Meterythrops tenuispinis, Fukuoka & Murano, 2006

Fukuoka, Kouki & Murano, Masaaki, 2006, Taxonomy of the genus Meterythrops (Crustacea: Mysida: Mysidae), with a redescription of M. microphthalmus and descriptions of two new species, Journal of Natural History 40 (27 - 28), pp. 1641-1674 : 1665-1670

Meterythrops tenuispinis sp. nov.

( Figures 10 , 11 , 12H )

Material examined

Holotype: NSMT-Cr 16761, male (8.6 mm), dissected, Stn 13, Enshu-nada (34 ° 279N, 137 ° 309E), central Japan, 247 m, sledge net, 17 July 1996, R / V Tansei-Maru cruise . Paratypes: NSMT-Cr 16762, one female (8.4 mm), dissected, same data as holotype ; NSMT-Cr 16763, one male (9.0 mm) and five immature females (4.7–6.3 mm), same data as holotype ; NSMT-Cr 16764, one male (9.2 mm), one immature male (5.3 mm), and two females (8.3 mm and broken), Stn B 11-1, East China Sea (31 ° 15.79N, 128 ° 20.69E), 364– 369 m, beam trawl, 5 August 1974, R / V Hakuho-Maru cruise .

Description

Body smooth, constricted between thorax and abdomen. Thoracic somites without sternal processes. First to fifth abdominal somites subequal, sixth somite 1.3–1.5 times as long as fifth one; in male anterior four somites with papilliform process at posterior end along ventral median line, these processes gradually becoming smaller posteriorly.

Carapace anteriorly produced into linguiform rostral plate not reaching base of antennular peduncle; lateral margin of rostrum deeply concave ( Figure 10A, B ). Anterolateral corner of carapace acute. Posterior margin of carapace emarginate, leaving last thoracic somite exposed dorsally.

Eyes well developed, depressed dorsoventrally, four-fifths as long as broad in dorsal view; cornea occupying half to three-fifths of eye; eyestalk with small papilla on dorsal surface ( Figure 10A, B ).

Antennular peduncle: third segment as long as preceding two segments combined, with developed appendix masculina in male ( Figure 10A ).

Antennal scale extending slightly beyond distal end of antennular peduncle, 3.5–3.8 times as long as broad, with suture near apex; lateral margin naked, terminating in spiniform process; apical lobe occupying one-fourth of scale length, as long as broad at base, 2.8–2.9 times as long as spiniform process on lateral margin ( Figure 10 A–D).

Labrum without spiniform process on frontal margin.

Mandibular palp: second segment cylindrical, armed with numerous setae on lateral margin; third segment half of second one in length ( Figure 10E ).

Maxillule: lateral lobe armed with 13 spines on distal margin and with three long setae near distal end of ventral surface, lateral margin with small swelling in middle ( Figure 10F ).

Maxilla: exopod extending slightly beyond distal margin of proximal segment of endopod; endopod two-segmented, distal segment 2.3–2.4 times as long as broad, armed with long setae on margin ( Figure 12G ).

First thoracopodal endopod short, robust ( Figure 11A ); second to eighth thoracopodal endopods missing. Thoracopodal exopods with 10-segmented flagellum, with blunt lobule at lateral distal angle of basal plate.

Penis 3.3 times as long as broad in lateral view, armed with nine plumose setae on posterolateral margin; distal end bilobed, anterior part overreaching posterior one, armed with 11 long, mesially curved setae on distal margin ( Figure 11B ).

Female with reduced oostegite on sixth thoracopod, and with developed oostegite on seventh and eighth thoracopods.

All pleopods of male biramous; endopod of first pleopod reduced to unsegmented single lobe; endopod of second to fifth pleopods seven-segmented, with rectangular pseudobranchial lobe; exopod of all pleopods seven-segmented ( Figure 11 C–E). Endopod of fifth pleopod armed on distal end with two modified setae, which are thicker and longer than other normal plumose setae, and furnished with setules on distal half except for distal seventh part naked; exopod of fifth pleopod without modified setae ( Figure 11E ). All pleopods of female reduced to unsegmented single lobe; first pleopod six-sevenths length of second pleopod, second to fourth pleopods subequal, fifth pleopod 1.9 times as long as fourth pleopod.

Endopod of uropod extending beyond apex of telson for half of its length, armed with single slender spine on ventral surface close to statocyst ( Figure 11G, F ). Exopod of uropod 1.4–1.5 times as long as endopod ( Figure 11G ).

Telson short triangular with narrowly truncated apex, 0.9–1 times as long as last abdominal somite, 1.1–1.3 times as long as maximum width; lateral margin slightly concave, unarmed with spines, barely serrulated on distal third; distal margin armed with median pair of plumose setae and two pairs of spines, mesial pair of spines 1.6–2 times as long as lateral one ( Figure 11G, H ).

Etymology

The specific name is derived from Latin tenuis, slender, and spina, spine, referring to a slender spine on the uropodal endopod.

Remarks

Meterythrops tenuispinis resembles M. japonicus in the short telson, but differs by having a single slender spine on the ventral statocyst region of the uropodal endopod, dorsoventrally depressed eyes, and the long apical lobe of the antennal scale. Furthermore, it is distinct from the other species of the genus by having a small body size, short telson, and a single slender spine in the statocyst region of the uropodal endopod.

Meterythrops tenuispinis is similar to Pleurerythrops monospinosa in the shape and armature of the telson, and the presence of a single spine on the uropodal endopod. However, the lateral margins of the rostral plate are deeply concave in M. tenuispinis , whereas they are slightly or not at all concave in P. monospinosa . The fifth pleopod in the male is armed with modified setae on the distal end of the endopod in M. tenuispinis , whereas P. monospinosa males lack modified setae. The eyes of M. tenuispinis are larger than those of P. monospinosa .

Figures

Figure 10. Meterythrops tenuispinis sp. nov. (A, C, E–G) Male (8.6 mm, holotype, NSMT-Cr 16761); (B, D) female (8.4 mm, paratype, NSMT-Cr 16762). (A, B) Anterior end of body, dorsal; (C, D) left antenna, ventral; (E) left mandible; (F) left maxillule, posterior; (G) left maxilla, posterior.

Figure 11. Meterythrops tenuispinis sp. nov. (A, H) Female (8.4 mm, paratype, NSMT-Cr 16762); (B–G) male (8.6 mm, holotype, NSMT-Cr 16761). (A) Endopod of right first thoracopod; (B) left penis, lateral; (C) left first pleopod, posterior; (D) right fourth pleopod, anterior; (E) left fifth pleopod, posterior; (F) left uropod, ventral; (G) uropod and telson, dorsal; (H) posterior part of telson, dorsal.

Figure 12. (A) Meterythrops japonicus, from Ogasawara, Japan; (B) M. microphthalmus, from Alaska; (C) M. microphthalmus, from the Japan Sea; (D) M. pictus, from Scotland; (E) M. robustus, type, from Massachusetts Bay, USA; (F) M. robustus, from Onagawa, Japan; (G) M. intermedius, from Alaska; (H) M. tenusipinis, from Enshunada, Japan.