these spines arranged in clusters, each cluster composed of one to five small spines and one large spine.

Remarks
Comparisons of important characters among Orientomysis and related genera are shown in Table II.
Orientomysis is distinguished from Acanthomysis by the male pleopods and the telson as follows: (1) in Orientomysis the two terminal setae of the fourth male pleopod are subequal in length with normal plumose termination, whereas in Acanthomysis these are unequal, one seta 1.3-1.5 times longer than the other, and have a slender, naked and spiniform termination; (2) the pseudobranchial lobe of the pleopods is not as well developed in Orientomysis as in Acanthomysis; and (3) the telson of Orientomysis is elongate linguiform or triangular and armed with spines along the entire lateral margin, while that of Acanthomysis is linguiform with markedly dilated basal part, and the lateral margin has a naked part inserted between one to three spines on the dilated part and numerous spines on distal two-thirds.

Type locality
Port Tainan, southwestern Taiwan.

Remarks
Location of type specimens is unknown.

Distribution
Known only from the type locality in Taiwan (Ii 1964).

Type locality
Atsu, mouth of Kojima Bay, Seto Inland Sea, western Japan.

Material examined
Syntype: 33 males (7.2-13.6 mm) and 17 females (8.5-11.1 mm), Atsu, mouth of Kojima Bay, Okayama, western Japan, 18 March 1936, Ii's coll. no. 216. Others. One immature female (7.9 mm), Ryubai Island, Haeju Bay, Hwanghaenamdo, western Korea, 13 April 1936, Ii's coll. no. 33 (part). Two males (6.2 and 6.4 mm) and two females ( Ii (1964) described the outer pair of the apical spines of the telson as more strongly curved medially in females than in males. In the present specimens a similar feature is observed. Furthermore, these apical spines of the female are larger and more robust than those of the male ( Figure 1). This species most closely resembles O. aokii, but is distinguished from it only by the hispid body (smooth in O. aokii).

Distribution
This species is recorded from the Japanese embayments, Kojima Bay (Ii 1964), Tokyo Bay (Ii 1964;present study), the Ariake Sea, Omura Bay, Ise Bay, Mikawa Bay and Osaka Bay (present study); the Chinese coastal waters of the Bohai Sea (the Gulf of Zhili), the Yellow Sea and the East China Sea (Shen et al. 1989;Wang and Liu 1997); southern and western coastal waters of Korea (Jo and Ma 1996); and Sacramento-San Joaquin estuary, California (Modlin and Orsi 1997). The occurrence in the Sacramento-San Joaquin estuary of this species is surely a result of the transportation from Asian waters with ballast water of ships (Modlin and Orsi 1997).
The present specimens from Japan were collected from depths of 10-40 m in eutrophic embayments.

Remarks
Direct examination of this species was not made. This species, however, clearly belongs to Orientomysis because it is in agreement with the new generic diagnosis.
Orientomysis crassispinosa was originally described by Liu and Wang (1980) on the basis of two female specimens collected from the South China Sea off Guangdong; no male specimens have been recorded.
This species is similar to O. aokii and O. aspera in that the telson is armed with two pairs of robust spines on the apex and the abdominal somites are without any spine rows. However, it is distinguished from the latter two species by the absence of spines in the statocyst region of the endopod of the uropod (single spine present in O. aokii and O. aspera), and from O. aokii by the hispid body (smooth in O. aokii ).

Remarks
Location of type specimens is unknown. Orientomysis fujinagai is characterized by fifth and sixth abdominal somites with one and two transverse spine rows, respectively, the lateral margin of the telson armed with small subequal spines between larger ones, and the apex of the telson with two pairs of stout spines. This species is similar to A. koreana and A. robusta which also have the abovementioned characters. Morphological differences among these three species are summarized in Table III.

Distribution
This species has been collected from western and southeastern coasts of Korea (Ii 1964;Jo and Ma 1996) and Zhejiang, Chinese coast of the East China Sea (Wang and Liu 1997).
In the East China Sea, this species was collected from a depth of 11 m (Wang and Liu 1997).

Remarks
The number of minute spines inserted between inner and outer pairs of larger apical spines on the telson is one or two, rarely three ( Figure 2) against one in the original description by Ii (1964). This species is distinguishable from the other species of Orientomysis by the armature of the telson: the apical margin is armed with two pairs of large spines, which are subequal to the larger spines on the lateral margin, and with one to three minute spines between the inner and outer larger spines.

Distribution
This species has been recorded from southern and western coasts of Korea (Ii 1964;Jo and Ma 1996) and Chinese coast of the Bohai Sea, the Yellow Sea and the East China Sea (Shen et al. 1989;Wang and Liu 1997;present study). Modlin and Orsi (2000) reported this species from the San Francisco Bay estuary, California, resulting from the transportation with ship ballast water from East Asia. Marukawa, 1928 (

Type locality
Sagami and Suruga Bays, central Japan.

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).
Orientomysis koreana (Ii, 1964) N, number of individuals examined; +, a single dorsal fold present; ++, two dorsal folds present; 2, fold absent. The inside of the brackets shows the items.
Others. Two females (damaged), Yongyoodo, Inchon, western Korea, dip net by diver, 27 April 1986, on loan from S.-G. Jo. Ii (1964) described the first to fourth abdominal somites having a lateral fold which is discontinuous dorsally. On the other hand, Shen et al. (1989) illustrated the fourth abdominal somite with a lateral fold as well as a transverse dorsal fold. In the type specimens, an obscure transverse dorsal fold or groove was recognized on the fourth abdominal somite. Shen et al. (1989) and Jo and Ma (1996) observed the presence of spines on the dorsal surface near the base of the telson, which were neither described nor illustrated by Ii (1964). Re-examination of the type specimens confirmed that there are two or three pairs of spines.

Remarks
Orientomysis koreana shows a sexual dimorphism in the size of apical spines of the telson as illustrated by Ii (1964), although he did not describe it. The outer pair of the large apical spines of the telson is only slightly longer than the apicalmost lateral spine in the male, compared with 1.5 times as long in the female.
Orientomysis koreana is allied to O. fujinagai and O. robusta. Morphological differences among these three species are summarized in Table III.

Distribution
This species has been recorded from the western Korea (Ii 1964;Jo and Ma 1996) and Chinese coasts of the Bohai Sea, the Yellow Sea and the East China Sea (Shen et al. 1989;Wang and Liu 1997).
This species was collected from depths of 11 to 41 m in Chinese waters (Wang and Liu 1997).

Remarks
Orientomysis leptura is similar to O. pseudomitsukurii and O. tenella in the abdominal somites without spines and the shape of the telson. Morphological differences among the three species are summarized in Table V.

Distribution
This species is known only from China: the South China Sea off Guangdong (Liu and Wang 1980) and the East China Sea off Zhejiang (Wang and Liu 1997). This species was collected from depths of 6-32 m in the South China Sea (Liu and Wang 1980) and from depths of 11-63 m in the East China Sea (Wang and Liu 1997).

Material examined
One male (5.2 mm) and one immature female (4.6 mm), Wuchuan, Guangdong, southern China, coastal water, 2 May 1978, on loan from Liu and Wang.

Remarks
The specimens examined in this study are a part of those collected together with the type specimens of O. meridionalis. These specimens are slightly different from the original description by Liu and Wang (1983) in the maxilla, the endopod of the fourth male pleopod and the telson. The maxilla illustrated by Liu and Wang (1983) is not armed with tiny spines on the outer margin of the second endopod segment, whereas in the present specimens the outer margin is armed with five tiny spines in the male and two in the female among long setae ( Figure 5B). The exopod of the fourth male pleopod is not armed with setae on the distal end of the proximal segment in the original illustration, while in the present specimen the proximal segment bears a long plumose seta at the inner distal angle and a tiny seta at the outer distal angle ( Figure 5C, D). The dorsal surface of the telson is smooth in the original illustration, whereas it is armed with a pair of short spines in the present specimens ( Figure 5E). Orientomysis meridionalis is allied to O. okayamaensis and O. serrata in the following characters: last one or two abdominal somites are furnished with a row of spines; the larger lateral spines of the telson increase in size posteriorly; and the apex of the telson is armed with two pairs of strong spines. Differences among these species are summarized in Table  VI.

Remarks
Location of type specimens is unknown. Orientomysis mitsukurii is readily distinguished from related species by the presence of a number of small spines on the abdominal somites (Ii 1964).

Distribution
This species is distributed in Pacific coastal waters of Japan from Aomori to Miyazaki (Nakazawa 1910;Ii 1936Ii , 1964Yamada et al. 1994;present study).
Along the Sanriku coast, northern Japan, this species predominates in mysid assemblages in waters shallower than 10 m deep, especially in depths of 7-10 m, in sandy bottom areas facing the open sea (Yamada et al. 1994).

Type locality
Atsu, mouth of Kojima Bay, Seto Inland Sea, western Japan.

Material examined
Syntype: one male (18.6 mm) and two females ( Remarks Shen et al. (1989) and Jo and Ma (1996) noted the presence of dorsal spines on the telson, a character not mentioned in the original description by Ii (1964). The presence of these spines was confirmed in the type specimens. Orientomysis okayamaensis is related to O. meridionalis and O. serrata. Differences among the three species are shown in Table VI.

Distribution
This species has been collected from Japanese embayments, Kojima Bay (Ii 1964) and Osaka Bay (present study); Chinese coastal waters, Qingdao (Shen et al. 1989) and the mouth of the Yangtze River (Wang and Liu 1997); and West Korean coastal waters (Jo and Ma 1996).
This species was collected from a depth of 30 m in the mouth of the Yangtze River (Wang and Liu 1997) and from a depth of 40 m in Osaka Bay (present study).

Type locality
Coast near Marine Biological Center of Tokyo Bunrika University (now University of Tsukuba), Shimoda, Shizuoka, central Japan.

Remarks
Location of type specimens is unknown. Some differences exist between Ii's description and the present specimens. According to Ii (1964), the eyestalk was not armed with a papilliform process on the dorsal surface, whereas a small but distinct process is present in our material. Ii (1964) also described a single distinct transverse dorsal fold on each of the second to fourth abdominal somites, but the state of the abdomen varies with individuals in the present specimens ( Figure 6; Table VII). The folds on the second to fifth abdominal somites are frequently indistinct.
Orientomysis pseudomitsukurii resembles O. leptura and O. tenella. Differences among these three species are summarized in Table V.

Distribution
This species is recorded only from the Japanese warm water region: Shizuoka (Ii 1964), Yamaguchi, Tottori (Hirota et al. 1989), Nii-jima Island of Izu Islands, Chiba, and western Kagoshima (present study).
In Japan Sea coasts, this species is abundantly distributed in depths of 4-5 m (Hirota et al. 1989). They were collected from near shoreline and among seaweeds (present study).

Remarks
It was observed for the first time that the carpopropodus of the endopod of the third to eighth thoracic limbs is divided into five or six subsegments.
Orientomysis robusta is similar to O. fujinagai and O. koreana in the abdominal somites with a spine row, the lateral margin of the telson with several subequal small spines between larger ones, and the apex of the telson with two pairs of strong spines. Morphological differences among these three species are shown in Table III.

Distribution
This species has been recorded from Japan Sea coastal waters of Japan, from Hokkaido to Niigata (Ii 1964;Murano 1984;Hirota et al. 1989;present study).

Remarks
When Acanthomysis longicauda was established, Murano (1991) mentioned three points distinguishing this species from a similar species, O. rotundicauda: the length of the rostrum, the size of the eye, and the length-breadth ratio of the telson. The rostrum of the present specimens collected from the South China Sea extends to the proximal fourth to middle of the first segment of the antennular peduncle, as seen in the original description by Liu and Wang (1980) (Figure 7A). On the other hand, in the majority of Japanese specimens the rostrum does not overreach the base of the antennular peduncle as described by Murano (1991) (Figure 7G), but in some individuals it extends to the proximal fourth of the first antennular peduncle segment ( Figure 7J). As to the eyes, Murano (1991) noted that those of A. longicauda were large and far extending laterally beyond the lateral margin of the carapace, whereas those of O. rotundicauda were rather small and barely extending beyond the lateral margin of the carapace. In the present specimens from the South China Sea, the eyes were observed to be an intermediate form between the two species ( Figure 7A). The telson also is not distinctly different; 2.3 times as long as broad in the female from the South China Sea (Figure 7F), whereas 2.3-2.6 times as long as broad in those from Japanese waters ( Figure 7I). Other differences between O. rotundicauda and A. longicauda are also observed in the original descriptions and illustrations. The plumose seta at the inner distal angle of the proximal exopodal segment of the fourth male pleopod is slightly longer than the distal segment in O. rotundicauda (Liu and Wang 1980, Figure 4(4)), whereas it is twice as long as the distal segment in A. longicauda (Murano 1991, Figure 3I, J). In the present specimen from the South China Sea, however, it is nearly twice as long as the distal segment ( Figure 7E). The larger apical spines of the telson are subequal in size to the larger spines on the lateral margin in O. rotundicauda (Liu and Wang 1980, Figure 4(6)), while these are shorter than the larger spines on the lateral margin in A. longicauda (Murano 1991, Figure 4A-D) and the present specimens from the South China Sea ( Figure 7F).
Differences between these two species are small and considered to fall within the range of intraspecific variation. Consequently, A. longicauda is declared to be a junior synonym of O. rotundicauda.

Distribution
China: the South China Sea off Hainan and Guangdong (Liu and Wang 1980). Japan: Tateyama Bay, Chiba; between Jusan-ko and Ajigasawa, Japan Sea side of Aomori; Kyoto (Murano 1991).
This species was collected from depths of 6-31 m in the South China Sea (Liu and Wang 1980) and from depths of 1-10 m in Japanese waters (Murano 1991).

Description
Body robust, not hispid. All thoracic somites without sternal process. First to fifth abdominal somites subequal in length, sixth somite slightly longer than preceding one; first somite with two dorsal folds, second to fourth somites with one dorsal fold, fifth and sixth somites smooth. Carapace produced frontally into long triangular rostral plate with bluntly pointed apex and concave lateral margins; apex extending to middle to distal margin of first segment of antennular peduncle ( Figure 8A, B); anterolateral corner of carapace rounded; posterior margin of carapace emarginate, leaving last two thoracic somites exposed dorsally, furnished with minute setae on medial part.
Eye somewhat flattened dorsoventrally, about 1.2 times as long as broad in dorsal view; cornea reniform and occupying two-fifths of eye in dorsal view; eyestalk hispid, with minute papilliform process on dorsal surface ( Figure 8A, B).
Antennular peduncle with first segment 1.2 times as long as broad; third segment almost same in length with proximal two segments combined, about 1.4 times as long as broad; in male distal segment with developed appendix masculina ( Figure 8A, B).
Antennal scale lanceolate with rounded apex, about 1.5 times longer than antennular peduncle, 5.6-6 times as long as broad, setose on entire margin, with suture near apex ( Figure 8A-C). Antennal peduncle extending to middle of scale; second segment 1.4 times as long as broad in male, and 1.7 times as long as broad in female; third segment slightly shorter than second, 1.3 times as long as broad in male and 1.6 times as long as broad in female ( Figure 8C). Antennal sympod with spiniform process at outer distal corner ( Figure  8C).
Labrum armed with forwardly directed, long, spiniform process. Mandibular palp three-segmented; second segment slightly expanded, 2.3 times as long as broad; third segment about half of length of second ( Figure 8D).
Outer lobe of maxillule armed with 12 stout spines on distal margin and with three setae on ventral surface; middle of outer margin with hump-like process armed with a few tiny spines ( Figure 8E).
Endopod of maxilla two-segmented; distal segment 1.5 times as long as broad, armed with two to six minute spines among plumose setae on outer margin ( Figure 8F). Exopod of maxilla reaching distal margin of proximal segment of endopod, armed with plumose setae on outer and apical margins ( Figure 8F). Basal endite with spinous surface ( Figure 8F).
Endopod of first thoracic limb short and robust; preischium, ischium and merus with expanded inner lobe ( Figure 8G). Endopod of second thoracic limb short; merus 3.2 times as long as broad; carpopropodus slightly shorter than merus ( Figure 8H). Endopods of third to eighth thoracic limbs long and slender; carpopropodus divided into five subsegments in third to seventh limbs and into six subsegments in eighth limb; dactylus with long, slender claw ( Figure 9A, B). Exopods with flagellum eight-segmented in first and eighth limbs and nine-segmented in second to seventh limbs; basal plate with outer distal corner rounded with several minute spines, inner and outer margins partly spinulose ( Figures 8G, H, 9A, B).
Penis 1.5 times as long as broad in lateral view, armed with six to eight short, plumose and five or six long, naked setae on posterior margin, four or five inwardly curved setae on distal margin, and three long plumose setae on distal half of anterior margin ( Figure 9C).
Marsupium composed of two pairs of developed oostegites; oostegite on seventh limb with baling lobe.
First to third pleopods in both genders reduced to uniramous, unsegmented lobes, gradually increasing in size from first to third ( Figure 9D-F). Fourth pleopod of male biramous; endopod rudimentary, unsegmented; exopod extending to middle to posterior third of last abdominal somite, two-segmented; proximal segment 1.7 times longer than endopod, armed at outer distal corner with one short seta and at inner distal corner with one long plumose seta which is 1.6 times longer than distal segment; distal segment onefifth of length of proximal segment, armed with short seta on inner and outer distal corners and with two long, stout, barbed terminal setae, one of which is slightly longer than other (1.1 times) and 2.5 times as long as distal segment ( Figure 9G). Fourth pleopod of female uniramous, reduced to unsegmented lobe, 1.1 times as long as third pleopod. Fifth pleopod of both genders uniramous, reduced to unsegmented lobe, about 1.5 times as long as third pleopod ( Figure 9H). Pseudobranchial lobe poorly developed in all pleopods ( Figure 9D-H).
Endopod of uropod slightly shorter than telson, armed with two to five spines on inner ventral surface of statocyst region; spines increasing in size distally ( Figure 9I, J). Exopod of uropod 1.4 times as long as endopod ( Figure 9J).
Telson triangular with rounded apex, 2.1 times as long as last abdominal somite, 2.2-2.6 times as long as broadest part near base ( Figure 9K, J). Lateral margin of telson armed with five to nine spaced spines on proximal one-fifth, followed by about oneseventh of margin naked, distal three-fourths densely armed with 10-13 clusters of spines, each cluster composed of one larger spine and one to four subequal smaller spines ( Figure 9K, J). Distal margin of telson armed with two pairs of stout spines; in male these spines almost straight and 1.4-1.6 times as long as larger lateral spines, those of female slightly curved medially and 1.8-2.2 times as long as larger lateral spines ( Figure 9K, J).

Remarks
Orientomysis sagamiensis was established by Nakazawa (1910), as Metamysis sagamiensis, on the basis of specimens collected from near Enoshima, Sagami Bay, central Japan. However, his description was brief with only one illustration, and after that there is no record of collection. Ii (1964: 488), who examined a number of mysid specimens in Japanese coastal waters, noted that ''no specimens which should rightly be referred to this species were found in any hauls, which made not only in the waters near the type locality but also done in those around Japan''. The location of Nakazawa's specimens is unknown (they were probably lost during World War II), so that we cannot know the morphological aspect of this species at present. This species seems to be characterized by (1) the abdominal somites being smooth; (2) the endopod of the uropod with seven spines in the ventral statocyst region; (3) the lateral margin of the telson with a spineless part; and (4) the broad apex of the telson with four uniform and remarkably strong spines (Nakazawa 1910). The present specimens identified as O. sagamiensis agree well with the original description of this species in two characters of the telson, but are not in agreement in characters of the abdominal somite and the uropodal endopod. In the majority of the present specimens, a fold exists on each of the anterior four abdominal somites, but in a few specimens, the fold is so obscure that its observation is difficult. The abdominal fold may have been overlooked in previous investigation or may vary intraspecifically as is observed in O. japonica and O. pseudomitsukurii. The uropodal endopod is armed with two to five spines in the present specimens compared with seven in the original description. The number of spines varies with individuals, but those which have as many spines as seven were not found in the present specimens.
The present specimens closely resemble O. tamurai, but cannot be identified with this species in the apical spines of the telson: these spines in O. sagamiensis are uniform and more than twice as long as the larger lateral spines ( Figure 9J, K), whereas in O. tamurai the inner pair of apical spines is slightly shorter than the outer pair in most cases, and as long as and rarely longer than the outer pair in some cases; the outer pair is slightly longer than the larger lateral spines ( Figure 11D, E).
The present specimens are identified with O. sagamiensis with some doubts and a full description is given here.

Distribution
This species is recorded only from Japanese coastal waters of the Pacific side, from Miyagi to Kanagawa (Nakazawa 1910;present study).
This species was collected from a depth of 8 m in Sendai Bay, northern Japan (present study).

Remarks
Orientomysis serrata resembles O. meridionalis and O. okayamaensis in characters of the abdominal somites and the armature of the telson. Differences among these three species are summarized in Table VI.

Distribution
This species is known only from the depths of 6-8 m in the South China Sea off Guangdong, southern China (Liu and Wang 1980).

Type locality
North Yellow Sea off Yantai, Shandong, China.

Remarks
In the present examination, some characters, which were probably overlooked by Wang and Liu (MS in Shen et al. 1989), were observed. The second segment of the endopod of the maxilla is armed with three tiny spines in the male and seven in the female, among long setae on the outer margin ( Figure 10B). Sternal processes are present on the second to sixth and eighth thoracic somites in the female ( Figure 10A). Those on the second to sixth somites are large and blunt, and covered with numerous spinules. The seventh somite has no process. The process on the eighth somite is knob-like without spinules. Figure 10. Orientomysis sheni (Wang and Liu, 1989)

Distribution
This species has been recorded only from Chinese waters: the Bohai Sea off Hebei and the Yellow Sea off Shangdong (Wang and Liu, in Shen et al. 1989).

Remarks
Orientomysis tamurai is readily distinguishable from the other species of the genus except O. sagamiensis by the telson, with a hiatus in the spine rows of the lateral margin ( Figure 11D, E). This species is distinctly different from O. sagamiensis in the size of the apical spines of the telson.

Distribution
This species has been collected only from coastal water of Chiba, central Japan (Ii 1964;present study).

Material examined
One male (4.6 mm) and one female (4.5 mm), coastal water of Wuchuan, Guangdong, southern China, 18 May 1978, on loan from Liu and Wang.

Remarks
Orientomysis tenella resembles O. leptura and O. pseudomitsukurii. Differences among them are shown in Table V.

Distribution
Known only from the South China Sea off Guandong, southern China (Liu and Wang 1983).

Remarks
Orientomysis tenuicauda is closely related to O. sheni in the shape of the telson, but differs from the latter species in the following points: (1)  tenuicauda the outer pair of apical spines of the telson is as long as or a little shorter than the larger lateral spines, while in O. sheni it is markedly longer than the larger lateral spines.

Distribution
This species has been collected from embayments and shallow water regions around Japan and Korea: the Ariake Sea (Ikematsu 1963), Suruga Bay, Ise Bay, Mikawa Bay, Enshunada, Osaka Bay, Kii Channel, the Seto Inland Sea (present study), the East China Sea (Murano 1984), andPyonsan andBoryong, western Korea (Jo andMa 1996). This species was collected from a depth of 42 m in the East China Sea (Murano 1984) and from depths of 14-74 m in the Japanese waters (present study).
Penis 1.7 times as long as broad in lateral view, armed with five short, plumose and four long, naked setae on posterior margin, with five long, medially curved setae on distal margin, and with four long, plumose setae on distal half of anterior margin ( Figure 13C).
Female with setose tuft on basis of third to sixth thoracic limbs and developed oostegites on seventh and eighth limbs; oostegite on seventh limb with baling lobe.
First to third pleopods of both genders uniramous, reduced to unsegmented lobes, gradually increasing in length from first to third ( Figure 13D-F). Fourth pleopod of male biramous; endopod short, unsegmented; exopod extending to middle of last abdominal somite, two-segmented; proximal segment 1.8 times longer than endopod, armed with one tiny seta at outer distal corner and one long plumose seta at inner distal corner, latter seta 1.4 times as long as distal segment; distal segment one-fifth length of proximal segment, armed with one tiny seta at outer distal corner, one short seta at inner distal corner, and two long, barbed terminal setae which are subequal in length and 3.3 times as long as distal segment ( Figure 13G). Fourth pleopod of female uniramous, reduced to unsegmented lobe, 1.2 times as long as third pleopod. Fifth pleopod of both genders reduced to uniramous and unsegmented lobe, 1.7 times as long as third ( Figure 13H). Pseudobranchial lobe poorly developed in all pleopods ( Figure 13D-H).
Endopod of uropod slightly shorter than telson, armed in ventral statocyst region with two or three spines increasing in size distally ( Figure 13I-K). Exopod of uropod 1.3 times as long as endopod ( Figure 13J, K).
Telson elongate triangular with rounded apex, twice as long as last abdominal somite, 2.3 times as long as maximum breadth near base, armed with spines throughout margins; lateral spines in proximal two-fifths spaced and subequal, in distal three-fifths these arranged in about 13 clusters, each cluster consisting of one larger spine and one to four subequal smaller spines, larger spines almost same in length; distal margin with two pairs of subequal spines, which are almost as long as apicalmost larger lateral spines ( Figure  13J, K).

Etymology
The specific name is derived from Latin arenarius, sandy, referring to the habitat, sandy beach.

Remarks
Orientomysis arenaria is characterized by the following points: (1) the antennal scale is rather long, more than 1.5 times as long as the antennular peduncle; (2) the labrum is armed with a rather short spiniform process on the anterior margin; (3) the second endopod segment of the maxilla is armed with a few tiny spines among long plumose setae on the outer margin; (4) all the abdominal somites lack spine rows; (5) the telson is armed with numerous spines on the entire lateral margin; and (6) the distal margin of the telson is armed with two pairs of spines which are as long as the larger lateral spines.
Orientomysis arenaria is allied to O. robusta and O. koreana in the shape and armature of the telson. However, O. arenaria is distinctly different from the latter two species by the abdominal somites without a row of spines and by the absence of dorsal spines near the base of the telson.
This species is also similar to O. sagamiensis and O. tamurai, but is distinguished from the latter two species by having the spines along the entire length of the lateral margin of the telson.

Distribution
This species was collected from the Pacific coast of the Japanese waters from Miyagi to Shizuoka and the East China Sea coast of Kagoshima.
This species was collected from waters shallower than 10 m.
Concerning two Indian species, A. anomala and A. macrops, Holmquist (1981b) excluded them from her discussion on Acanthomysis, because she could not refer to the original descriptions of the two species. Acanthomysis anomala possesses characteristics of Acanthomsyis s. str. as follows: the three-subsegmented carpopropodus of the endopod of the third to eighth thoracic limbs, the exopod of the fourth male pleopod with two terminal setae unequal in length, and the telson with lateral margins with a naked portion between the proximal spine group in the expanded basal part and the distal spine group composed of numerous spines. However, this species is not in agreement with the genus in having an acute anterolateral corner of the carapace. Furthermore, features of the setae terminating the exopod of the fourth male pleopod and of the pseudobranchial lobe of the pleopods, which indicate the generic character, are not known from the original description and illustration. Acanthomysis macrops also has characteristics of Acanthomysis s. str. in the number of the subsegments of the carpopropodus and in the shape of the terminal setae of the exopod of the fourth male pleopod, but disagrees with the genus in the telson without a prominent basal dilation, and the feature of the pseudobranchial lobe of the pleopods is unknown. The decision on the taxonomic position of these two species must await further examination of specimens.
Acanthomysis thailandica is closely related to Acanthomysis s. str. with the exception of the telson without a prominent basal dilation. Suitable taxonomic position will be decided through the examination of the type specimens stored at NSMT in the near future.
Acanthomysis californica seems to be related to Orientomysis rather than Acanthomysis s. str. in the four-subsegmented carpopropodus of the endopod of the third to eighth thoracic limbs, the poorly developed pseudobranchial lobe of the pleopods, and the shape of the telson. However, it is different from Orientomysis in having two unequal terminal setae on the exopod of the fourth male pleopod. The shape of the terminal setae of the exopod of the fourth male pleopod is not known from the original description. Appropriate taxonomic position should be decided after the examination of the type specimens.
Acanthomysis ornata is characterized by the three-subsegmented carpopropodus of the endopod of the third to eighth thoracic limbs, the exopod of the fourth male pleopod with two long terminal setae different in length, the uropodal endopod armed with a row of large, evenly spaced and curved spines throughout the inner margin, and the long and narrow telson armed with regularly spaced, downwardly directed, curved spines on the lateral margin. These characters indicate clearly that A. ornata is different from Acanthomysis s. str., Orientomysis or any of their related genera.
As for the two boreal species, A. borealis and A. stelleri, Holmquist (1979) stated that they were similar to Exacanthomysis in the state of the abdominal somites and the shape and armature of the telson. However, she hesitated to conclude their systematic position because the male of both species was unknown at that time. Later, Petryashov (1992) described the fourth male pleopods of A. borealis and A. stelleri, and also pointed out that the specimens identified by Holmquist (1979) as A. borealis and Exacanthomysis arctopacifica Holmquist 1979, were A. stelleri and A. borealis, respectively. It appears that these two species should be transferred to Exacanthomysis.
When Holmquist (1981b) established Pacifacanthomysis, she noted that this genus was distinguished from Acanthomysis by the following characters: the carpopropodus of the endopod of the third to eighth thoracic limbs is divided into four or five subsegments; the exopod of the fourth male pleopod reaches slightly beyond the posterior end of the last abdominal somite and bears two long, subequal terminal setae; the fifth pleopod is not exceptionally long; the pseudobranchial lobe of the pleopods is small; and the telson is long, triangular and armed with unequal spines along the lateral margins. However, these characters are the same as those of Orientomysis except for the length of the fourth male pleopod (see Table II). Re-examination of the generic value of Pacifacanthomysis is needed.  Murano and Chess, 1987 incertae sedis A. macrops Pillai, 1973 incertae sedis A. ornata O. Tattersall, 1965 incertae sedis A. stelleri (Derzhavin, 1913) incertae sedis A. thailandica Murano, 1988 incertae sedis