Pachynus chelatum Bulycheva, 1955

Lowry, J. K. & Stoddart, H. E., 2012, The Pachynidae fam. nov. (Crustacea: Amphipoda: Lysianassoidea) 3246, Zootaxa 3246 (1), pp. 1-69 : 38-40

Pachynus chelatum Bulycheva, 1955

( Fig. 26 )

Pachynus chelatum Bulycheva, 1955: 194 , fig. 1. ―J.L. Barnard, 1958: 97. ― Gurjanova, 1962: 346, figs 116A, B. ― Lowry, 1984: 78, figs 21–23. ― Barnard & Karaman, 1991: 510.

Material examined. 1 specimen, NMCC 1990-0548 , Swanson Bay , Graham Reach , British Columbia, Canada, 52°17.4'N 127°44.8'W, 90 m, 19 August 1974, C.D. Levings, stn 51B-038. –1 ovigerous female (with 3 eggs), 4.0 mm, NMCC 1990-0546 , 300 m south of Pilot Cove, Strait of Georgia, British Columbia, Canada, 49°20'N 123°15'W, 44–50 m, mud and woody detritus, dredged, 2 November 1977, E.L. Bousfield, stn P1 . – 1 specimen, NMCC 1990-0544 , French Creek , north of Nanaimo, Vancouver Island, British Columbia, Canada, [approx. 49°10’N 123°56’W], 23 August 1977, P. O'Rourke, stn FC-7 . – 1 specimen, NMCC 1990-0545 , same data, stn FC- 8 . – 2 specimens, NMCC 1990-0549 , same data, stn FC-66 . – 1 female, NMCC 1990-0547 , Trevor Channel , off Execution Rock, Vancouver Island, British Columbia, Canada, 48°48’N 125°11.2’W, 44–54 m, sandy mud, algae and debris, dredged, 25 May 1977, E.L. Bousfield, stn B14 . – 2 specimens, NMCC 1990-0550 , McCauley Point , Vancouver Island, British Columbia, Canada, 48°24.2'N 123°23.85'W, 66 m, 26 August 1976, G.W. O'Connell, stn W4-B .

Diagnostic description. Head lateral cephalic lobe small, subacute; ventral margin curved. Antenna 1 peduncular article 1 without posterodistal spine; accessory flagellum present, 2-articulate, article 1 short; flagellum with weak 1-field callynophore in female, [male not known]. Mandible lacinia mobilis a simple robust seta; lamina dentata absent; accessory setal row present, not on raised setose ridge; molar a nonsetose flap; palp attached midway; palp article 2 slender, as long as article 3; article 3 with subparallel margins. Maxilla 1 outer plate with 8 setalteeth in 5/3 arrangement, setal-teeth smooth; palp well developed, 2-articulate, with apical robust setae. Maxilliped inner plate small; outer plate small; palp 4-articulate, article 4 small. Gnathopod 1 chelate; ischium smaller than propodus, not swollen posteroproximally; carpus extremely compressed; propodus length about 1.5 × breadth, posterior margin well developed, palm with produced corner, corner of palm with complex robust seta; dactylus reaching to corner of palm. Gnathopod 2 propodus, length more than 2 × breadth, palm minute, obtuse. Pereonite 4 without dorsal spines. Pereonite 5 without dorsal spines. Pereopod 4 merus expanded anteriorly, produced about halfway along carpus. Pereopod 5 merus with posteroproximal shoulder. Pereopod 6 basis with posteroventral lobe produced beyond ischium; merus broader than long, with posteroproximal shoulder. Pereopod 7 basis subovate, about as long as wide with posterior margin weakly dentate. Epimeron 3 posterior margin without projections, posteroventral corner subquadrate. Uropod 3 biramous, inner ramus extending beyond article 1 of outer ramus. Telson longer than broad, entire.

Remarks. According to Gurjanova (1962: 345, footnote), Bulycheva’s original diagnosis of P. chelatum was incorrect in stating that the mandible has no molar. Gurjanova says that all the specimens she examined have a narrow, oblong molar process with a pointed apex and no triturating surface. Gurjanova (1962: figure 116A) shows such a structure. When Lowry (1984) diagnosed and illustrated P. chelatum (from a specimen from the original collections) he did not record or illustrate any molar on the mandible. We have re-examined the slides used by Lowry and found that there probably is a molar present. This molar is very thin and is lying on top of the body of the mandible; it is extremely difficult to see. In the 4 mm female recorded here from Vancouver Island (NMCC 1990-0546) the molar is unmistakable.

Some of the present material was originally identified as P. barnardi . These two species are very similar, as noted by both Hurley (1963) and Lowry (1984). Apart from the presence of a molar in P. chelatum but not in P. barnardi , the two species can be distinguished by: the relative length of mandibular palp articles 2 and 3 (subequal in P. chelatum , article 3 about 0.8 times article 2 in P. barnardi ); the relative length to breadth of gnathopod 2 propodus (length more than 2 times breadth in P. chelatum , less than 2 times in P. barnardi ); and the shape of the posteroproximal corner on the posteroventral lobe of coxa 4 (acutely rounded in P. chelatum , broadly rounded in P. barnardi ).

Distribution. Sea of Japan, western North Pacific Ocean, 88–125 m depth; Vancouver Island, eastern North Pacific Ocean, 44–66 m depth.

Figures

FIGURE 26. Pachynus chelatum Bulycheva. Female, 4.0 mm, NMCC 1990-0546, Vancouver Island, British Columbia, Canada. Scales represent 0.1 mm.