Ericthonius megalopus
Creators: name: Ariyama, Hiroyuki
name: Fujiwara, Yoshihiro
Publisher: name: Zenodo
Year: 2011
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Amphipoda
Ischyroceridae
Ericthonius
Ericthonius megalopus

description: Ericthonius megalopus (Sars, 1879)
(Figures 1–12)
Cerapus megalopus Sars, 1879:461–462.
Ericthonius megalopus: Sars, 1885:210–212, plate 17, figure 4; Stebbing, 1906:673–674; Gurjanova, 1951:949–950, figure 660; Myers and McGrath, 1984:397–398, figures 12–13.
Ericthonius tolli Brüggen, 1909:41–42, plate 1, figure 6, plate 3, figures 29–34; Gurjanova, 1951:952, figure 663.
Material examined
Sagami Bay, Japan (OMNH-Ar 8541, 8542). Dissected: one male (11.5 mm, six slides and one vial) and one ovigerous female (11.8 mm, five slides and one vial); 34 ◦ 59.267’ N, 139 ◦ 18.378’ E, 1456 m, 2.4 ◦ C, inhabiting a single mud tube attached to a small branch (Figure 1), 9 December 2007, coll. JAMSTEC.
Norwegian Sea (ZMUC-CRU-20157). Dissected: one male (9.0 mm, four slides and one vial) and one ovigerous female (9.6 mm, three slides and one vial), not dissected: five males (9.1, 8.7, 8.5, 6.8, 6.0 mm) and five females (8.1, 6.8, 5.7, 5.5, 4.7 mm); Ingolf St. 44, 61 ◦ 42’ N, 9 ◦ 36’ W, 1026 m, 4.8 ◦ C, 9 August 1895, identified as Ericthonius megalopus probably by Stephensen (1944b), also examined by Myers and McGrath (1984).
Arctic Ocean (ZMUC-CRU-20162). Dissected: one male (8.3 mm, four slides and one vial), not dissected: four males (10.1, 10.1, 8.6, 7.2 mm); Sibir. Polar Sea, Russian Polar-Expedition 1900–1903, Zool. Mus. Leningrad, identified as Ericthonius tolli by E. Brüggen.
Baffin Bay (ZMUC-CRU-20161). Dissected: one ovigerous female (11.2 mm, three slides and one vial), not dissected: two males (9.9, 9.8 mm) and two females (6.1, 6.0 mm); Godthaab St. 114, 76 ◦ 40 N, 76 ◦ 20’ W, 85 m, 16 August 1928, identified as Ericthonius tolli probably by Stephensen (1933).
Colouration in life (male and female from Sagami Bay, Japan)
Eyes dark red; head, pereonites 1 and 2 and posterior parts of pereonites 3–7 pale brown; articles 1 and 2 of antenna 2, mouthparts and carpus–dactylus of gnathopod 1 orange; coxa–merus of gnathopod 1 and coxa, carpus–dactylus of gnathopod 2 pale brown; eggs purple; other parts white.
Description of male (11.5 mm from Sagami Bay, Japan)
Body (Figure 2) slender, relatively depressed dorsoventrally. Rostrum short; antennal sinus deep. Eyes large, width about 0.45 times head length.
Antenna 1 (Figure 3A, A 1) slender, long; ratio of lengths of peduncular articles 1–3 1: 1.6: 1.4, posterodistal corner of peduncular article 1 with one robust seta, posterior margins of peduncular articles with many long setae; accessory flagellum vestigial, with three short setae on tip; flagellum with 14 medium-sized articles and one tiny distal article, posterior surface bearing many long setae. Antenna 2 (Figure 3B) slender, a little shorter than antenna 1; ratio of lengths of peduncular articles 3–5 1: 1.9: 2.0, article 3 with one posterodistal robust seta, posterior margins of articles 3–5 setose; flagellum with 14 medium-sized articles and one tiny distal article, setose posteriorly.
Upper lip (Figure 3C) subrounded, bare; epistome strongly projected anteriorly. Mandible (Figure 3D, D 1, E, E1), left and right similar; incisor and lacinia mobilis each with four cusps; palp strong, article length ratio 1: 3.6: 3.3, ventral surfaces of articles 2 and 3 and distal margin of article 3 heavily setose. Lower lip (Figure 3F), inner and outer lobes covered with minute setae, mandibular lobes short. Maxilla 1 (Figure 3G, G 1), inner plate ovoid, bluntly pointed, bearing three normal and many thin setae; outer plate bearing 10 robust setae distally; tip of palp article 2 with seven robust and nine normal setae. Maxilla 2 (Figure 3H), inner plate setose on apical and medial margins, dorsal surface covered with thin setae; outer plates setose apically. Maxilliped (Figure 3I, I 1, I 2), inner plate bearing three anterior and one ventral robust setae; outer plate with 14 robust setae (five distal long and nine medial short setae); palp with four articles, article 2 long, article 4 with one long spine apically.
Gnathopod 1 (Figure 4A) small, subchelate; coxal plate slightly projected anteroventrally, bearing a few short setae; basis slightly curved anteriorly, anterior margin with two short setae, posterior margin with several short setae; ischium setose posterodistally; merus setose posteriorly and distally, posterodistal corner slightly projected; carpus triangular with length of 1.6 times width, posterior margin heavily setose; propodus short, rounded, posterior margin, anterodistal corner and medial surface setose; dactylus curved. Gnathopod 2 (Figure 4B, B 1) extremely large, carpochelate; coxal plate slightly projected posteriorly, bearing a few short setae anteroventrally, ventral margin without stridulating ridges, gill absent; basis relatively slender, anterior and posterior margins with sparse short setae; ischium rounded anterodistally; merus narrow, extended along posterior margin of carpus, with two short setae on posterodistal corner; carpus greatly enlarged, posterodistal corner bearing large process reaching about 0.67 times propodus length, anterior margin of process without tooth, slightly swollen at mid-length; propodus wide, posterior margin with four roundish projections (two large, one small and one middle), anterodistal corner sparsely setose; dactylus stout, slightly curved posteriorly, posterior margin with many short setae.
Pereopods 3 and 4 (Figure 4C, D) similar; coxal plates protruded posteriorly, each bearing several ventral setae, gills longish ovoid; bases relatively wide, longish ovoid, anterior margins with several setae, posterodistal corners each with two short setae; meri projected anterodistally; carpi and propodi with several setae on anterodistal corners; dactyli narrow, almost straight. Pereopod 5 (Figure 4E) short; coxal plate wide, anterior lobe broad, with three plumose setae on inner surface, ventral margin with several short setae, posterior lobe narrow, with one short seta on posteroventral corner, gill ovoid; basis 0.69 times as wide as long, posterior margin expanded, posterodistal corner not produced, anterodistal corner setose, posterior and posterodistal margins with sparse setae; merus relatively short, posterodistal corner projected, anterodistal and posterodistal corners with several setae; carpus short, anterodistal and posterodistal corners setose; propodus with one robust seta on anterodistal corner, posterodistal margin setose; dactylus short, strongly curved. Pereopod 6 (Figure 4F, F 1) about 1.5 times pereopod 5 length; coxal plate smaller than coxa 5, lobate, anteroventral margin with one seta, posterodistal corner bearing one short seta, gill ovoid; basis 0.57 times as wide as long, posterior margin expanded, anterior margin with one robust and two normal setae, anterodistal corner with three setae, posterior margin with sparse setae; merus long, anterodistal and posterodistal corners setose; carpus relatively short, anterodistal and posterodistal corners setose; propodus with one robust seta on anterodistal corner and several long setae on distal margin; dactylus short, strongly curved. Pereopod 7 (Figure 5A, A 1) about 1.1 times pereopod 6 length; almost similar to pereopod 6, but gill absent, basis without robust seta on anterior margin, and propodus with two robust setae on anterodistal corner.
Pleopods (Figure 5B–D) relatively long, pleopod 3 shortest; peduncles each with two coupling hooks, peduncles of pleopods 1 and 2 with six and five plumose setae, respectively; outer rami shorter than inner, outer and inner rami with 15 and 14 articles, respectively, inner basal margins of inner rami each with five or six clothespin setae. Uropod 1 (Figure 5E) long; peduncle bearing nine lateral and six medial robust setae; outer ramus longer than inner, outer ramus with six lateral and four terminal robust setae, inner ramus with four medial and two terminal robust setae. Uropod 2 (Figure 5F) short; lateral margin of peduncle bearing five robust setae, mediodistal corner with one long robust seta; outer ramus longer than inner, outer ramus with three lateral and four terminal robust setae, inner ramus with two medial and two terminal robust setae. Uropod 3 (Figure 5G) small, uniramous; peduncle stout, with two lateral, two dorsomedial and two or three laterodistal setae; single ramus short, about half length of peduncle, lateral margin with two lateral setae, tip with three minute protuberances and two short setae. Telson (Figure 5G) entire, short, with a pair of densely spinulose lobes each bearing two lateral and one distal setae; posterior margin between lobes almost straight.
Description of female (11.8 mm from Sagami Bay, Japan)
Gnathopod 1 (Figure 6A) similar to that of male. Gnathopod 2 (Figure 5H, H 1) proportionally smaller than that of male, subchelate; coxal plate inflated posteriorly, anteroventral margin bearing a few short setae, ventral margin without stridulating ridges, gill absent; basis broadened at mid-length, anterior and posterior margins with sparse short setae; ischium short; merus truncate distally, distal margin setose; carpus with large posterodistal lobe bearing four robust and many normal setae posteriorly; propodus enlarged, posterior margin with six robust and several normal setae; dactylus stout, gradually curved posteriorly, anterior and posterior margins each with several short setae. Pereopods 3 and 4 (Figure 6B, C), coxae wider, gills and bases longer than those of male. Pereopod 5 (Figure 6D), anterior lobe of coxal plate more slen- der than that of male, ventral margin bearing many plumose setae, posterior lobe with three plumose setae on posteroventral corner. Pereopods 6 and 7 (Figure 6E, F), anterior lobes of coxal plates with many plumose setae ventrally, inner surfaces of posterior lobes bearing several setae; bases, meri and propodi longer than those of male. Oostegites broad, present on pereopods 2–5.
Descriptions of the specimens deposited in the Natural History Museum of Denmark
Morphological characters of all the specimens dissected are similar to those of the Japanese specimens: antennae, mouthparts, gnathopod 1 except for merus, male gnathopod 2 except for basis, pleopods, uropods and telson. However, several characters are different. In the male from the Norwegian Sea (9.0 mm), (1) merus of gnathopod 1 projected posterodistally (Figure 7A), (2) bases of pereopods 3 and 4 shorter and wider (Figure 7C, D), and (3) coxae 5–7 each with shallow notch on posterodorsal margins (Figure 7E–G). In the female from the Norwegian Sea (9.6 mm), (1) merus of gnathopod 2 weakly projected posterodistally (Figure 8B), (2) carpus of gnathopod 2 with three robust setae on posterodistal margin (Figure 8B 1), (3) propodus of gnathopod 2 shorter and wider (Figure 8B), (4) dactylus of gnathopod 2 shorter (Figure 8B), and (5) bases and meri of pereopods 3–6 shorter and wider (Figure 8C–F). In the male from the Arctic Sea (8.3 mm, previously identified as E. tolli), (1) basis of gnathopod 2 shorter and wider (Figure 9B), (2) bases of pereopods 3 and 4 shorter and much wider (Figure 9C, D), and (3) meri of pereopods 3 and 4 shorter and wider. In the female from Baffin Bay (11.2 mm, previously identified as E. tolli), (1) merus of gnathopod 1 weakly projected posterodistally (Figure 10B), (2) carpus of gnathopod 2 with five robust setae on posterodistal margin (Figure 10C 1), (3) propodus of gnathopod 2 shorter and wider (Figure 10C), (4) propodus of gnathopod 2 with seven robust setae on posterior margin (Figure 10C 1), (5) dactylus of gnathopod 2 shorter (Figure 10C), (6) bases of pereopods 4 and 5 shorter and wider (Figure 10E, F), and (7) coxa 7 with shallow notch on posterodorsal margin (Figure 10H).
Remarks
The morphological characters are similar among the dissected specimens, including the specimens previously identified as E. tolli, except for several characters. In the shapes of the bases of pereopods 3 and 4, the specimens deposited in the Natural History Museum of Denmark are different from the Japanese specimen, namely those of the former are shorter and wider than those of the latter. In males, the body lengths are 11.5 mm in the Japanese specimen, 9.0 mm in the specimen from the Norwegian Sea, and 8.3 mm in the specimen from the Arctic Ocean; therefore, the difference of the shapes of the pereopods 3 and 4 is estimated to be a result of the difference in body size. In fact, Myers and McGrath (1984) and Ariyama (2009) recognized that shapes of male appendages vary markedly with growth in Ericthonius species. Also in females, the difference in the shapes of the gnathopod 2 and the pereopods between the Japanese specimen (11.8 mm) and the specimen from the Norwegian Sea (9.6 mm) is probably caused by body size, whereas the shapes of the specimen from Baffin Bay resemble those of the specimen from the Norwegian Sea in spite of its large size (11.2 mm).
To clarify variation of the width of pereopod basis, the widths of pereopod 4 bases were measured in 24 individuals (six dissected specimens and 18 additional specimens). Figure 11 shows the relationship between body length and the width of the basis in pereopod 4. The width is larger with longer body length, but the relationship differs according to the locality of the specimens. The widths in the specimens previously identified as E. tolli from the Arctic Ocean and Baffin Bay are relatively larger than those in the specimens identified as E. megalopus from the Norwegian Sea; however, the two groups cannot be obviously separated. These facts suggest that all the specimens collected from various localities are included in the same species, whereas the width of the pereopod basis varies with the locality. With respect to other differences, the posterodistal projection of the merus of gnathopod 1 and the number of robust setae on female gnathopod 2 could be within intraspecific variation, and the posterodorsal notches on the coxae 5–7 seem to be changeable in the process of preparation.
The present specimens match the original descriptions and figures of both E. megalopus and E. tolli, although only three and seven figures were presented, respectively (Sars 1885; Brüggen 1909). Because all the dissected specimens, including the male specimen identified as E. tolli by Brüggen, are assigned to the same species as stated above, E. tolli is synonymized with E. megalopus and all the specimens are identified as E. megalopus. Brüggen (1909) described E. tolli without citing Sars (1879, 1885) so he probably did not know of the existence of E. megalopus.
Distribution (Figure 12)
Ericthonius megalopus has been recorded in many localities of the world. All of the records up to the present are shown as follows ([m]: record as E. megalopus; [t]: record as E. tolli).
Arctic Ocean. Foxe Basin (Atkinson and Wacasey 1989, 49– 92 m [t]), Hudson Bay (Shoemaker 1926, 128 m [t]), Hudson Strait (Dunbar 1954, 18– 275 m [t]; Stewart et al. 1985, 106– 145 m [t]; Wacasey and Atkinson 1987, shore– 255 m [t]), Baffin Bay (Ohlin 1895, 9– 37 m [m]; Stephensen 1933, 85 m [t]), Davis Strait (Stephensen 1915, 360– 410 m [m]), Greenland Sea (Hansen 1895, 176 m [m]; Stephensen 1942 [m]; Stephensen 1944a, 185 m [m]; Brandt 1997, 45 m [t]), Barents Sea (Gurjanova 1931 [m]; Gurjanova 1951 [m]), White Sea (Derjugin 1928 [m]; Gurjanova 1951 [m]), Kara Sea (Gurjanova 1936, 10 m [m], 368 m [t]; Jørgensen et al. 1999, 30 m [m]), Laptev Sea (Gurjanova 1932 [t]; Gurjanova 1934, 24– 40 m [m]), East Siberian Sea (Brüggen 1909, 42 m [t], type locality of “ E. tolli ”), Chukchi Sea (Gurjanova 1951 [m]), Chukchi Sea or Beaufort Sea (Shoemaker 1955, 38– 136 m [t]).
Atlantic Ocean. Gulf of Saint Lawrence (Saint-Marie and Brunel 1985, 119 m [t]), Labrador Sea (Hansen 1888, 88– 335 m [m]; Stephensen 1933, 314 m [m?]; Stephensen 1944b, 1096 m [m]; Myers and McGrath 1984 [m]; Stransky and Brandt 2010, 109– 154 m [m]), Denmark Strait (Stephensen 1912, 92 m [m]; Stephensen 1944b, 600–621 m [m]; Myers and McGrath 1984 [m]; Stransky and Svavarsson 2010, 161 m [m]; Stransky and Brandt 2010, 162 m [m]), Norwegian Sea (Sars 1879, 1885, 128– 1134 m [m], type locality of E. megalopus; Stebbing 1906, 179– 1217 m [m]; Stephensen 1912, 174– 1159 m [m]; Stephensen 1942, 911 m [m]; Stephensen 1944b, 1026–1200 m [m]; Myers and McGrath 1984 [m]).
Pacific Ocean. Sagami Bay (present study, 1456 m), Sea of Japan (Derjavin 1930 [t]; Gurjanova 1938, 35– 105 m [t]; Gurjanova 1951 [t]; Gurjanova 1959 [t]; Ivanova et al. 2009, intertidal [t]), Sea of Okhotsk (Gurjanova 1951 [t]; Gurjanova 1959 [t]; Kudrjashov 1968, 54– 225 m [t]; Kudrjashov 1972, 0–235 m [t]; Budnikova and Bezrukov 2003, 42– 232 m [t]; Arhipova 2009, 15– 500 m [t]), Bering Sea (Gurjanova 1951 [m, t]; Shoemaker 1955 [t]), Gulf of Alaska (Hoberg and Feder 2002, 30– 100 m [t]).
This species is widely distributed in the northern seas of the world with a wide depth range. Briggs (1995) stated that the fauna of the Arctic Region is “essentially homogeneous biota” and this circumpolar distribution supports his opinion. The present occurrence in Sagami Bay is the southernmost (34 ◦ 59’ N) and deepest (1456 m) record of the species. Because the cold Intermediate Oyashio Water enters into Sagami Bay through the narrow deep Oshima east channel (Yang et al.1993a, b), some of the subarctic species were recorded in relatively deep water of the bay. Okutani (1972) reported several probable subarctic megabenthos which might be transported by the Oyashio Undercurrent. The subarctic copepod Neocalanus cristatus and chaetognath Sagitta elegans were also recorded there (Oh et al,. 1991; Terazaki, 1998). Our finding of E. megalopus at the deep-sea bottom in Sagami Bay is consistent with the occurrence of subarctic benthos and plankton in the bay in the previous studies.
description: Published as part of Ariyama, Hiroyuki & Fujiwara, Yoshihiro, 2011, First record of Ericthonius megalopus (Sars, 1879) from bathyal Sagami Bay, central Japan, including synonymization of Ericthonius tolli Brüggen, 1909 with Ericthonius megalopus (Crustacea: Amphipoda: Ischyroceridae), pp. 2795-2814 in Journal of Natural History 45 (45 - 46) on pages 2796-2811, DOI: 10.1080/00222933.2011.614424, http://zenodo.org/record/5204787