Neobirsteiniamysis inermis

Neobirsteiniamysis inermis

( Willemoes-Suhm, 1874)

( Figs. 1‒10 View Figure 1 View Figure 2 View Figure 3 View Figure 4 View Figure 5 View Figure 6 View Figure 7 View Figure 8 View Figure 9 View Figure 10 )

Petalophthalmus inermis Willemoes-Suhm, 1874: 575 . — Willemoes-Suhm, 1876 b: 588.

Petalophthalmus armiger View in CoL — Willemoes-Suhm, 1875: 41 (only female), pl. 7, fig. 1, 3–14. — E. Perrier, 1893: 1026.

Boreomysis scyphops G.O. Sars, 1879: 428 . — G.O. Sars, 1884: 34; 1885b: 56. — G.O. Sars, 1885c: 178, pl. XXXII, figs. 10–20. — G.O. Sars, 1886: 14. — Filhol, 1885: 1 (passim). — Norman, 1886: 9. — Norman, 1905: 10. — Stebbing, 1893:268.— Calman, 1901: 23. — Calman, 1909: 171 (passim). — Gerstaecker and Ortmann, 1901: 602 (passim). — Ohlin, 1901 b: 71. — Zimmer, 1904: 433. — Zimmer, 1905: 149 (passim). — Zimmer, 1909: 57. — Hansen, 1908: 99, fig.1–8. — Hansen, 1921: 71. — Linko, 1908: 41. — Stephensen, 1918: 65. — Illig, 1930: 559. — Nouvel, 1943: 46. — Nouvel, 1950: 4. — Zenkewitch, 1954: 82. — Brattegard and Meland, 1997: 78.

Boreomysis suhmi Faxon, 1893: 218 (foot note).

Boreomysis distinguenda Hansen, 1908: 100 , fig. 2a, b. — W.M. Tattersall, 1913: 869. — Zimmer, 1927: 623. — Illig, 1930: 559.

Boreomysis inermis View in CoL — W.M. Tattersall, 1951: 46. — O.S. Tattersall, 1955: 75. — Gordan, 1957: 342 (catalogue). — Birstein and Tchindonova, 1958: 282, fig. 9. — Birstein and Tchindonova, 1962: 62. — Ii, 1964: 19. — Mauchline and Murano, 1977: 49 (list). — Kathman et al., 1986: 108, 109, figs. a‒e. — Laubitz, 1986: 15. — Ledoyer, 1990: 40. — 1995: 603. — Müller, 1993: 25. — Castellani et al., 2017: 479, figs. O–R.

Birsteiniamysis inermis Tchindonova, 1981: 28 , 29 (passim).— Tchindonova, 1993:153‒155 (passim). — Fukuoka, 2007: 418. — Petryashov, 2009: 67, pl. II, fig. 1, pl. III, figs. 7, 8. — Petryashov, 2014: 187.

Birsteiniamysis scyphops — Tchindonova, 1981: 28, 29 (passim).

Neobirsteiniamysis inermis — Hendrickx et al., 2020: 21 (genus nomen nudum). — Hendrickx and Tchindonova, 2020: 2 (genus nomen novum).

Material examined. 168 specimens: 44 males, 123 females, 1 ovigerous female. TALUD IV. St. 26 (24°56’24”N 109°05’36”W), August 26, 2000 GoogleMaps , 1 male (CL 13.56 mm) and 3 females (CL 13.13–18.08 mm), BS operated at 1200–1274 m (ICML-EMU-12581). TALUD V. St. 26 (24°56’18”N 109°11’48”W), December 16, 2000 GoogleMaps , 1 female (CL 14.88 mm), BS operated at 1280–1310 m (ICML-EMU-12582). TALUD VI. St. 18 (24°14’56”N 108°16’17”W), March 15, 2001 GoogleMaps , 1 male (CL 8.24 mm), BS operated at 850–890 m (ICML-EMU-12583). TALUD VII. St. 13 (23°30’18”N 107°44’00”W), June 6, 2001 GoogleMaps , 1 female (CL 9.14 mm), BS operated at 1400–1450 m (ICML-EMU-12584-A); St. 18 (24°14’30”N 108°16’24”W), June 7, 2001, 4 males (CL 9.00– 9.82 mm), 51 females (CL 6.19–12.05 mm) and 1 ovigerous female (CL 11.55 mm), BS operated at 950–1010 m (ICML-EMU-12584-B); St. 25 (24°52’48”N 108°58’00”W), June 8, 2001, 1 male (CL 10.18 mm) and 14 females (CL 6.57–10.98 mm), BS operated at 780–850 m (ICML-EMU-12585). TALUD VIII. St. 20 (25°56’56”N 110°43’00”W), April 19, 2005 GoogleMaps , 7 males (CL 6.73–12.04 mm) and 2 females (CL 8.08– 10.15 mm), BS operated at 1140–1150 m (ICML- EMU-12586). TALUD IX. St. 10 (24°56’24”N 110°16’42”W), November 12, 2005 GoogleMaps , 1 female (CL 11.41 mm), BS operated at 969–1225 m (ICML- EMU-12587-A); St. 16 (25°23’48”N 110°36’42”W), November 13, 2005, 3 females (CL 11.04 – 11.81 mm), BS operated at 997–1021 m (ICML-EMU-12587-B). TA LUD X. St. 10 (27°48’30”N 112°17’12”W), February 10, 2007 GoogleMaps , 2 females (CL 11.29–15.44 mm), BS operated at 1396–1422 m (ICML-EMU-12588-A); St. 18 (27°09’06”N 111°46’54”W), February 12, 2007, 1 female (CL 17.19 mm), BS operated at 1526 m (ICML-EMU-12588-B). TALUD XII. St. 9 (17°10’26”N 101°37’37”W), March 29, 2008 GoogleMaps , 4 males (CL 13.88–16.36 mm) and 4 females (CL 11.72–18.55 mm), BS operated at 1392–1420 m (ICML-EMU-12598-A); St. 15 (17°25’33”N 102°07’20”W), March 30, 2008, 1 female (CL 20.14 mm), BS operated at 2080–2125 m (ICML-EMU-12589-B). TALUD XV. St. 8 (24°25’48”N 112°38’06”W), July 30, 2012 GoogleMaps , 2 females (CL 15.03–15.92 mm), BS operated at 1212– 1235 m (ICML-EMU-12590-A); St. 9 (24°25’12”N 112°52’48”W), July 30, 2012, 1 female (CL 13.23 mm), BS operated at 1425–1494 m (ICML-EMU-12590-B); St. 13 (25°02’12”N 112°54’06”W), July 30, 2012, 1 male (CL 14.35 mm) and 3 females (CL 14.24–18.32 mm), BS operated at 1210–1245 m (ICML-EMU-12590-C). TALUD XVI-B. St. 5 (28°48” N 115°24.1 ”W), May 24, 2014 , 4 males (CL 8.05–10.12 mm) and 3 females (CL 10.14–11.72 mm), BS operated at 772–776 m (ICML-EMU-12591-A); St. 7 (29°21.2” N 115°39.14 ”W), May 31, 2014, 1 male (CL 8.77 mm) and 5 females (CL 10.74–12.87 mm), BS operated at 710–750 m (ICML-EMU-12591-B); St. 8 (29°23.8” N 115°45.2 ”W), May 24, 2014, 1 female (CL 12.76 mm), BS operated at 1416–1480 m (ICML- EMU-12591-C); St. 9 (29°20.89” N 115°51 ”W), May 31, 2014, 1 male (CL 14.49 mm) and 1 female (CL 15.65 mm), BS operated at 1848–1860 m (ICML- EMU-12592-A); St. 17 (29°54.3” N 116°01.5 ”W), May 27, 2014, 12 males (CL 7.73–11.35 mm) and 14 females (CL 8.10–12.21 mm), BS operated at 734– 774 m (ICML-EMU-12592-B); St. 18 (30°39.3” N 116°25.9 ”W), May 26, 2014, 2 males (CL 10.00– 10.66 mm) and 2 females (CL 11.84–12.61 mm), BS operated at 740–785 m (ICML-EMU-12592-C); St. 19 (30°38” N 115°31.67 ”W), May 25, 2014, 2 females (CL 16.84–18.34 mm), BS operated at 1385–1433 m (ICML-EMU-12593-A); St. 20 (30°51.26” N 116°42.18 ”W), May 26, 2014, 1 male (CL 15.50 mm), BS operated at 2075–2090 m (ICML-EMU-12593-B); St. 21 (30°49.4” N 116°47.8 ”W), May 28, 2014, 1 male (CL 16.22 mm) and 1 female (CL 15.04 mm), BS operated at 2018–2093 m (ICML-EMU-12593-C); St. 23 (30°56.04” N 116°40.92 ”W), May 27, 2014, 1 male (CL 16.20 mm) and 3 females (CL 12.69–15.92 mm), BS operated at 1296–1340 m (ICML-EMU-12594-A); St. 26 (31°46.1” N 116°58.35 ”W), May 26, 2014, 2 males (CL 9.84–10.67 mm), BS operated at 982–989 m (ICML-EMU-12594-B); St. 27(31°42.6” N 117°13 ”W), May 27, 2014, 1 female (CL 15.80 mm), BS operated at 1394–1397 m (ICML-EMU-12594-C).

Description. Carapace ( Fig. 1A, C View Figure 1 ) elongated, slightly produced forward as small triangular process, occasionally acute; antero-lateral margins slightly produced; cervical groove well marked anteriorly and laterally. Eyes ( Fig. 1B View Figure 1 ) large, elongated, outer surface concave, oblong in lateral view, without pigmentation, one small dorsal papilla. Antennular peduncle ( Fig. 1E View Figure 1 ) short, 3-articulate; first rectangular, as long as combined length of second and third, 3 short plumose setae on outer margin, inner margin with 2 short, plumose setae, ventral surface with 4 short, plumose setae; second article smaller than other two, one short, plumose setae on outer margin, 3 short, plumose setae on ventral surface; third article almost as wide as long, 4 short, plumose setae on outer margin, a small bump in middle of distal margin bearing small setae. Antennal peduncle ( Fig. 1D View Figure 1 ) little more than 1/2 scale length; first article short, with prominent spine near inner margin; second article long, 2 slender, simple setae in middle of outer margin, 3 setae on outer distal edge; third article slightly more than 1/2 length of second article, approximately 1/6 scale length, with 4 long setae in middle of outer margin, 2 setae on outer distal edge, 1 setae on inner margin. Antennal scale

( Fig. 1D View Figure 1 ) extended well beyond distal edge of antennal peduncle, approximately 4.5 times as long as broad, wider near its base, straight outer margin without setae, ending in one strong spine, apex truncated, all margins setose. Mandibles ( Fig.2A View Figure 2 ) palp 3-articulated;first article short, without spines; second longer than third, both margins setose, almost same width throughout its length; third article approximately 3 times as long as broad, short comb-shaped setae on distal 2/3 of inner margin, one series of short, sub-marginal setae on proximal 1/3, apical border with 2 long, plumose setae, one series of short, sub-marginal setae on distal 1/4, outer margin with 7 thin setae. Right mandible ( Fig. 2A View Figure 2 ) well developed, incisor process small, composed of one chitinous ridge with four teeth; lacinia mobilis small, located between incisor and pars centralis, represented by one row of thick, long apical setae and one series of small, thin setae covering both base of molar process and outer margins ( Fig. 7A–C View Figure 7 ); molar process sub-rectangular, with one series of parallel lamellae on crushing surface formed by strong sharp spines attached at base, one series of small pores at edge of molar process. Left mandible ( Fig. 7D‒F View Figure 7 ) with strong incisive process, consisting of one chitinous ridge with 4 teeth; lacinia mobilis present, well developed, consisting of 5 teeth, one bifid, two small, and one large, well marked at extremity; par centralis represented by one series of 6 flattened elements, composed of one row of long setae, united at their base; molar process similar to the right mandible.

Maxilla ( Fig. 2B View Figure 2 ) exopod wide, extending to proximal 1/3 of endopod distal article, 38 long, plumose, marginal setae; distal segment of endopod oval, densely setose on both margins, setae long, plumose; 3 sub-rectangular, elongated endites armed with numerous long, distal setae.

Maxillula ( Fig. 2C View Figure 2 ) external lobe armed with 13 strong, indented, apical setae, 8 long plumose setae on ventral surface; inner lobe with 3 long, stout and 2 thin apical setae, all plumose, 4 small plumose setae on inner margin, 5 slender, plumose setae on outer margin, 4 small, simple setae on ventral surface. Labrum ( Fig.2D View Figure 2 ) symmetric, bilobed anterior margin with simple short setae.

First thoracopod ( Fig. 3A,B View Figure 3 ) small, thin; gnathobase with elongate lobe, well developed, covered with one series of long, plumose setae on outer margin, 14 short, simple setae on ventral surface; pre-ischium with 10 plumose setae; ischium with 9 long, plumose setae on inner margin, 4 single setae on outer margin, several single setae on ventral surface; merus elongated, with 17 long, plumose setae on inner margin, several single setae on outer margin and on ventral surface; carpopropodus with one long setae, serrated on inner margin; short, triangular dactyl with several long thin setae on inner and outer margins; exopodite almost 3 times longer than endopodite, 29-articulated, each article with long plumose setae; epipod well developed.

Second thoracopod ( Fig. 3C, D View Figure 3 ) longer than first, shorter than combined length of thoracopods 3, 4; endopod elongated, without distal subchela; short pre-ischium, with few short, ventral, simple setae; ischium with long, simple seta on inner margin; merus and carpus approximately of same size, armed with one series of long, simple setae on inner margin and on distal part of external margin; dactyl covered with simple setae, one series of short, serrated spines on inner margin; exopod much longer than endopod, 36-articulated, each article with long, plumose setae.

Thoracopod3–8 ( Figs.3E, F View Figure 3 , 4A–F View Figure 4 ) with long, thin endopods; ischium armed with long, simple setae on inner margin; merus armed with simple setae on inner margin; carpus with one series of tufts of short setae, distally hook-shaped, few long, plumose setae on inner margin; propodus longer than dactyl, 1–3 long setae on distal end of outer margin; dactyl fused with nail, forming long claw; exopod longer than endopod, 34– 40 articles, each with long, plumose setae. Penis ( Fig. 4F View Figure 4 ) at base of male’s eighth thoracopod, elongated, with one series of tiny spines on ventral surface, short setae on inner margin. Marsupium of females made of 7 pairs of oostegites.

Abdominal somites 1–6 without spines. Male pleopods ( Fig. 5A–F View Figure 5 ) biramous, well developed, all endopods with proximal lobe armed with short, simple setae; endopod of first pair ( Fig. 5A View Figure 5 ) covered with long, thin setae on inner margin, exopod approximately twice as long as endopod, covered with long, plumose setae on both margins; endopod of second pair of pleopods ( Fig.5B View Figure 5 ) approximately 1/2 length of exopod, both covered with long, plumose setae on both margins; exopod of third pair of pleopods ( Fig. 5C View Figure 5 ) considerably longer than exopod of other pleopods, endopod approximately twice as long as exopod, both covered with long, plumose setae on both margins, distal joints with shorter, stout, spiniform setae ( Fig. 5D View Figure 5 ); fourth pair of exopod ( Fig. 5E View Figure 5 ) slightly longer than endopod, both armed with long, plumose setae on both margins; endopod and exopod of fifth pair ( Fig.5F View Figure 5 ) almost of same size, long, plumose setae on both margins. Female pleopods ( Fig. 5G–K View Figure 5 ) uniramous, well developed, increasing in size posteriorly, covered with long, plumose setae on inner margin, one series of short, simple setae on proximal part of both margins.

Uropods ( Fig. 6A View Figure 6 ) long, robust; exopod about 1.3 times as long as telson, about 4 times as long as wide, outer margin armed with 2 proximal spine-like setae, inner about twice as long as the other; endopod shorter than exopod, one statocyst at base, 2‒3 spine-like setae on proximal 1/3 of inner margin.

Telson ( Fig. 6B View Figure 6 ) wide, approximately 2.7 times as long as wide, distal half with convex sides, proximal 1/3 of lateral margins unarmed, remaining 2/3 armed with about 37 spine-like setae, distal 1/3 with series of 3–5 short spine-like setae between slightly larger one, spines reducing in size towards posterior margin, the latter with moderately deep cleft, V-shaped, approximately 1/9 of total telson length, posterior lobes sub-triangular, 1 strong, terminal, long spine-like seta in each lobe, cleft armed with numerous small teeth, 2 long, plumose setae at basal edge of cleft.

Size. Males, CL 6.73–16.36 mm; females, CL 6.19–20.14 mm; ovigerous female (1 specimen) CL 11.55 mm.

Geographic distribution (previous records as Boreomysis inermis or Birsteiniamysis inermis ). South Atlantic, Antarctic, and Pacific (W.M. Tattersall, 1951; Tchindonova, 1958; Ii, 1964). Registered by G.O. Sars (1885) at three locations in the South Pacific and by W.M. Tattersall (1913) in the Weddell Sea. W.M. Tattersall (1951) reported this species in Pacific deep water, from the Bering and Okhotsk Seas to the southeast coast of California. Off Japan (Fukuoka, 2007). North Atlantic and Norwegian Sea ( Castellani et al., 2017). Off western Mexico ( Fig. 8 View Figure 8 ) (this study).

New records in Mexico. Neobirsteiniamysis inermis is recorded for the first time in the Mexican Pacific, in 28 localities ( Fig. 8 View Figure 8 ). It occurs both in the central and southern Gulf of California (11 localities), off the west coast of the Baja California Peninsula (15 localities), and off SW Mexico (two localities), thus indicating it is a widespread and rather frequent species in the region.

Bathymetric distribution. Because stratified (discrete) samples were not available during our study, the material examined herein was collected at unknown depths during sampling operations within the water column, from surface to 710–2125 m. Tchindonova (1958), Ii (1964), and W.M. Tattersall (1951) report a bathymetric range of 900–3800 m. However, unless automatic opening-closing gear was used, which is apparently not the case, the depth range at which specimens of N. inermis have been captured in the past remains imprecise. Brattegard and Meland (1997: 78) referred to B. scyphops as a probably benthopelagic species, occurring in 980–3570 m depth.

The rarity of previous records for western Mexico could be linked to the fact that most zooplankton and micronekton samples previously taken in the area have been obtained between surface and about 200 m depth (see Hendrickx and Estrada-Navarrete, 1996; Durán-Campos et al., 2020; Diaz et al., 2020). Although precise level of residence of N. inermis is still imprecise, it would seem that it is mainly a deep-water species, occurring well offshore, beyond the limit of neritic waters. A total of 166specimens were examined during this study, a very large proportion (78 %) in stations with depth comprised between 780–1450 m and 710–2093 m ( Tab.1). The maximum number of specimens in a single locality was 56 ( TALUD VII, St. 18; maximum sampling depth, 950–1010 m), the unique locality where an ovigerous female was found. Second to this is St. 17 of TALUD XVI-B cruise, with maximum sampling depth of 734–774 m, with 26.

Remarks. In her characterization of “ Birsteiniamysis ”, Tchindonova (1993) emphasized the structure of the eyes, stating: “Concave surface with microfibrous microstructure. Visual elements located inside proximal part of eye stalk. Degree of development of ocular rudimentary papilla and its location on stem species-specific” (translation available in Hendrickx et al., 2020). While examining the material collected during this study, is was possible to review in detail these characteristics. The ocular globe is indeed deeply concave on its lateral face ( Fig. 9A, B View Figure 9 ), with the upper and lower margin next to the concavity wide or narrow depending upon the angle at which the eye is illustrated (compared Fig. 9A, B View Figure 9 ). The “microfiber microstructures” described by Tchindonova (1993) correspond to a microscopically rippled bottom of the ocular concavity, showing rather regular undulations without apparent visual elements ( Fig. 9C View Figure 9 ). The position of the ocular papilla in our material was quite consistent, but there was a considerable variation of its size, from a proportionally very small structure ( Fig. 10C, I View Figure 10 ) to a much larger protuberance ( Fig. 10A, E, G View Figure 10 ). No clear relationship was observed between the size of the papilla and either the sex or the size of the specimens examined.

A slightly produced anterior margin of the carapace, forming a small triangular process, occasionally acute, was observed in the material examined ( Fig. 10 View Figure 10 ). In this case, all three males examined ( Fig. 10B, D, F View Figure 10 ) featured a rather sharp terminal process, which seems to be sex-related; indeed, the females featured an obtusely triangular projection ( Fig. 10H, J View Figure 10 ).

The number of ovigerous females captured for this species is notably low, and only one out of 124 females collected during the survey was carrying embryos inside the marsupium. There are very few records of large numbers of offshore mysids available and comparison with other oceanic species is rather difficult. In the case of Petalophthalmus armiger Willemoes-Suhm, 1875 , collected during the same survey in similar conditions, a total of 38 specimens was obtained, six males and 32 females, without a single ovigerous female ( Hendrickx and Hernández-Payán, 2018). In other studies, some covering longer periods of time or in shallower water, the number of ovigerous females collected has been reported as significantly higher. For example, in Boreomysis oparva Saltzman and Bowman, 1993 , up to 7 % of females collected between surface and 3400 m depth carried larvae, but values were highly variable according to depth. In the case of Metamysidopsis neritica Bond-Buckup and Tavares, 1992 , Calil and Borzone (2008) reported about 10 % of ovigerous females among the specimens (including juveniles), and 66 % of adult females were ovigerous. In Schistomysis assimilis (G.O. Sars, 1877) , the percentage of brooding females varied from 1.0 to 15.4 % according to the period of the year ( San Vicente and Sorbe, 2003).

The very low incidence of ovigerous females in our sample might be related to variation in the depth range at which ovigerous females reside compared to males and non-ovigerous females, or to the period of the year when samples were obtained. However,

samples analyzed herein were collected from February to July, in November and in December, thus covering most of the year. Unfortunately, for the time being, there is no clear explanation for the absence of a larger proportion of ovigerous females in our samples.