Boreoacanthomysis, Fukuoka & Murano, 2004

Fukuoka, Kouki & Murano, Masaaki, 2004, A new genus for Acanthomysis schrencki (Czerniavsky, 1882) (Crustacea: Mysida: Mysidae) with the first description of the male, Journal of Natural History 38 (17), pp. 2135-2143 : 2136-2143

Genus Boreoacanthomysis gen. nov.

Type species Mysis schrencki Czerniavsky, 1882 .

Diagnosis

Carapace produced anteriorly to triangular rostrum; anterolateral corner rounded; posterior margin emarginate. Eye developed, slightly depressed dorsoventrally. Antennal scale lanceolate with rounded apex, armed with setae on entire margin; subapical suture present. Antennal sympod with spiniform process at outer distal angle. Maxilla with distal segment of endopod longer than wide. Carpopropodus of endopods of third to eighth thoracic limbs divided into five or six subsegments. Penis armed with several long plumose setae on anterior margin near apex, with several smooth, inwardly curved setae on distal margin, and with several long and short setae on posterior margin. Marsupium composed of two pairs of developed oostegites on seventh and eighth limbs; oostegite on seventh limb with small posterior lobe. First to third and fifth pleopods of male and all pleopods of female uniramous, reduced to unsegmented lobe, increasing in length from first to fifth; fifth pleopod of male not reaching middle of last abdominal somite. Fourth pleopod of male biramous; endopod reduced to unsegmented lobe; exopod long, two-segmented, proximal segment long, armed at inner and outer distal corners with long plumose setae extending to middle of long terminal setae, distal segment short, armed with two long, strong, subequal terminal setae and with short setae at inner and outer distal corners. Pseudobranchial lobe of pleopods poorly developed. Endopod of uropod armed densely with 20–30 spines on ventral surface near inner margin of statocyst region. Telson elongated, linguiform with narrow apex; lateral margin armed on anterior half with equal spines set rather sparsely and on posterior half with grouped spines set densely, each group consisting of one larger spine and one to three smaller ones; apex with two pairs of spines, outer spines much longer than inner.

Etymology The generic name is derived from Greek boreas, northern, and Acanthomysis .

Gender Female.

Remarks

When Holmquist (1981) established Pacifacanthomysis for Acanthomysis nephrophthalma , she assumed Acanthomysis schrencki (now Boreoacanthomysis schrencki ) to be a congener of Pacifacanthomysis , because the shape and armature of the telson and uropod show more in common with the species P. nephrophthalma than with other allied species, such as Disacanthomysis dybowskii ( Derzhavin, 1913) , Hyperacanthomysis longirostris ( Ii, 1936) or Hemiacanthomysis dimorpha ( Ii, 1936) . Boreoacanthomysis , however, is distinguished from Pacifacanthomysis as follows: the fourth pleopod of the male is armed only with a very small seta on the outer distal angle of the proximal segment of the fourth male pleopod in Pacifacanthomysis , instead of two long plumose setae on each side in Boreoacanthomysis , and the uropodal endopod is armed with five to seven spines in the ventral statocyst region in Pacifacanthomysis , compared with 20–30 spines in Boreoacanthomysis .

In the uropodal endopod with many spines and the spination of the telson, Boreoacanthomysis is similar to Disacanthomysis Holmquist, 1981 , but the former genus differs in the following respects: (1) the anterolateral corner of the carapace is rounded in Boreoacanthomysis while acutely produced in Disacanthomysis ; (2) the exopod of the fourth pleopod of the male is armed on each distal angle of the proximal segment with a long plumose seta, which reaches the middle of the terminal setae in Boreoacanthomysis , while it is armed with a long seta slightly longer than the proximal segment on the inner distal angle and with a short seta on the outer distal angle in Disacanthomysis ; (3) the fifth male pleopod is so short that the distal end does not reach the posterior margin of the last abdominal somite, and is three-quarters of the length of the fourth pleopod in Boreoacanthomysis , whereas it extends from middle to distal third of the telson and is as long as the fourth pleopod in Disacanthomysis ; and (4) the terminal seta of the fifth male pleopod of Disacanthomysis is stouter than the rest and is armed with marginal spinules, while in Boreoacanthomysis it is not specialized.

Boreoacanthomysis is easily distinguishable from Acanthomysis and the genera established for accommodation of species detached from Acanthomysis such as Alienacanthomysis Holmquist, 1981 , Exacanthomysis Holmquist, 1981 , and others by the characters of the fourth male pleopod, the uropodal endopod and the telson.

Boreoacanthomysis schrencki ( Czerniavsky, 1882) comb. nov.

( figures 1–3 )

Mysis schrencki Czerniavsky, 1882: 20 , 21, pl. 19, figures 8–23.

Mysis (?) schrencki: Zimmer, 1904: 472 , figures 171, 172.

Orientomysis schrencki: Derzhavin, 1913: 198 .

Neomysis schrenckii: Tattersall, 1932: 317 (key).

Acanthomysis schrencki: Ii, 1936: 589 (list); Banner, 1948: 87 (key); Gordan, 1957: 338 (list); Mauchline and Murano, 1977: 45 (list); Müller, 1993: 198 (list).

Acanthomysis schrencki (?): Ii, 1964: 484–486, figure 123.

Type locality The Castries Bay , Tatar Strait, eastern Siberia. Material examined

Ii’s Coll. No. 147: two immature males (damaged) and one juvenile (5.3 mm), Poromoshiri Island , northern Kurile, 12 June 1932. NSMT-Cr 15111: four males (9.1–10.0 mm), two immature males (6.2 and 6.6 mm), three females (9.1–13.3 mm) and one immature female (6.4 mm), Akkeshi Bay, eastern Hokkaido, northern Japan, seagrass beds ( Zostera asiatica ), sledge net, 15 April 1997, coll. K. Takahashi.

Description

Body smooth. All thoracic somites without sternal processes. No folds or spines on abdominal somites; first to fifth somites subequal in length; sixth somite 1.3–1.4 times as long as fifth.

Carapace produced anteriorly to form triangular rostral plate with obtusely pointed apex and concave lateral margins; apex of rostrum extending near or to base of antennular peduncles; anterolateral corner rounded; posterior margin smooth, emarginate, leaving last three thoracic somites exposed dorsally ( figure 1A, B ).

Eye slightly depressed dorsoventrally, 1.2–1.3 times as long as broad in dorsal view; cornea reniform in dorsal view, occupying two-fifths of eye; eyestalk spinulose proximally, without papilla on dorsal surface ( figure 1A, B ).

Antennular peduncle robust; in male, first segment 1.2 times as long as broad, third segment as long as first, with well-developed appendix masculina ( figure 1A ); in female, first segment 1.7 times as long as broad, third segment as long as first ( figure 1B ).

Antennal scale lanceolate with rounded apex, extending beyond distal end of antennular peduncle but not reaching apex of appendix masculina in male, and beyond distal end of antennular peduncle by one-third of its length in female, 4.5 times as long as broad, armed with long, spiniform, plumose setae along entire margin; subapical suture present ( figure 1C, D ). Antennal peduncle extending to distal two-fifths of scale in male and distal half in female; second segment longest; third segment four-fifths of second in length in male and three-fifths in female ( figure 1C, D ). Antennal sympod with spiniform process at outer distal angle ( figure 1C, D ).

Labrum with short acute anterior process ( figure 1E ).

Mandibular palp three-segmented; second segment expanded, 1.8–2 times as long as broad; third segment half of second segment in length ( figure 1F ).

Outer lobe of maxillule with about 13 robust spines on distal margin and with three slender setae on posterior surface; outer margin with hump-like process at about middle ( figure 1G, H ). Inner lobe of maxillule armed with one long, spiniform seta on inner margin, and three long spiniform, one stout plumose and numerous slender setae on distal margin ( figure 1G, H ).

Endopod of maxilla two-segmented; distal segment 1.3–1.5 times as long as broad, armed on margin with setae but lacking spines ( figure 2A ). Exopod of maxilla extending near or to distal margin of proximal segment of endopod, armed with numerous plumose setae on outer margin and long plumose seta at apex ( figure 2A ).

Endopod of first thoracic limb short and robust; pre-ischium, ischium and merus expanded medially ( figure 2B ). Endopod of second thoracic limb short and robust ( figure 2C ). Endopods of third to eighth thoracic limbs long and slender; carpopropodus divided into five subsegments by transverse articulations; dactylus with long slender claw terminally ( figure 2D, E ). Exopods of thoracic limbs with flagellum eight-segmented in first and eighth pairs, and nine-segmented in second to seventh pairs; basal plate with outer distal corner rounded without spinules ( figure 2E ).

Penis 2.3 times as long as broad in lateral view, armed with two long, inwardly curved, plumose setae on anterior margin near apex, five naked, inwardly curved setae on distal margin, and four long naked, one short naked and seven short plumose setae on posterior margin ( figure 2F ).

Female with setose tuft on basis of fourth to sixth thoracic limbs and with developed oostegite on seventh and eighth limbs; oostegite on seventh limb with small lobe posteriorly.

First to third and fifth pleopods of male and all pleopods of female uniramous, reduced to unsegmented lobe, increasing in length from first to fifth, with hardly developed pseudobranchial lobe ( figure 3 A–C, E, G). Fifth pleopod of male extending to posterior one-quarter of last abdominal somite, three-quarters of exopod of fourth pleopod in length, 1.4–1.5 times as long as third pleopod ( figure 3E, G ). Fifth pleopod of female 1.6 times as long as third, and 1.2 times as long as fourth. Fourth pleopod of male biramous; endopod reduced to unsegmented lobe; exopod long, extending to middle of last abdominal somite, twosegmented; proximal segment long, 1.6–1.9 times as long as endopod, armed at each distal angle with plumose seta, these setae subequal in length, extending to middle of terminal setae, 2.6–3.5 times as long as distal segment; distal segment short, oneeighth of proximal segment in length, armed on each distal angle with short seta and on terminal end with two long spiniform setae subequal in length and 4.3–5.1 times as long as distal segment ( figure 3D, F ).

Endopod of uropod shorter than telson, armed with row of 21–28 spines arranged densely on inner ventral surface close to statocyst; spines becoming longer posteriorly ( figure 3H, I, K ). Exopod of uropod exceeding apex of endopod by onefifth to one-quarter of its length ( figure 3I, K ).

Telson elongated, linguiform with narrowly truncate distal margin, 1.7 times as long as sixth abdominal somite, 2.3–2.4 times as long as broadest width, armed throughout with marginal spines. Spines on anterior two-fifths to half of lateral margin subequal, rather sparsely set, those on posterior half to three-fifths grouped into about 15, each spine group composed of one larger spine and one to three smaller spines of same size, larger spines subequal in length. Distal margin armed with two pairs of spines, outer pair longer, more than three times as long as inner spines, 1.5–1.7 times as long as distalmost larger lateral spines; inner spines equal to smaller lateral spines in size ( figure 3 I–K).

Remarks

The present specimens collected from Akkeshi Bay, northern Japan, generally agree with the original description, but there are recognized slight differences. The carpopropodus of the endopod of the third to eighth thoracopods in the present specimens is divided into five subsegments instead of six in the type specimens. The flagellum of the thoracopodal exopods is divided into eight or nine segments in the present specimens compared with nine or ten segments in the type specimens. The number of spines near the statocyst of the uropodal endopod is 28 at most in the present specimens while 30 in the type specimens. These differences are judged to be intraspecific variation .

Distribution

This species is known from the Castries Bay, Tatar Strait, eastern Siberia ( Czerniavsky, 1882), Kamchatka ( Derzhavin, 1913), Poromoshiri Island, northern Kurile ( Ii, 1964) and Akkeshi Bay, eastern Hokkaido, northern Japan (present study).

Figures

FIG. 1. Boreoacanthomysis schrencki (Czerniavsky, 1882) comb. nov.: NSMT-Cr 15111. (A, C, E–G) Male (9.1 mm); (B, D, H) female (13.3 mm). (A, B) Anterior end of body, dorsal; (C) antennal scale of male, dorsal; (D) antennal scale of female, ventral; (E) labrum, lateral; (F) mandible; (G, H) maxillule, posterior surface.

FIG. 2. Boreoacanthomysis schrencki (Czerniavsky, 1882) comb. nov.: NSMT-Cr 15111, male (9.1 mm). (A) Maxilla, posterior surface; (B) endopod of first thoracic limb, posterior surface; (C) endopod of second thoracic limb, posterior surface; (D) endopod of sixth thoracic limb, posterior surface; (E) eighth thoracic limb, posterior surface; (F) penis, lateral.

FIG. 3. Boreoacanthomysis schrencki (Czerniavsky, 1882) comb. nov.: NSMT-Cr 15111. (A– E, H–J) Male (9.1 mm); (F, G) male (9.5 mm); (K) female (13.3 mm). (A–C) First to third pleopod, anterior surface; (D) fourth pleopod, posterior surface (seta at outer distal angle of proximal segment broken); (E) fifth pleopod, anterior surface; (F) fourth pleopod, posterior surface; (G) fifth pleopod, anterior surface; (H) basal part of endopod of uropod, ventral; (I) telson and uropod of male, dorsal; (J) distal part of telson, dorsal; (K) telson and uropod of female, dorsal.