Taxonomy of the genus Meterythrops (Crustacea: Mysida: Mysidae), with a redescription of M. microphthalmus and descriptions of two new species

A taxonomic revision of the genus Meterythrops is provided, including a redescription of M. microphthalmus and newly reported characters of M. japonicus, M. pictus, and M. robustus. Adult males of all species (excluding M. japonicus and M. megalops because no adult males of these species have been collected) have pleopods with modified setae, which are found on the fifth pleopod in M. microphthalmus and M. robustus, and on the fourth and fifth pleopods in M. pictus. Two new species, M. intermedius and M. tenuispinis, from Alaskan and East Asian waters, respectively, are also described. Males of both species also have modified setae on the fifth pleopod. Meterythrops microphthalmus and M. robustus are recorded for the first time from Alaskan waters and Japanese waters, respectively.


Introduction
The genus Meterythrops was established by Smith (1879) to describe M. robusta Smith, 1879 from the northeastern coast of North America. Since then, four species, M. picta Holt and W. Tattersall, 1905, M. microphthalma W. Tattersall, 1951, M. megalops Ii, 1964, and M. japonica Murano, 1977, have been added. In 1977 Meterythrops sp. on the basis of two female specimens from the stomach of a tadpole sculpin, Malacocottus gibber Sakamoto, 1930. We report new taxonomic information for all known Meterythrops species, with exception of M. megalops, and two new species.
This paper revises the genus Meterythrops, including a redescription of M. microphthalmus, supplemental descriptions of M. japonica, M. picta, and M. robusta, and first descriptions of the two new species.
Body length was measured from the tip of the rostrum to the posterior end of the telson, excluding spines. The following abbreviations are used: NSMT, National Science oostegite in Meterythrops sp . Murano, 1977 and M. tenuispinis, and a tuft of setae in M. microphtahlmus, M. pictus, and M. intermedius on the sixth thoracopod. Ii (1964) reported that male Meterythrops have pleopods lacking modified setae. However, we report modified setae on the fifth pleopod in M. microphthalmus and M. robustus, and on the fourth and fifth pleopods in M. pictus. It is unknown whether the other two species, M. japonicus and M. megalops, have these modified setae because adult males have not yet been collected.
Meterythrops resembles Amathimysis Brattegard, 1969, Katerythrops Holt and W. Tattersall, 1905, and Parerythrops Sars, 1869 in the shape and armature of the telson. It differs, however, from Amathimysis and Katerythrops by having a median pair of plumose setae on the telson apex. It differs from Parerythrops in that the first pleopod of males has a multi-segmented exopod.
Meterythrops is also similar to Pleurerythrops Ii, 1964 in the shape and armature of the telson. However, there is taxonomic confusion within Pleurerythrops, which currently includes five species: P. inscita Ii, 1964, P. secunda Murano, 1970, P. constricta Panampunnayil, 1977, P. americana Zoppi de Roa and Delgado, 1989, and P. monospinosa Liu and Wang, 1986. Pleurerythrops inscita and P. secunda have a long, bare, or slightly barbed seta on the ultimate and penultimate segments of the fifth pleopod in males (Ii 1964;Murano 1970a;Panampunnayil 1998). Pleurerythrops americana males also have two modified setae on the ultimate segment of the endopod of the fifth pleopod, but unlike all other Pleurerythrops species, do not have a pair of plumose setae on the apex of the telson (Zoppi de Roa and Delgado 1989). The pleopods of male Pleurerythrops monospinosa lack modified setae (Liu and Wang 1986). Only female Pleurerythrops constricta are known (Panampunnayil 1977). Meterythrops is distinct from Pleurerythrops s. str. (comprising P. inscita and P. secunda) in both body shape and in the characters of the pleopods of males. Meterythrops has a cylindrical body, as opposed to the somewhat depressed body in Pleurerythrops. The pleopods of male Meterythrops are densely covered in minute, modified setae, whereas those of Pleurerythrops are remarkably long and either bare or sparsely barbed distally. Murano, 1977 (Figures 1, 12A) Meterythrops japonica Murano 1977, p 169-171, Figure 18;Mü ller 1993, p 123 (list).

Distribution
Meterythrops japonicus had been known only from the type locality, Tateyama Bay, central Japan (Murano 1977). The present occurrence from adjacent waters of Ogasawara Islands, southern Japan, is the second record. The specimens were collected from the near-bottom layer at 220-543 m deep with a sledge net (Murano 1977;present study). Murano (1977) established this species on the basis of an adult female specimen and assigned it temporarily to Meterythrops because he could not determine whether it belonged to Meterythrops or Parerythrops without a male specimen. The two immature males collected in the present study from adjacent waters of the Ogasawara Islands support the original description by Murano (1977). The exopod of the first pleopod in these immature male specimens, although undeveloped, is divided into multiple segments ( Figure 1A). Thus, this species likely belongs to Meterythrops, although adult male specimens need to be examined to confirm this classification. Murano (1977) did not report on the marsupium in the original description. The type specimen and the new specimens from the Ogasawara Islands have a developed oostegite on the seventh and eighth thoracopods, and the oostegite on the seventh thoracopod bears a baling lobe. Ii, 1964 Meterythrops megalops Ii 1964, p 320-322, Figure 83;Mü ller 1993, p 123 (list).

Distribution
This species is known only from Sagami Bay, central Japan (Ii 1964). (1964) established M. megalops on the basis of a single immature female (6 mm) in fragmentary condition collected from Sagami Bay, central Japan. This species has not been collected since then, in spite of intensive collection efforts near the type locality. Murano (1977) suggested that M. megalops may be a young form of M. pictus. The type material is missing.

Description
Body robust, constricted between thorax and abdomen. Integument smooth. First to third abdominal somites with blunt small process on median line of ventral side; first somite 1.4 times as long as second, second to fifth somites subequal, and sixth somite twice as long as preceding one. Carapace produced anteriorly into triangular rostral plate with blunt apex extending slightly beyond base of antennular peduncles; lateral margins of rostrum slightly convex; anterolateral corner pointed; posterior margin emarginate, leaving last thoracic somite exposed dorsally (Figure 2A, B).
Eyes small, subcylindrical, equal to or narrower than width of proximal segment of antennular peduncle, 1.3-1.5 times as long as broad in dorsal view; cornea occupying about two-fifths of eye, only slightly wider than stalk; eyestalk with papilliform process on dorsal surface (Figure 2A, B).
Antennular peduncle of male more robust than that of female, third segment 1.3 times as long as first, 1.3 times as long as broad, with developed appendix masculina; third segment of female 1.4 times as long as broad (Figure 2A, B).
Antennal scale extending beyond antennular peduncle for one-third of its length, about four times as long as broad; lateral margin naked, terminating in spiniform process; apical lobe occupying three-sevenths of scale in length, twice as long as broad at base, with suture near apex (Figure 2A-C). Antennal peduncle robust, extending to distal two-fifths of scale ( Figure 2C). Antennal sympod with spiniform process at lateral distal corner ( Figure 2C).
Labrum without frontal process. Mandibular palp: second segment 3.5 times as long as broad, armed with numerous setae on mesial and lateral margins; third segment two-fifths of second in length ( Figure 2D).
Maxillule: lateral lobe armed with about 12 spines on distal margin and three barbed setae on ventral surface, lateral margin with fine setae on distal half and small lobe in middle ( Figure 2E).
Maxilla: second segment of endopod 1.7 times as long as broad, without spines on margins ( Figure 2F).
Female with tuft of setae on coxa of sixth thoracopod and with developed oostegite on seventh and eighth thoracopods; oostegite on seventh thoracopod with baling lobe.
All pleopods of male well developed, biramous ( Figure 3B-F). First pleopod with endopod reduced to unsegmented lobe extending to distal margin of third segment of exopod; exopod 14-segmented ( Figure 3B). Second to fourth pleopods with 12-segmented endopod and 12-segmented exopod, both rami almost same length, without modified setae ( Figure 3C-E). Fifth pleopodal endopod 10-segmented, longer than exopod, armed on ultimate segment with pair of modified setae, which are slightly longer and thicker than normal setae, and furnished with rather long, fine setae on proximal half and short setae on distal half except for distal one-fifteenth naked ( Figure 3F, G). Fifth pleopodal exopod 10segmented, terminating in normal plumose seta and modified seta, latter seta 1.6 times as long as former one, furnished with fine setae on proximal two-thirds and spinulated on distal one-third except for distal one-eleventh naked ( Figure 3F).
All pleopods of female reduced to unsegmented single lobe, gradually increasing in length towards posterior pair.
Telson elongated triangular with narrowly truncate apex, 1.2 times as long as last abdominal somite, 1.7-2 times as long as maximum width; lateral margin without spines, almost straight; posterior margin narrow, armed with two pairs of slender spines, mesial pair of which is 1.4 times longer than lateral one, and with median pair of plumose setae ( Figure 3J).

Distribution
This species has been recorded from the cold-water region of the western North Pacific: the Sea of Okhotsk (Tattersall 1951;Birstein and Tchindonova 1958), the Japan Sea (Tattersall 1951;Murano 1977;Jo et al. 1998), the Pacific Ocean off the Kurile Islands (Birstein and Tchindonova 1958;Murano 1977) and the Pacific Ocean off northeastern and central Japan (Taniguchi 1969;Murano 1977). The present collection from Alaskan waters is the first record from the eastern North Pacific.
This species is distributed in the mesopelagic zone (Tattersall 1951;Murano 1977;Ikeda 1991). Murano (1977) reported this species from depths of 350 to 2000 m in the daytime and 0 to 1650 m in the nighttime in the northern part of the Japan Sea. Ikeda (1991) also reported the major population of this species inhabited consistently below 250 m in the Toyama Bay, the Japan Sea.

Supplemental description
Penis 2.5 times as long as broad in lateral view, armed with about 15 short plumose setae on posterior margin and three short, naked setae near apex of posterolateral part; apex rounded armed with three long, mesially curved setae and a short seta ( Figure 4A). Female with tuft of setae on sixth thoracopod and with developed oostegites on seventh and eighth thoracopods; oostegite on seventh thoracopod with baling lobe.
Fourth pleopodal endopod of male six-segmented; third, fourth, and fifth segments armed with modified seta at mesial distal angle, these setae extending beyond tip of terminal setae on ultimate segment, slightly thicker than others and furnished with fine setae on distal one-fourth ( Figure 4B-D). Fourth pleopodal exopod of male six-segmented, as long as endopod, without modified setae ( Figure 4B).
Fifth pleopodal endopod of male six-segmented, without modified setae ( Figure 4E). Fifth pleopodal exopod of male six-segmented, as long as endopod; fourth and fifth segments armed with modified seta at mesial distal corner, these modified setae with same setulation as those of endopod of fourth pleopod ( Figure 4E).

Remarks
The presence of modified setae on the male pleopods and the components of the marsupium are reported for the first time.

Supplemental description
Body constricted between thorax and abdomen. Each of anterior four abdominal somites with small, blunt, papilliform process on ventral median line, these processes gradually becoming smaller posteriorly. Antennal sympod with single, robust, spiniform process at lateral angle ( Figure 5B). Labrum without frontal spiniform process. Penis rectangular in lateral view, armed with about 15 short plumose setae on posterolateral margin; distal end divided into two portions, anterior portion overreaching posterior one, unarmed, posterior portion broadly rounded, armed with about seven long, mesially curved setae ( Figure 5F).
Female with developed oostegites on seventh and eighth thoracopods; oostegite on seventh thoracopod with baling lobe.
Fifth pleopodal endopod: ultimate segment about twice as long as that of exopod, terminating in two long setae, mesial seta slightly longer than lateral one, 4.3 times as long as own segment, extending to distal end of telson, both setae furnished with short setae on proximal two-thirds, densely with setules on following one-fifth, then naked on distal onetenth ( Figure 6E). Fifth pleopodal exopod slightly shorter than endopod, ultimate segment with one rather short, normal seta and one long, modified seta, latter is furnished with short  setae on proximal two-thirds and then densely with setules except for distal eighth portion being naked ( Figure 6E).
Endopod of uropod armed with 22-37 short, stout spines on mesial ventral margin from statocyst region to distal one-tenth, spines in statocyst region arranged in one or two rows ( Figure 6F, G).
Telson with lateral margin unarmed with spines but finely serrated on distal half ( Figure 6H, I).

Remarks
We analysed specimens collected from waters near Onagawa, Japan. They correspond with the original description by Smith (1879), with the exception of the uropodal endopod. In Smith's (1879) description, the uropodal endopod extends posteriorly only slightly beyond the tip of the telson. We found that it extends beyond the tip of the telson by one-fourth of its length.
We also report differences in the shape of the antennal scale and the spine arrangement in the statocyst region of the uropodal endopod. The antennal scale is 3-3.5 times longer than it is broad, compared to three times longer in the original description. The spines in the statocyst region are arranged in one or two rows, as opposed to the single row in the type specimens.
Males have modified terminal setae on the endopod and exopod of the fifth pleopod, but these setae could not be confirmed in the type specimens due to damage. Banner (1948) noted that M. robustus from the northeastern Pacific have small-eyed morphs similar to those of M. microphthalmus as well as intermediate forms. Banner (1954) tentatively assigned M. microphthalmus to a junior synonym of M. robustus. Birstein and Tchindonova (1958) recorded both M. microphthalmus and M. robustus from the Sea of Okhotsk, but none of the intermediate forms reported by Banner (1948) were found. We report that these two species are clearly distinguishable from each other in the characters of the eyes, the antennal scale, and the uropodal endopod. The eyes are large and slightly depressed dorsoventrally in M. robustus, whereas they are small and cylindrical in M. microphthalmus. The apical lobe of the antennal scale is as long as it is broad or up to 1.2 times longer that it is broad in M. robustus, as opposed to 1.7-2 times longer than it is broad in M. microphtalmus. The uropodal endopod is armed with a row of spines on the mesial margin from the statocyst region to the distal tenth in M. robustus, whereas it extends to the distal fourth in M. microphthalmus.

Description of female
First to seventh thoracic somites with sternal process; process on first somite knob-shaped, those of second to seventh somites bilobed and flattened, those of fifth to seventh gradually becoming smaller towards posterior somite ( Figure 7A). Abdominal somites smooth; first somite broken, second to fifth somites subequal in length, sixth somite 1.4 times longer than fifth.
Carapace produced anteriorly into low triangular rostral plate with obtuse apex extending to base of eyes; anterolateral corner acute; posterior margin emarginate.
Eyes globular, as long as broad; cornea occupying half of eye in dorsal view. Antennular peduncle robust, first segment slightly longer than broad, third segment 1.2 times as long as first.
Antennal scale extending beyond apex of antennular peduncle by two-fifths of its length, four times as long as broad; lateral margin smooth, terminating in spiniform process; apical lobe occupying three-eighths of scale length, three times as long as terminal spine of lateral margin, with indistinct subapical suture.
Labrum without frontal acute process. Mandibular palp: second segment three times as long as broad; third segment more than half of second segment in length ( Figure 7B).
Maxillule: lateral lobe armed with 13 robust spines on distal margin and with three setae on ventral surface ( Figure 7C).
Maxilla: second segment of endopod 1.8 times as long as broad, armed with numerous long setae on margin; exopod extending to distal margin of proximal segment of endopod ( Figure 7D).
First thoracopodal endopod short and robust ( Figure 7E). Second thoracopodal endopod long, carpopropodus slightly shorter than merus, dactylus armed densely with setae and with claw on distal end ( Figure 7F). Third to eighth thoracopodal endopods long, slender; carpopropodus divided into three-subsegments, proximal subsegment occupying three-fifths of carpopropodus in length and articulated obliquely from middle subsegment, distal subsegment slightly shorter than middle and articulated transversely from middle one ( Figure 7G, H). Exopod of thoracopod with flagellum 14-segmented in first pair, 16segmented in second to seventh pairs, and 15-segmented in eighth pair. Sixth thoracopod with reduced oostegite; seventh and eighth thoracopods with developed oostegite.
All pleopods reduced to unsegmented single lobe. Uropodal endopod extending beyond apex of telson by one-fifth of its length, armed with 42 spines on mesial ventral margin from statocyst region to distal one-seventh; spines in statocyst region lined in double rows ( Figure 7I-K). Uropodal exopod 1.5 times as long as endopod ( Figure 7K). Telson 1.3 times as long as last abdominal somite, twice as long as maximum width, abruptly narrowing near base, and then becoming gradually narrower toward apex; lateral margin naked throughout but finely serrated in distal half; apex narrowly truncated, armed with two pairs of spines and median pair of plumose setae ( Figure 7K). (1977) differentiated Meterythrops sp. from M. robustus based on morphological differences in the antennal scale, uropodal endopod, and telson. It was ascertained through the present study that two characters previously reported by Murano (1977) for Meterythrops sp. correspond with M. robustus: the lateral margin of the telson is finely serrated in the distal half, and the uropodal endopod has two rows of spines in the statocyst region. However, the specimens differ from M. robustus by the large body size, globular eyes, length-width ratio of the antennal scale, and shape of the telson.

Murano
It is important to acknowledge that no male specimens have been collected to date, and thus this classification should be verified by future studies on male characters.

Description
Body robust, constricted between thorax and abdomen. Thoracic somites without sternal processes. Anterior four abdominal somites gradually decreasing in length, fifth somite subequal with fourth, sixth somite 1.5-1.6 times as long as fifth; in male first to fourth somites with small, blunt, papilliform process along ventral median line, these processes gradually becoming smaller posteriorly. Carapace produced anteriorly into rounded rostral plate, extending to or slightly beyond base of antennular peduncle; anterolateral corner pointed; posterior margin emarginate, leaving last thoracic somite exposed dorsally ( Figure 8A, B).
Eyes as long as broad, not depressed dorsoventrally; cornea occupying half of eye; eyestalk with papilla on dorsal surface ( Figure 8A, B).
Antennular peduncle more robust in male than in female, distal segment slightly longer than combined length of proximal two segments, 1.1 times as long as broad in male, and 1.3 times as long as broad in female, with well-developed appendix masculina in male ( Figure 8A, B).
Antennal scale extending beyond apex of antennular peduncle by one-fourth of its length, 3.3 times as long as broad, with suture near apex; lateral margin smooth, terminating in strong spiniform process; apical lobe occupying one-fourth of entire length  of scale, 1.2 times as long as broad at base, three to four times longer than lateral terminal process ( Figure 8A-C). Antennal peduncle extending to distal two-fifths of scale in male and distal four-ninths in female ( Figure 8C). Antennal sympod with spiniform process at lateral distal angle ( Figure 8C).
Labrum without frontal spiniform process. Mandibular palp: second segment armed with numerous setae on lateral margin; third segment half to four-sevenths of second segment in length ( Figure 8D).
Maxillule: lateral lobe armed with 13 strong spines on distal margin and with four long setae on ventral surface, small lobe present in middle of lateral margin ( Figure 8E).
Maxilla: second segment of endopod twice as long as broad, armed with long setae on margin; exopod extending to distal margin of proximal endopodal segment ( Figure 8F).
Penis 2.6 times as long as broad in lateral view, armed with about six short setae on posterolateral margin, distally bilobed, anterior lobe overreaching apex of posterior one, posterior lobe armed with 10 long, mesially curved setae ( Figure 9B).
Female with tuft of setae on coxa of sixth thoracopod and with developed oostegite on seventh and eighth thoracopods; oostegite on seventh thoracopod with small lobe.
All pleopods of male developed, biramous ( Figure 9C-E). First pleopod with endopod reduced to unsegmented lobe and with exopod 14-segmented ( Figure 9C). Second to fourth pleopods with 12-segmented endopod and 13-segmented exopod slightly shorter than endopod ( Figure 9D). Endopod of fifth pleopod nine-segmented, ultimate and penultimate segments lengthened, penultimate segment 2.7 times as long as broad, ultimate segment 3.4 times as long as broad, armed on distal end with two long, modified setae, mesial seta slightly longer than lateral one, these setae thicker than other normal setae, and furnished densely with setules on distal two-fifths except for distal tenth naked ( Figure 9E, F). Exopod of fifth pleopod 10-segmented, shorter than endopod, ultimate segment armed with normal plumose seta and long, modified seta, latter seta thicker, 1.7 times longer than former, spinulated on distal third except for distal tenth naked ( Figure 9E). Pseudobranchial lobe of male pleopods rectangular ( Figure 9C-E). All female pleopods reduced to unsegmented single lobe, increasing in length towards posterior pair ( Figure 9G-I).
Endopod of uropod overreaching distal end of telson for one-fourth of its length, armed on mesial ventral margin from statocyst region to distal third with 12-18 small spines, which become sparser posteriorly ( Figure 9J, K). Exopod of uropod 1.4 times as long as endopod ( Figure 9J).
Telson elongated triangular with narrowly truncated apex, 1.3 times as long as last abdominal somite, 1.8-1.9 times as long as maximum width; lateral margin slightly concave in proximal half and almost straight in distal half, without spines throughout but serrated barely on distal third; apex armed with median pair of plumose setae and two pairs of spines, mesial pair 1.3-1.4 times as long as lateral pair ( Figure 9J).   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 10A-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 11C-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.

Discussion
The pleopods of male Mysida are often modified for copulation. Therefore, these features are considered important to the systematics of Mysida, especially at the genus and species levels. In the tribe Erythropini, however, modified setae are not as conspicuous as ordinary plumose setae; thus, this character has not been considered as important to the genera of this tribe as it is in other taxonomic groups, such as Leptomysini and Mysini.
The males in 13 of 47 known genera of Erythropini have modified setae on the pleopods in all the species (Table III). Eight genera contain species with and without the modified seta. The remainder comprise genera consisting of species without the modified setae, species in which the character is unknown, and other species in which no information on the male pleopod is given. Murano and Mauchline (1999) and Fukuoka and Murano (2002) reported modified setae on the endopod of the fifth pleopod in male Katerythrops oceanica Holt and W. Tattersall, 1905, and on the endopod of the fourth pleopod in male Hypererythrops spinifera (Hansen, 1910), respectively. These modified setae had been overlooked in previous studies, possibly because few specimens exist. Also, they are often damaged during sampling because Erythropini species inhabit meso-and bathypelagic zones (e.g. Mü ller 1993