Orientomysis sagamiensis ( Nakazawa, 1910 )

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 : 687-692

Orientomysis sagamiensis ( Nakazawa, 1910)

( Figures 8 , 9 )

Metamysis sagamiensis Nakazawa 1910: 251 , 252, Plate 8, Figure 32.

Orientomysis sagamiensis: Derzhavin 1913: 202 .

Neomysis sagamiensis: Illig 1930: 597 (key); W. Tattersall 1932: 317 (key); Gordan 1957: 369 (list).

Acanthomysis sagamiensis: Ii 1936: 589 (list); Banner 1948: 86 (key); Ii 1964: 488–489 (only remarks); Mauchline and Murano 1977: 45 (list); Müller 1993: 197 (list).

Type locality

Near Enoshima, Sagami Bay , central Japan .

Material examined

One male (8.6 mm), Uchiura Bay, Chiba , central Japan, 29 April 1975, NSMT-Cr 15591. Two males (10.9 and 11.4 mm), Uchiura Bay , Chiba, central Japan, 31 July 1976, NSMT- Cr 15592. Twelve males (9.9–13.0 mm) and seven females (10.1–11.9 mm), Sendai Bay , Miyagi, northern Japan, 8 m, sledge net, 22 November 1991, coll. H. Yamada, NSMT-Cr 15593. Four females (10.7–11.4 mm), Kashima, Ibaraki, central Japan, sledge net, 8 September 1996, coll. K. Kimoto, NSMT-Cr 15594. One female (9.7 mm), Kashima, Ibaraki, central Japan, sledge net, 16 October 1996, coll. K. Kimoto, NSMT-Cr 15595. One male (13.0 mm), Kashima, Ibaraki, central Japan, sledge net, 15 November 1996, coll. K. Kimoto, NSMT-Cr 15596. Ten females (12.3–14.2 mm), Kashima, Ibaraki, central Japan, sledge net, 4 December 1996, coll. K. Kimoto, NSMT-Cr 15597. Seven males (damaged, up to 15.2 mm) and 24 females (damaged, up to 17.0 mm), Kashima, Ibaraki, central Japan, sledge net, 16 January 1997, coll. K. Kimoto, NSMT-Cr 15598 .

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).

Figures

Figure 8. Orientomysis sagamiensis (Nakazawa, 1910). (A, C–H), Male (15.6 mm), NSMT-Cr 15598; (B) female (17.0 mm), NSMT-Cr 15598. (A, B) Anterior part of body, dorsal; (C) antenna, ventral; (D) mandible; (E) maxillule, posterior; (F) maxilla, posterior; (G) first thoracic limb, posterior; (H) second thoracic limb, posterior.

Figure 9. Orientomysis sagamiensis (Nakazawa, 1910). (A–J) Male (15.6 mm), NSMT-Cr 15598; (K) female (17.0 mm), NSMT-Cr 15598. (A) Third thoracic limb, posterior; (B) eighth thoracic limb, posterior; (C) penis, lateral; (D–H) first to fifth pleopods; (I) basal part of uropod, ventral; (J) uropod and telson, dorsal; (K) telson, dorsal.

Figure 11. Orientomysis tamurai (Ii, 1964) comb. nov. (A–D) Male (9.8 mm), NSMT-Cr 15599; (E) female (10.4 mm), NSMT-Cr 15599. (A) Penis, lateral; (B) fourth pleopod, posterior; (C) proximal part of uropod, ventral; (D, E) uropod and telson, dorsal.