Meterythrops robustus Smith, 1879

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 : 1655-1658

Meterythrops robustus Smith, 1879

( Figures 5 , 6 , 12E, F )

Meterythrops robusta Smith 1879, p 93 –98, Plate 12; Hansen 1908, p 106–107; Zimmer 1909, p 85–87, Figures 168–172; Stephensen 1912, p 79–80; 1933, p 12; W. Tattersall 1933, p 8; 1939, p 283; Banner 1948, p 377–379 (in part?); W. Tattersall 1951, p 113, Figure 35; Banner 1954, p 580–581 (in part?); Birstein and Tchindonova 1958, p 305– 306; Daly and Holmquist 1986, p 1208; Kathman et al. 1986, p 188, Figure (p 189); Müller 1993, p 124 (list).

Parerythrops robusta: Sars 1879, p 98 –102, Table 39.

Not Meterythrops robusta: Holt and W. Tattersall 1905, p 143 (5 Parerythrops obesa ( Sars, 1864)) ; Taniguchi 1969, p 47–48, Figure 5 (5 Meterythrops microphthalmus ).

Material examined

Type material. USNM 35202 , syntype, one male (13.0 mm), Stn 24, off Halfway Rock , Salem, Massachusetts Bay, northeastern USA, about 60 m, soft mud, 13 August 1877 . USNM 35203 , syntype, one male (damaged) and one female (broken), Stn 5, off Bakeris Island , Salem, Massachusetts Bay, northeastern USA, about 60 m, sandy mud, 4 August 1877 .

Japanese material. NSMT-Cr 16755, two females (12.0 and 12.6 mm), Stn 23, off Onagawa (38 ° 269N, 141 ° 579E), northern Japan, beam trawl, 286 m, 27 April 1992, coll. K. Taki. NSMT-Cr 16756, one male (14.2 mm), Stn 25, off Onagawa (38 ° 269N, 141 ° 429E), northern Japan, beam trawl, 150 m, 28 April 1992, coll. K. Taki. NSMT-Cr 16757, four females (9.8–12.0 mm) and one immature male (9.0 mm), Stn 29, off Onagawa (38 ° 269N, 141 ° 579E), northern Japan, beam trawl, 286 m, 28 April 1992, coll. K. Taki.

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 ).

Distribution

Meterythrops robustus has been known as a circumpolar species ( Birstein and Tchindonova 1958): northern North America ( Smith 1879; Stephensen 1933; Tattersall 1933, 1939, 1951; Banner 1948), the west coast of Norway, Spitsbergen ( Sars 1879), the Kara Sea, northwestern Russia ( Hansen 1908), eastern and western Greenland ( Hansen 1908; Stephensen 1912, 1933), and eastern Russia ( Tattersall 1951; Birstein and Tchindonova 1958). This species is here recorded from Japanese waters for the first time.

This species has been collected from the mesopelagic zone by dredge ( Whiteaves 1874; Tattersall 1951), trawl ( Stephensen 1933; present study), and vertically or obliquely towed net ( Tattersall 1939; Banner 1948).

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 .

Figures

Figure 5. Meterythrops robustus Smith, 1879. Male (14.2 mm, NSMT-Cr 16756) from off Onagawa. (A) Anterior end of body, dorsal; (B) right antenna, dorsal; (C) left mandible; (D) left maxillule, posterior; (E) left maxilla, posterior; (F) right penis, lateral.

Figure 6. Meterythrops robustus Smith, 1879. (A–G) Male (14.2 mm, NSMT-Cr 16756) from off Onagawa; (H, I) female (12.6 mm, NSMT-Cr 16755) from off Onagawa. (A–E) Left first to fifth pleopods, anterior; (F) uropod, ventral; (G) spiniation on anterior part of uropodal endopod, ventral; (H) uropod and telson, dorsal; (I) apical 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.