Exacanthomysis marsailiae and Nipponomysis neolingvura, two new species of mysid (crustacea: mysida: mysidae) from the east coast of Korea

Order Mysida Boas, 1883

Family Mysidae Haworth, 1825

Subfamily Mysinae Haworth, 1825

Tribe Neomysini Wittmann, Ariani & Lagardère, 2014

Genus ExacanthomysisHolmquist, 1981

Exacanthomysis marsailiae sp. nov. (Figs. 1–4)

Fig. 1. Exacanthomysis marsailiae sp. nov. Holotype, male (BL 14.0 mm: A, B, E), allotype, female (BL 15.5 mm: C, D). A, C: anterior part of body; B, D: antenna; E: labrum, ventral. Scale bars: 0.5 mm for A–D; 0.25 mm for E.

Download Original Figure

Fig. 2. Exacanthomysis marsailiae sp. nov. Holotype, male (BL 14.0 mm: A–E). A: lateral view of abdominal somites; B: second thoracopod; C: enlarged terminal part of second thoracopodal endopod; D: seventh thoracopod; E: enlarged terminal part of seventh thoracopodal endopod. Scale bars: 1.0 mm for A; 0.5 mm for B, D; 0.25 mm for C, E.

Download Original Figure

Fig. 3. Exacanthomysis marsailiae sp. nov. Holotype, male (BL 14.0 mm: A–E). A: first pleopod; B: second pleopod; C: third pleopod; D: fourth pleopod; E: fifth pleopod. Scale bars: 0.25 mm for A–E.

Download Original Figure

Fig. 4. Exacanthomysis marsailiae sp. nov. Holotype, male (BL 14.0 mm: A–C), allotype, female (BL 15.5 mm: D–F). A, D: dorsal view of telson and uropod; B, E: enlarged distal part of telson; C, F: uropd. Scale bars: 0.5 mm for A, C, D, F; 0.25 mm for B, E.

Download Original Figure

New Korean name: 진주곤쟁이

Acanthomysis sp. Ii, 1964: 511-513, fig. 132.

Holotype, adult male (14.0 mm in body length), MABIK CR00253006. Allotype, adult female (15.5 mm), MABIK CR00253007. Paratypes, 5 adult females (14.0–14.5 mm), 5 adult females (14.5–15.5 mm) MABIK CR00254248. Byeonggok-ri, Byeonggok-myeon, Yeongdeok-gun, Gyeongsangbuk-do (36°35’27.1”N, 129°27’29.6”E); all specimens collected by a light trap at 50 m depth, 12 April 2022.

Body robust (Figs. 1A, 1C, and 2A). Abdomen (Fig. 2A) 6-segmented with transverse dorsolateral folds: somites 1–3 usually with 1-fold, somite 4 with 1 or 2, somite 5 with 2 or 3, somite 6 always with 3. Carapace (Fig. 1A and 1C) with anterior margin produced into wide triangular rostral plate with edges slightly raised upward, completely exposing eyestalk dorsally; anterolateral corners of carapace acutely pointed; rostrum with pointed apex. Eyes (Fig. 1A and 1C) extending to middle of segment 3 of antennular peduncle; cornea much wider than eyestalk; eyestalk decreasing sharply in width from distally to proximally, with fine hairs on basal half. Antennular peduncle (Fig. 1A and 1C) slender, slenderer in females; segment 1 almost the same length as segment 3 in males, slightly shorter than segment 3 in females. Antennal peduncle (Fig. 1B and 1D) extending slightly beyond middle of antennal scale in males, slightly below middle of antennal scale in females; three-segmented, segment 2 the longest, 1.3 times segment 3. Antennal scale (Fig. 1B and 1D) slender and setose along entire margins, 7.5 times maximum width in females, 8.5 times in males; transverse suture distinct, extending slightly beyond appendix masculine. Labrum (Fig. 1E) armed with no spines on each side of anterior spinous process. Thoracopods all biramous; endopod and exopod slender and well developed; thoracopods 1 and 2 (Fig. 2B and 2C) similar in general form, with basal plate of exopod bearing denticle at outer distal corner; The remaining thoracopods similar in structure to each other as thoracopod 7 (Fig. 2D and 2E): thoracic endopods 3 to 8 with carpopropodus divided into five subsegments armed with only setae; dactylus with long and slender claw. Thoracic exopods 3 to 8 with basal plate smooth without denticles at outer distal corner; its flagellum segmented into 10. All male pleopods (Fig. 3A–3E) unsegmented and reduced to uniramous except for pleopod 4; pleopod 5 more slender and longer than the other pleopods (pleopods 1 to 3), extending posteriorly by three-quarters of abdominal segment 6. Pleopod 4 biramous, extending posteriorly to middle of abdominal segment 6; endopod rudimentary and unsegmented, half length of proximal segment of exopod; exopod two-segmented, with proximal segment about 7 times longer than distal segment: distal segment having two long subequal terminal setae, which are more than 3 times longer than distal segment. Telson (Fig. 4A, 4B, 4D, and 4E) narrow and elongated in overall appearance, about 2.5 times longer than width at its base; telson width decreasing rapidly in the proximal quarter, becoming parallel in next quarter, gradually decreasing in distal half. Lateral margins armed with spines throughout, with proximal half being relatively sparsely armed with about 16 spines; distal half more densely armed with spines arranged in 11 to 13 groups with two to five smaller spines of increasing size between two larger ones, large spines becoming slightly smaller toward apex; apex narrow and round, armed with two pairs of spines, inner pair much smaller than outer one. Uropods (Fig. 4C and 4F) fringed with plumose setae along margins; endopod slightly shorter than telson, armed with 5 to 7 spines near statocyst; spines increasing in size distally; exopod elongated, about 1.3 times longer than telson, about 1.4 times longer than endopod.

The species name comes from the Gaelic equivalent of pearl and was named to celebrate the birth of Jo’s granddaughter.

Holmquist (1981) established the genus Exacanthomysis to accommodate Acanthomysis davisiBanner, 1954 and Acanthomysis alaskensisBanner, 1954, and at the same time included the new species Exacanthomysis arctopacifica. Afterwards, Petryashov (1992) organized E. arctopacificaHolmquist, 1981 as the junior synonym of Acanthomysis stelleri (Derzhavin, 1913), and Sedova et al. (2016) changed Acanthomysis borealisBanner, 1954 into species of the genus Exacanthomysis. According to Mees & Meland (2023), the number of species in the genus Exacanthomysis is currently known to be three, namely Exacanthomysis davisi (Banner, 1948), Exacanthomysis alaskensis (Banner, 1954) and Exacanthomysis borealis (Banner, 1954). With the report of this new species, the genus Exacanthomysis consists of a total of four species.

E. marsailiae sp. nov. was first reported as Acanthomysis sp. by Ii (1964) based only on female specimens collected near Ulsan on the east coast of Korea. He believed that this species was probably a new species, although it was similar to E. alaskensis (Banner, 1954), E. borealis (Banner, 1954), as well as Paracanthomysis kurilensisIi, 1936. However, since his specimens at that time did not contain any specimens of males, which are more taxonomically important than females, the species was left unnamed as a new species.

The specimens in this study were collected from the coast approximately 100 km north of the specimen collection site of Ii (1964). The male specimen collected in this study is similar to P. kurilensis in the rostrum, telson spine arrangement, and folds on the abdominal somites, but can be easily distinguished by the male’s fourth pleopod. That is, in P. kurilensis, the exopod of the fourth pleopod is unsegmented, whereas in this species, it is 2-segmented. As previously mentioned, E. marsailiae sp. nov. resembles the remaining species of the genus Exacanthomysis in some morphological characteristics, namely the rostrum, the fourth male pleopod, the form of the telson, the arrangement of the spines of the telson, the several-segmented carpopropodus of the endopod of the third to eighth thoracopods, etc. However, they can be distinguished by a combination of characters such as antennae, exopods of the first and second thoracopods, and the fourth male pleopod. Table 1 provides a comparison of external morphological characteristics that can be helpful in distinguishing between species belonging to the genus Exacanthomysis.

Until now, all species of the genus Exacanthomysis have been reported only from the high-latitude northeast Pacific or northwest Pacific (Table 1). In this study, E. marsailiae was added as a new member of the genus Exacanthomysis from the East Sea of Korea, expanding the distribution range of this genus further south to the mid-latitude waters of the Pacific Ocean.

Genus NipponomysisTakahashi & Murano, 1986

Nipponomysis neolingvura sp. nov. (Figs. 5 and 6)

New Korean name: 가시곤쟁이

Holotype, adult male (9.0 mm body length), MABIK CR00253008. Galnam Port, Galnam-ri, Wondeok-eup, Samcheok-si, Gangwon-do (37°16’52.04”N, 129°19’26.63”E); light trap, 18 May 2022.

Abdominal segments smooth without grooves or folds; segment 6 longest, 1.4 times as long as segment 5. Carapace (Fig. 5A) with anterior margin produced into triangular rostral plate, dorsally leaving eyestalk exposed; rostrum with obtusely angled apex, reaching middle of segment 1 of antennular peduncle. Eyes (Fig. 5A) extending beyond segment 2 of antennular peduncle; eyestalk broader than length; cornea wider than eyestalk. Antennular peduncle (Fig. 5A) robust, three-segmented, segment 3 as long as basal and second segments combined, 2.6 times longer than segment 2 at inner margin; appendix masculine densely covered with long sensory hairs. Antennal peduncle (Fig. 5B) three-segmented, segment 2 longest, 1.3 times segment 3. Antennal scale (Fig. 5B) with distinct transverse suture, lanceolate, setose along entire margins, 3.5 times as long as broad, 1.2 times longer than antennal peduncle, not extending beyond distal margin of appendix masculine. Labrum (Fig. 5C) armed with no spines on each side of strong median frontal process. Thoracopods all biramous with well-developed endopod and exopod; thoracopods 3 to 8 similar to each other as thoracicopod 7 (Fig. 5D and 5E); edopods with carpopropodus divided into 3 subsegments armed with only setae, of which most proximal subsegment longest, second longer than third; exopods with basal plate smooth without denticle at outer distal margin, flagellum 10-segmented. Pleopods (Figs. 5F and 5G, 6A-6C) uniramous except for pleopod 4; pleopod 4 biramous, reaching middle of abdominal segment 6, not extending beyond end of pleopod 5, with exopod three-segmented; segment 1 of exopod armed with one long seta extending beyond segment 2, slightly less than twice as long as endopod, 3.6 times longer than remaining two segments combined; segment 2 slightly longer than segment 3, segment 3 terminating into two strong setae of slightly different lengths. Telson (Fig. 6D) tongue-shaped, 1.6 times as long as maximum width near base; lateral margin slightly concave in proximal half, armed with spines without any spineless parts; proximal one-third of lateral margin armed with spines of approximately equal size, distal two-thirds more densely armed with spines arranged in eight or nine groups with one to five small spines of different sizes between two larger spines; larger spines near apex gradually increase in size; apex armed with two pairs of spines, with outer spines much larger than inner ones. Uropods (Fig. 6D and 6E) fringed with plumose setae along lateral margins; endopod armed with 17 graded spines on inner ventral margin near statocyst, extending beyond telson by about one-fifth of length of telson; exopod extending by about half length of telson.

Fig. 5. Nipponomysis neolingvura sp. nov. Holotype, male (BL 9.0 mm: A–G). A: anterior part of body; B: antenna; C: labrum, ventral; D: seventh thoracopod; E: enlarged distal part of seventh thoracopodal endopod; F: first pleopod; G: second pleopod. Scale bars: 0.5 mm for A, B, D; 0.25 mm for C, E–G.

Download Original Figure

Fig. 6. Nipponomysis neolingvura sp. nov. Holotype, male (BL 9.0 mm: A–E). A: third pleopod; B: fourth pleopod; C: fifth pleopod; D: dorsal view of telson and uropod; E: uropod. Scale bars: 0.5 mm for B–E; 0.25 mm for A.

Download Original Figure

The species name comes from Nipponomysis lingvura, which has a remarkably similar external morphology.

In this survey, female individuals of N. neolingvura sp. nov. was not collected, so only male characteristics are described. N. neolingvura is remarkably similar characteristics to N. lingvura (Murano, 1977) except for a few characters. However, the present species is justified as a new species on the basis that the telson is armed with spines on all margins, that the rostral apex of carapace extends slightly beyond the distal half of the antennular peduncle, and that the fourth male pleopod has a single elongated seta, which extends beyond the second segment, at the end of the first segment of the exopod.

On the other hand, in N. lingvura, there is about 1/6 of the spineless part on both sides of the telson, which is an important characteristic that distinguishes this species from other species in the genus Nipponomysis. In addition, the rostral apex of carapace in N. lingvura reaches near the distal end of the first segment of antennular peduncle and the fourth male pleopod has two short setae, which do not extend beyond the second segment, at the end of the first segment of the exopod.

N. neolingvura sp. nov. also resembles Nipponomysis calcarataTakahashi and Murano, 1986, Nipponomysis fusca (Ii, 1936), and Nipponomysis sandoi (Ii, 1964), but can be distinguished by combinations such as the rostral plate, the fourth male pleopod, the arrangement of spines on the telson, and the number of spines near the statocyst of the endopod of uropod. A detailed comparison between these species is shown in Table 2.