Orientomysis Derzhavin, 1913

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 : 658-663

publication ID	https://doi.org/ 10.1080/00222930400001418
publication LSID	lsid:zoobank.org:pub:94C1EB6D-F636-485E-931F-8A217B2B0DD3
persistent identifier	https://treatment.plazi.org/id/0A1A87E5-FFB5-9B11-FE29-25964375E9BB
treatment provided by	Felipe
scientific name	Orientomysis Derzhavin, 1913
status

Treatment

Genus Orientomysis Derzhavin, 1913 View in CoL

Metamysis Nakazawa 1910: 250 .

Orientomysis Derzhavin 1913: 200–202 View in CoL .

Type species

Metamysis mitsukurii Nakazawa, 1910 .

Diagnosis

Carapace anteriorly produced to triangular rostral plate; anterolateral corner rounded; posterior margin emarginate. Eye somewhat dorsoventrally compressed. Antennal scale lanceolate with rounded apex, setose throughout margins; subapical suture present. Antennal sympod with spiniform process at outer distal angle. Labrum with anterior spiniform process. Carpopropodus of endopod of third to eighth thoracic limbs divided into three to eight subsegments. Penis armed with several setae on posterior margin, several medially curved setae on distal margin, and several plumose setae on distal half of anterior margin. Marsupium composed of two pairs of developed oostegites; oostegite of seventh limb with baling lobe. First, second, third, and fifth pleopods of male and all pleopods of female reduced to unsegmented single lobe, gradually increasing in length from first to fifth; pseudobranchial lobe poorly developed. Fourth pleopod of male biramous; endopod reduced to unsegmented lobe; exopod elongate, extending to last abdominal somite, but not overreaching distal end of abdomen, two-segmented, proximal segment long, distal segment short, one-seventh to one-fourth of length of proximal segment, with two long, strong, subequal terminal setae (except O. leptura and O. tenuicauda , being unequal). Endopod of uropod armed with one to six spines on inner ventral surface of statocyst region (except in O. crassispinosa with no spine). Telson elongate triangular or linguiform; lateral margin with spines throughout (except O. sagamiensis and O. tamurai , with spineless part), on distal half to three-fourths these spines more or less arranged in clusters.

Other species

General morphology

Integument. Body not hispid except O. aspera and O. crassispinosa ; thoracic somites without sternal process except O. sheni , in which a process is present on each of second to sixth and eighth somites of female; abdominal somites with or without transverse grooves, folds and/ or row of spines ( Table I).

Carapace. Rostral plate triangular with concave lateral margins; anterolateral corner rounded; posterior margin emarginate, exposing posterior one to four thoracic somites dorsally, with or without minute setae on median part ( Table I).

Eye. Well-developed, slightly depressed dorsoventrally; eyestalk with or without papilla on dorsal surface ( Table I).

sm, smooth; gr, groove present; fo, fold present; sp, spine present; ++, present; 2, absent.

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.

Kingdom	Animalia
Phylum	Arthropoda
Class	Malacostraca
Order	Mysida
Family	Mysidae