Bathyraja trachura ( Gilbert, 1892 )

Knuckey, James D. S. & Ebert, David A., 2022, A taxonomic revision of Northeast Pacific softnose skates (Rajiformes: Arhynchobatidae: Bathyraja Ishiyama), Zootaxa 5142 (1), pp. 1-89 : 46-53

Bathyraja trachura ( Gilbert, 1892)

Figures 35–37 ; Tables 7 , 8 , 9

Roughtail Skate

Raja abyssicola Gilbert, 1892: 539 [Proceedings of the United States National Museum] v. 14 (no. 880). Holotype: USNM 46930 . Off Santa Barbara , California, USA, 32°40'30"N, 117°31'30"W, Albatross station 2923, depth 822 fathoms.

Raja trachura: Gilbert, 1892: 539 ; Goode & Bean, 1895: 509 (listed); Gilbert, 1896: 398; Garman, 1913: 344 (compiled); Jordan & Evermann, 1896: 75 (compiled); Townsend & Nichols, 1925: 6 (description); Fowler, 1930: 502 (listed); Jordan et al., 1930: 26 (listed); Schultz & DeLacy, 1935: 368 (range); Barnhart, 1936: 13 (compiled); Schultz, 1936: 132 (key); Schultz & DeLacy, 1936: 214 (reference); Taranetz, 1937: 51 (key); Grinols, 1965: 28 (listed); Isakson et al., 1971: 668 (listed); Miller & Lea, 1972: 44 (description, figure, key); Quast & Hall, 1972: 4 (listed); Robins et al., 1980: 14 (listed).

Bathyraja trachura: Stehmann, 1978: 53 (reference); Eschmeyer et al., 1983: 51, fig. 11, pl. 4 (description); Stehmann, 1986: 263; McAllister, 1990: 35; Castro-Aguirre & Espinosa Pérez, 1996: 27 (listed); McEachran & Dunn, 1998: 286 (listed); Dolganov, 1999: 429; Compagno, 1999 (listed): 489; Sheiko & Fedorov, 2000: 15; Mecklenburg et al., 2002: 105 (listed); Hoff, 2002: 145; Ebert, 2003: 202–203 (description, distribution); Fedorov et al., 2003: 20; Stevenson et al., 2004: 311; Stehmann, 2005a: S35; Stehmann, 2005b: S53; Ebert & Compagno, 2007: 117 (listed); Ebert & Davis, 2007: 9–10 (egg case description); Ruiz-Campos et al., 2010: 366 (description, figure, range); Parin et al., 2014 (listed): 34; Dyldin, 2015: 66 (listed); Del Moral-Flores et al., 2016: 110 (listed); Weigmann, 2016: 97 (listed); Last et al., 2016: 27, 423 (figure, listed); Ebert et al., 2017: 21, 59, 68 (description, distribution, key, listed); Dyldin & Orlov, 2018: 171; Ehemann et al., 2018: 24; Burton & Lea, 2019: 33 (listed).

Diagnosis. Moderately sized skates (910 mm TL), disc length (28.7–56.7% TL), pectorals are broadly rounded, head length short (15.0–24.1% TL), preorbital length short (9.0–13.5% TL); claspers short, tip of clasper relatively rounded, large pseudosiphon present, possesses a strong, short pseudorhipidion, not reaching the tip, V-shaped cleft, projection absent; teeth in 27–36 and 26–36 rows on upper and lower jaw, respectively; pectoral fin radials 81; pelvics, 23; total vertebrae 109; dorsal surface covered with rough prickles, prickles on tail are larger and more robust; thorns present on dorsal surface, mature males with alar thorns, malar thorns absent, middorsal thorns weak or absent (0–1); nuchal thorns small or obsolete (0–3); tail thorns moderate (21–30); interdorsal thorns small to occasionally absent (0–1); dorsal coloration dark purple, brown, or grey, spiracles occasionally pale, ventral coloration a lighter grey than dorsal surface, often possessing white blotching on mouth, gills, cloaca, and scattered across the body.

Description. A moderately sized skate with a rhomboidal disc, 1.1–2.0 times as broad as long; anterior margin moderately concave in adult males, straight to moderately convex beside and just forward of eyes; apex broadly rounded; posterior margin slightly convex; free rear tip broadly rounded. Head length relatively short 15.0–24.1% TL; preorbital snout length very short 9.0–13.5% TL; preoral length short 8.0–13.5% TL. Snout tip pointed, possessing no fleshy process at apex. Eye length moderate 2.5–5.0% TL; interorbital width short 3.7–5.6% TL. Spiracles average 1.5–3.9% TL, oval shaped. Mouth width short 6.7–8.1% TL. Nasal curtain length relatively short 2.1–4.1% TL, width average 5.9–9.7% TL, its posterior margin highly fringed at the corners; anterior margin of curtain lobe-like. Interspiracular space short 4.7–7.9%. Internarial distance relatively long 5.1–8.7% TL; first gill slit length relatively long 1.8–2.7% TL; fifth gill slit length 0.3–2.9% TL; distance between gill slits moderate, distance between first gill slits 12.3–21.2% TL, and distance between fifth gill slits 7.7–13.3% TL. Upper jaw moderately well arched, possessing a symphysis; lower jaw convex. Teeth similar in both jaws; teeth unicuspid, with a strong, bluntly pointed posteriorly directed cusp; arranged in longitudinal rows; teeth relatively high in number and similar in number, upper teeth (27–36) and lower teeth (26–36).

Pelvic fins moderate overall; anterior lobe 7.4–10.7% TL, posterior lobe short 8.3–15.2% TL, and similar between sexes and maturities, inner margin deeply incised. Tail relatively moderately sized 42.7–61.2% TL, stout, wider at base, tapering to the first dorsal fin origin, not expanded in the middle. Lateral tail fold short, 8.6–17.9% TL, similar in both sexes; not obviously broader at any point along its length. Dorsal fins moderate in size and shape, first dorsal fin slightly taller than second dorsal fin, 2.0–3.0% TL and 1.7–2.7% TL, respectively; first dorsal fin slightly longer than the second dorsal fin, 3.0–4.0% TL and 2.7–3.9% TL, respectively; anterior margins of both fins concave, apices rounded; free rear tip rounded; interdorsal space absent or short 0.0–1.6% TL; rear tip of first dorsal fin not overlapping base of second dorsal fin. Caudal fin large, low, height 0.6–2.0% TL; its dorsal margin weakly concave; connected to second dorsal fin by a small membranous ridge.

Dorsal surface covered with prickles and some thorns, with the prickles on the tail being larger than elsewhere; ventral surface smooth. Middorsal, nuchal, interdorsal, and tail thorns present, males with a well-developed set of alar thorns; malar thorns absent; thorns vary in size, from very weakly to moderately well-developed. Middorsal thorns absent or weakly developed (0–1); nuchal thorns absent or weakly developed (0–3); tail thorns moderate in size (21–30); interdorsal thorns weakly developed and occasionally absent (0–1). Thorns in a single, non-continuous row; no multiple rows of thorns on body. Alar thorn patches possess 4–6 rows and 20–23 columns on both pectoral fins.

Mature claspers short and robust, base length 1.9–2.5% TL, inner length 19.1–25.7% TL, tip of clasper bulbous and relatively rounded ( Figure 38 ). Clasper inner length 41.7–44.7% of tail length; a large pseudosiphon is present near the outer lateral edge of dorsal lobe, its length 33.0% of the length of the clasper; inner surface of dorsal lobe with robust, shortened pseudorhipidion that does not reach the tip of the clasper; V-shaped cleft; projection absent from tip.

Clasper skeleton consists of 3 dorsal terminal, 1 accessory terminal, ventral terminal and axial cartilages; dorsal terminal 1 large and rectangular, covering almost the entire dorsal surface, curved around clasper onto the ventral side and united with ventral terminal; dorsal terminal 1 forms the pseudosiphon externally; tip of dorsal marginal is pointed, forming the relatively short pseudorhipidion externally; accessory terminal 1 narrow and does not form a sentinel externally; tip of the ventral marginal is pointed and does not form a projection externally.

Dermal denticles possess 4 points to the base; very well-developed on the posterior third of the dorsal surface; denticles on both the first dorsal fin and the head stout, curved towards posterior, found in low densities ( Figure 39 ).

Length of rostral cartilage 41.7–47.2% of cranial length; prefontanelle rostral length 44.9–50.4%; cranial width 89.4–92.1%; least interorbital width 16.5–20.3%; length of anterior prefontanelle 17.3%; length of posterior prefontanelle 8.7%; length of rostral appendices 11.8–15.4%. Rostral appendices nearly straight; anterior fontanelle dagger-shaped; posterior fontanelle gourd-shaped and longer than anterior one ( Figure 40 ).

Coloration. Dorsal coloration dark purple, brown, or dark grey. Spiracles sometimes pale; claspers externally dark, but internally pale; pelvic fins brown-purple. Ventral coloration grey, lighter in color overall than the dorsal surface, often with white blotches around mouth, gill area, cloaca, and scattered across the body; cloaca light pink to brown. Coloration after preservation is uniform brown dorsally and lighter brown ventrally, with the pale blotches still being present.

Egg case description. Egg cases are small in size (62–78 mm TL), dark brown in coloration, becoming a light golden brown on keels, smooth and plush-like when touched. Possess long, slender horns at each corner of the case. Horns curve inwardly and slightly overlap at their tips. A distinct crossbar originating midway along posterior horns is present in some specimens ( Ebert, 2003; Ebert & Davis, 2007).

Distribution. Bathyraja trachura has been confirmed as occurring in the eastern North Pacific, specifically from the Bering Sea, south to the Gulf of California, Mexico, and as far west as the Sea of Okhotsk ( Ebert, 2003; Kuhnz et al., 2019). It is reported to occur at depths of 213–2,550 m, with most specimens coming from below 600 m in California ( Ebert, 2003), but from deeper than 500 m in Alaska ( Stevenson et al. 2004).

Biological notes. Size at maturity for males 74 cm TL; 80 cm TL for females ( Winton et al., 2013). Males grow to at least 90 cm TL; females grow to at least 89 cm TL. Size at birth is 9–16 cm TL ( Ebert, 2003). Maximum size to at least 90 cm TL. Maximum age has been estimated to be 20 years for males and 17 years for females ( Davis et al., 2007). Feed mostly on benthic organisms, including annelids, shrimps, crabs, and fishes. Adults tend to prefer fishes, whereas juveniles consumer mostly invertebrates ( Ebert, 2003).

Habitat. Found in deeper waters than hardnose skates that share similar ranges ( Ebert, 2003). Reported to prefer medium to cold temperatures and low oxygen environments compared to its congeners ( Kuhnz et al., 2019). Unlike other members of the bathyrajids in the ENP, it is found over a wide range of substrates, from fine sediment to rocky environments (Kuthnz et al., 2019).

Etymology. The species was named after the Greek trachys, meaning rough, referring to the thorny dorsal surface.

Comparisons. Bathyraja trachura is a wide-ranging, medium-sized skate that can be easily separated from its congeners in the ENP. Preorbital snout length significantly shorter than every species but B. spinosissima (F 6,104 = 12.4, p <0.0001). Large-bodied skates in the region (e.g., B. abyssicola , B. aleutica and B. spinosissima ) display obvious coloration differences that separate them from B. trachura . Bathyraja interrupta , a similarly sized species, possesses middorsal, nuchal, and scapular thorns, which B. trachura lacks and has a pale ventral side compared to the dark surface of B. trachura . Bathyraja kincaidii , which has similar thorns to B. interrupta , distinguishes itself from B. trachura in its long interdorsal space. In fact, B. trachura often lacks an interdorsal space entirely.

Bathyraja trachura is closely related to B. microtrachys morphologically, but can be distinguished from the later species based on several characteristics. Besides the coloration, dermal denticle, and morphological differences listed in the prior B. microtrachys comparison, both species possess short and robust mature claspers, but B. microtrachys has a very long pseudorhipidion that nearly touches the tip of the clasper. Conversely, B. trachura has a short pseudorhipidion that does not approach the tip. Further mature clasper differences exist, in that B. trachura possesses an external projection, whereas the structure is absent in B. microtrachys . As with other species, the thorn counts can indicate differences between species. Bathyraja microtrachys does not possess nuchal thorns, but the thorns are present in B. trachura . Bathyraja trachura is the only species in the study that possesses nuchal thorns, but lacks middorsal or scapular thorns. Other species either possess thorns in all those locations, or lack them entirely.

Remarks. Reported to be one of the most abundant skates in its range and distribution and has the widest habitat preference when compared to its congeners ( Kuhnz et al., 2019). This species has been reported to exhibit leucism, with a partially albino specimen coming from the coast of California ( Bigman et al., 2015).

Figures

FIGURE 35. Bathyraja trachura, PSRC Btra 062212-2, 805 mm TL, mature male, dorsal view, fresh. Photo by J.D.S. Knuckey. Scale bar = 100 mm.

FIGURE 36. Bathyraja trachura, PSRC Btra 062212-2, 805 mm TL, mature male, ventral view, fresh. Photo by J.D.S. Knuckey. Scale bar = 100 mm.

FIGURE 37. Bathyraja trachura, holotype, 457 mm TL, female, dorsal view, preserved. Photo by D.A. Ebert. Scale bar = 70 mm.

FIGURE 38. Components of the clasper for Bathyraja trachura, Btra 030613-2, 899 mm TL, mature male. CL- cleft, PJ- projection, PR- pseudorhipidion, PS- pseudosiphon. Photo by J.D.S. Knuckey.

FIGURE 39. Scanning electron microscope images of the dermal denticles of Bathyraja trachura, Btra 030613-2, 899 mm TL, mature male, from A. the first dorsal fin and B. the head region, behind the eyes. Photos by Justin Cordova (MLML).

FIGURE 40. Ventral view X-radiograph of Bathyraja trachura, CAS 243650, mature male. X-radiograph by Jon Fong (CAS).

Tables

TABLE 7. Morphometrics for Bathyraja trachura expressed as % of total length (n = 19).

Total length (mm)	294–899
Disc length (%)	28.7–56.7
Disc width	47.3–68.7
Anterior disc length	32.8–49.3
Anterior anterior disc length (males only)	20.0–25.3
Anterior posterior disc length (males only)	12.8–21.4
Posterior disc length	20.7–34.0
Inner disc length	4.9–10.4
Disc base length	9.5–15.0
Snout to maximum disc width	22.5–35.4
Head length	14.9–24.1
Preorbital snout length	9.0–13.5
Eye length	2.5–5.0