Bathycalanus unicornis Björnberg, 1968

Bradford-Grieve, Janet M., Blanco-Bercial, Leocadio & Boxshall, Geoffrey A., 2017, Revision of Family Megacalanidae (Copepoda: Calanoida), Zootaxa 4229 (1), pp. 1-183 : 123-127

Bathycalanus unicornis Björnberg, 1968

( Figs 46 , 81–86 )

Bathycalanus unicornis Björnberg, 1968 , pp 73–75, figs 1–14.

Type locality. 40.767o S, 76. 800o W.

Material examined. MV 73-I, Stn 53, IKMT, 0–2000 m, 2♀ (9.8, 10.2 mm), 2♂ (8.3, 8.8 mm). ANTXXIV/1, Stn 8, MOC10, 2990– 3992 m, 2♀ (9.3, 9.6 mm). Eltanin, Stn 175, IKMT, 2893 m, 1♀ Holotype USNM 122566. Records from Natural History Museum, London: Discovery Stns, RMT8: 8508#78, 2500–3100 m, 3♀ (9.3, 9.3, 9.4 mm) BMNH 1993.859-861; 8509#20, 3000–3500 m, 1♀ (9.35 mm), BMNH 1993.858; 8509#27, 3500–4000 m, 2♀ (10.0 mm, 1 damaged) BMNH 1993.862-863.

Morphological description. Following description based on specimen from MV73-1, Stn 53. As for genus with following specific level features.

Female ( Fig. 81 A–C). Total length 10.5 mm (mean = 9.6 mm, range = 9.3–10.2 mm, n = 9). Anterior margin of head in dorsal view produced into single spine-like process, curved ventrally with ventral barb near tip. In lateral view, posterolateral corners of pedigerous somite 5 symmetrical, short and rounded, not extending posterior to widest part of genital double-somite. Genital double-somite symmetrical in dorsal view, widest at about anterior one eighth, 1.2 times longer than wide, seminal receptacles not observed.

Antennule ( Fig. 82 ) extending beyond caudal rami by about 5–6 segments. Lengths of antennule segments (µm) as follows. Measurements taken along posterior border of each segment but two (posterior (shortest) and anterior) measurements taken of ancestral segment I. I (308, 623); II–IV (365); V (217); VI (256); VII (315); VIII (305); IX (303); X–XI (537); XII (345); XIII (367); XIV (429); XV (552); XVI (675); XVII (737); XVIII (892); XIX (926); XX (1032); XXI (1059); XXII (778); XXIII (727); XXIV (872); XXV (842); XXVI (389); XXVII (741); XXVIII (38). Anterior and posterior borders of ancestral segments XVI–XXI smooth.

Antenna ( Fig. 83 A) exopod segment IV with short seta extending to distal border of segment VIII and bearing short setules.

Maxillule ( Fig. 83 D) praecoxal arthrite with 14 setae including 3 on posterior surface and 2 setae on anterior surface, one of them small; coxal endite without setae, basal endites 1 and 2 with 2 and 4 setae respectively; endopod segments with 2/3, 2, 5+1 smaller anterior surface seta.

Maxilliped ( Fig. 81 E) syncoxal endite 4 longest, toothed seta extending beyond distal border of endopod segment 2.

Leg 1 ( Fig. 81 D) exopod with articulation between exopod segments 2 and 3 well developed; distolateral corner of endopod segment 1 bilobed.

Male ( Fig. 85 A–C). Total length 8.8, 8.3 mm. Anterior margin of head in dorsal view produced into single spine-like process curved ventrally, with ventral barb near tip. In lateral view, posterior corners of pedigerous somite 5 rounded, extending almost to posterior border of urosomite I.

Antennule ( Figs 85 D, 86) extends beyond caudal rami by about 5–6 segments on left. Left antennule similar to that of female. Right ancestral segment XVIII widened by anterior border ridge—2ms, 1a; XIX with proximal ridge—1ms, 1 fused element with free tip, 1a; XX—with 1 long fused element with free tip, 1ms, 1a; XXI–XXIII—2 fused elements with free tips (proximal element largely overlapping distal element), 1a, 1 vestigial s?, 1ms.

Leg 5 ( Fig. 85 E–G) extending just beyond third free urosomite, asymmetrically developed; left leg slightly longer than right leg, outer borders of exopod segments without surface thickenings. Right exopod segment 2, inner border seta in form of short thickened proximal part and apparently articulated tapering distal part. Inner border of right exopod segment 3 naked apart from proximal setules. Left exopod segment 2 inner border specialised seta in form of spherical proximal part with outer naked rounded lobe and inner pair of long lashes clothed in very long setules. Inner border of left exopod segment 3 irregular in shape, bordered by setules from proximal to near inner spine.

Remarks. This is the second time Ba. unicornis has been recorded since its original discovery off Valdivia, Chile ( Björnberg 1968). Here, we have corrected some observations, highlighted additional characters distinguishing the female of this species from other Bathycalanus and added a description of the male.

Morphological variation. There appears to be some variation in the setation of the maxillule as there may be 2 or 3 setae on endopod segment 1 in the same individual, although it is not possible to be sure if this observed variation is due to damage.

Distribution. The present records extend the distribution into the Atlantic Ocean over the abyssal plain off Namibia and northwards into the eastern Pacific to off the coast of Mexico ( Fig. 46 , Table 1 ) ( Björnberg 1968). Bathycalanus unicornis is an upper abyssopelagic species taken from depths <2000 to> 3000 m.

Species comparisons. Bathycalanus unicornis is unique amongst Bathycalanus in several respects ( Table 10 ): 1) the anterior head has 1 large barbed spine-like process projecting anteriorly and curving ventrally; 2) the genital double-somite, in dorsal view, is 1.2 times longer than wide and the widest part is at the anterior one eighth; 3) the posterolateral borders of pedigerous somite 5 are very short and rounded in lateral view; 4) the maxillule has 3 posterior surface setae on the praecoxal arthrite; 5) the male leg 5 is unique among currently known Bathycalanus in having a setal remnant on the inner border of the right exopod segment 2; and 6) in having 2 moderately long setulose lashes on the inner specialised seta of left exopod segment 2.

Figures

FIGURE 46. Distribution of Bathycalanus species: filled triangle = Bathycalanus bradyi; open triangle = Ba. milleri n. sp.; filled square = Ba. richardi; open square = Ba. dentatus n. sp.; open circle = Ba. tumidus n. sp.; open diamond = Ba. adornatus n. sp.; filled star = Ba. pustulosus; filled diamond = Ba. unicornis; filled circle = Ba. bucklinae n. sp.; cross = Ba. eximius.

FIGURE 81. Bathycalanus unicornis Björnberg, 1968 female: A, dorsal view; B, lateral view; C, lateral view of anterior head; D, anterior view of leg 1; E, maxilliped. Scale bars 1.0 mm on all figures. Illustrated specimen from MV 73 - 1, Stn 53.

FIGURE 82. Bathycalanus unicornis Björnberg, 1968 female antennule ancestral segments: A, segments I – XV; B, segments XVI – XVIII; C, segments XIX – XXII; D, segments XXIII – XXVIII; (E) detail of segment XXVIII. Scale bars represent 1.0 mm on figures A – D; 0.1 mm on remaining figure. Illustrated specimen from MV 73 - 1, Stn 53.

FIGURE 83. Bathycalanus unicornis Björnberg, 1968 female: A, antenna; B, mandibular palp; C, mandibular gnathobase; D, maxillule; E, maxilla; F, endopod of maxilla. Scale bars represent 1.0 mm on figure E; 0.1 mm on remaining figures. Illustrated specimen from MV 73 - 1, Stn 53.

FIGURE 84. Bathycalanus unicornis Björnberg, 1968 female: A, posterior view of leg 2; B, anterior view of leg 3; C, anterior view of leg 4; D, anterior view of leg 5. Scale bar represents 1.0 mm on all figures. Illustrated specimen from MV 73 - 1, Stn 53.

FIGURE 85. Bathycalanus unicornis Björnberg, 1968 male: A, dorsal view; B, lateral view; C, lateral view of anterior head; D, ancestral segments XVIII – XXIII of right antennule; E, posterior view of leg 5; F, inner distal corner modified seta of right exopod segment 2 of leg 5; G, inner distal specialised seta of left exopod segment 2 of leg 5. Scale bars represent 1.0 mm on figures A, B, E; 0.1 mm on remaining figures. Illustrated specimen from MV 73 - 1, Stn 53.

FIGURE 86. Bathycalanus unicornis Björnberg, 1968 male ancestral antennule segments. Left antennule: A, segments I – XVIII; B, segments XIX – XXI; C, segments XXII – XXVIII; D, detail of segment XXVIII. Right antennule: F, segments I – XV; G, segments XVI – XXIV; H, segments XXV – XXVIII. Scale bars represent 1.0 mm on figures A – C, F – H; 0.1 mm on remaining figure. Illustrated specimen from MV 73 - 1, Stn 53.