Orientomysis arenaria, Fukuoka & Murano, 2005

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 : 697-704

Orientomysis arenaria sp. nov.

( Figures 12 , 13 )

Material examined

Holotype: male (9.6 mm), dissected, Tateyama Bay , Chiba, central Japan, 0.7–3 m, sledge net, 20 May 1980, NSMT-Cr 15618 . Paratypes: one female (9.2 mm), dissected, Tateyama Bay , Chiba, central Japan, 0.7–3 m, sledge net, 23 April 1980, NSMT-Cr 15619 ; two males (10.0 and 10.4 mm) and five females (14.0– 17.8 mm), collection data same as NSMT-Cr 15619, NSMT-Cr 15620.

Others. Three males (9.4–10.1 mm), six immature males (6.9–9.1 mm) and 20 females (11.7–15.8 mm), collection data same as paratype (NSMT-Cr 15619), NSMT-Cr 15621. Fifty-five males (8.9–14.8 mm), 15 immature males (7.6–9.3 mm), 40 females (9.8– 13.9 mm) and 23 immature females (7.0–10.0 mm), Fukiage-hama Beach, East China Sea side of Kagoshima, western Japan, 21 March 1988, coll. T. Noichi, NSMT-Cr 15622. Three males (10.0– 11.4 mm) and two females (10.4 and 11.7 mm), mouth of Abukuma River , Miyagi, northern Japan, 1.5–4 m, 25 August 1989, NSMT-Cr 15623. Five males (10.0– 12.8 mm) and one immature female (8.3 mm), Sendai Bay , Miyagi, northern Japan, 8 m, sledge, 23 October 1991, coll. H. Yamada, NSMT-Cr 15624. Two males (10.0 and 13.0 mm), three immature males (6.0–8.0 mm) and two immature females (5.2 and 8.2 mm), Sendai Bay , Miyagi, northern Japan, 8 m, sledge, 22 November 1991, coll. H. Yamada, NSMT-Cr 15625. Three males (10.1–12.8 mm), seven immature males (6.3– 7.6 mm), three females (12.0– 13.2 mm) and three immature females (8.0– 9.6 m), Sendai Bay , Miyagi, northern Japan, 8 m, sledge, 23 October 1992, coll. H. Yamada, NSMT-Cr 15626. One male (15.7 mm) and one female (15.2 mm), mouth of Hamaoka River , Shizuoka, central Japan, beach seine, 9 April 1997, NSMT-Cr 15627. One male (damaged), Kashima, Ibaraki, central Japan, sledge net, 16 January 1997, coll. K. Kimoto, NSMT-Cr 15628 .

Description

Body robust; surface not hispid. All thoracic somites without sternal processes. All abdominal somites subequal in length; first abdominal somite with lateral fold, second to fourth somites with lateral groove, fifth and sixth somites without folds or spines.

Carapace with anterior margin produced to long triangular rostral plate with obtusely pointed apex, extending near distal margin of first segment of antennular peduncle; lateral margin of rostrum concave ( Figure 12A, B ); anterolateral corner of carapace rounded; posterior margin of carapace emarginate, leaving last one or two thoracic somites exposed dorsally, furnished medially with minute setae.

Eye somewhat depressed dorsoventrally, about 1.2 times as long as broad in dorsal view; cornea comprising two-fifths of eye in dorsal view; eyestalk with very small, blunt papilliform process on dorsal surface, hispid proximally ( Figure 12A, B ).

Antennular peduncle of male more robust than that of female; in male first segment 1.2 times as long as broad, third segment almost same length of first and second segments combined and 1.3 times as long as broad, with developed appendix masculina ( Figure 12A ); in female first segment 1.3 times as long as broad, third segment slightly longer than proximal two segments combined and 1.7 times as long as broad ( Figure 12B ).

Antennal scale lanceolate with rounded apex, 1.6 times longer than antennular peduncle, 6–6.5 times as long as broad, setose on entire margin; subapical suture present ( Figure 12C ). Antennal peduncle extending to proximal two-fifths of scale; second segment longest, 1.5 times as long as broad in male, and twice as long as broad in female; third segment four-fifths of length of second, 1.3 times as long as broad in male, and 1.5 times as long as broad in female ( Figure 12C ). Antennal sympod with spiniform process at outer distal angle ( Figure 12C ).

Labrum with short, spiniform process on anterior margin ( Figure 12D ).

Mandibular palp three-segmented; second segment expanded in middle, 2.2 times as long as broad; third segment two-fifths of second in length ( Figure 12E ).

Outer lobe of maxillule armed with 13 stout spines on distal margin and with three setae on ventral surface; outer margin with hump-like process armed with a few minute spines on distal base ( Figure 12F ).

Endopod of maxilla with second segment armed with five or six small spines among long setae on outer margin; exopod not reaching distal margin of first segment of endopod, armed with plumose setae on outer and apical margins; basal endite armed with many small spines on surface ( Figure 12G ).

Endopod of first thoracic limb short and robust; preischium, ischium and merus slightly expanded inwards ( Figure 12H ). Endopod of second thoracic limb robust; merus 3.2 times as long as broad; carpopropodus slightly shorter than merus ( Figure 12I ). Endopods of third to eighth thoracic limbs with five-subsegmented carpopropodus; dactylus with long, slender claw terminally ( Figure 13A, B ). Exopods of thoracic limbs with flagellum eightsegmented in first and eighth limbs and nine-segmented in second to seventh limbs; basal plate with outer distal corner rounded armed partly with many spinules on margin ( Figures 12H, I , 13A, B ).

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.

Discussion

The current taxonomic positions of the nominal Acanthomysis species revised by Holmquist (1979, 1980, 1981a, 1981b) and Fukuoka and Murano (2000, 2001a, 2001b, 2002, 2004, present study) are listed in Table VIII. Seven species, A. anomala Pillai, 1961 ( India) , A. borealis Banner, 1954 (East Russia and Alaska), A. californica Murano and Chess, 1987 (California) , A. macrops Pillai, 1973 ( India) , A. ornata O. Tattersall, 1965 ( Malacca Strait), A. stelleri ( Derzhavin, 1913) (East Russia and Alaska), and A. thailandica Murano, 1988 ( Thailand) , still remain in Acanthomysis s. l. as insertae sedis.

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.

Figures

Figure 12. Orientomysis arenaria sp. nov. (A, C–I) Male (9.6 mm), holotype, NSMT-Cr 15618; (B) female (17.8 mm), paratype, NSMT-Cr 15619. (A, B) Anterior part of body, dorsal; (C) antenna, ventral; (D) labrum, lateral; (E) mandible; (F) maxillule, posterior; (G) maxilla, posterior; (H) first thoracic limb, posterior; (I) second thoracic limb, posterior.

Figure 13. Orientomysis arenaria sp. nov. (A–J) Male (9.6 mm), holotype, NSMT-Cr 15618; (K) female (17.8 mm), paratype, NSMT-Cr 15619. (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, K) uropod and telson, dorsal.