Elenacalanus princeps Brady 1883, new combination

Elenacalanus princeps (Brady, 1883) new combination

(Figs 87–94)

Calanus princeps Brady, 1883, pp 36–37, pl. IV, figs 3–7. Heterocalanus medius Wolfenden, 1906, pp 201–202, pl. XL, fig. 1–5. Megacalanus princeps: Farran, 1908, p. 21.

Macrocalanus princeps: With, 1915, pp 37–40, pl. 1, figs 2a, 2b, textfig. 7. Bathycalanus rigidus Sars, 1920, p. 2.

Bathycalanus rigidus: Sars, 1924 /25, pp 19–20, pl. 5, figs 7–15. Bathycalanus rigidus: Rose, 1929, pp 9–14, pl. 1, 25 figs. Bathycalanus princeps: Farran, 1939, pp 357–359.

Bathycalanus princeps: Grice & Hulsemann, 1967, p. 13.

Type locality. 38.567o N, 72.167o W (Stn 45 of HMS Challenger Expedition, designated here).

Material examined. RHB0603, Stn 1, MOC1, 200– 400 m, 3♀ Co024.1.1, Co024.1.2, Co024.1.4. Francis Drake III, Stn 2, IKMT, 0–3000 m, 2♀ (12.4, 12.6 mm). Oceanus Cr 473, MOC1: Stn 8, 801– 1001 m, 1♀ (damaged) Co024.2.1; Stn 21, 600– 798 m, 1CV (9.6 mm) Co024.3.1, Co024.3.2; Stn 26, 798– 1001 m, 1♀ (12.6 mm) Co024.5.1; Stn 26, 400– 600 m, 1♀ (12.9 mm) Co024.6.1; Stn 31, 400– 600 m, 1♀ (13.4 mm) Co024.7.1. LMG11-10, MOC1, Stn 11, 1500–2000 m, 1♀ Co441.3.1. Eltanin Cr 15, 23 Jan 1964, 1♂ (8.9 mm) USNM299526. Records from Natural History Museum, London: Syntypes; 2♀ (each dissected on 2 slides) Challenger Expedition Stn 50 and Stn 45, BMNH 1884.4 c.c.2/5. Discovery Stns, RMT8: 7406#6, 900– 1000 m, 3♀ (11.6, 12.3, 12.1 mm), 1CV (10.3 mm), BMNH 1994.5748; 7406#33, 990– 1250 m, 5♂ (9.1, 9.3, 9.1, 9.2, 9.1 mm), BMNH 1993.809-813; 7480, 1250– 1500 m, 1♂ (9.4 mm), BMNH 1993.875; 7709#35, 1010– 1250 m, 2♀ (11.5, 11.9 mm), 2♂ (9.3, 9.4 mm), BMNH 1993.786-789; 7709#76, 1250– 1500 m, 9♂ (9.9, 9.6, 9.8, 9.7, 10.0, 9.6, 9.8, 9.7, 9.8 mm), BMNH 1993.819-827; 7711#4, 800– 900 m, 2♀ (11.9, 12.9 mm), BMNH 1994.5757-5758; 7711#61, 1500–2000 m, 1♂ (9.9 mm), BMNH 1993.814; 8507#72, 1250– 1500 m, 1♂ (9.8 mm), BMNH 1993.873; 8507#73, 1500–2000 m, 8♂ (9.0, 9.8, 9.0, 9.1, 9.4, 9.2, 9.0, 9.5 mm), BMNH 1993.1435-1442; 8507#73, 1500–2000 m, 21♂ (9.6, 9.7, 9.5, 9.5, 9.8, 9.8, 9.5, 9.7, 9.6, 9.8, 9.9, 9.6, 9.6, 9.4, 9.7, 10.0, 9.7, 9.6, 9.5, 9.8 mm), BMNH 1993.1395-1404. Additional records from Smithsonian Institution, USNM numbers: 58954, 58955, 67211, 232142, 262439, 262441–42, 262461, 262468–73, 262475–77, 262484, 262488, 269442, 269446, 269460–70, 299520, 299523, 299526, 302048, 302050, 302061–62, 302067, 302070, 302072, 302074, 1132629.

Genetic material. Co024.1.1, Co024.1.2, Co024.1.4, Co024.2.1, Co024.3.1, Co 0 24.3.2, Co024.5.1, Co024.6.1, Co024.7.1. Genbank numbers in Table 6.

Morphological description. Following description based on female specimen from Francis Drake III, Stn 2. Male description based on that of Rose (1929) (as Ba. rigidus), and male, USNM 122568, augmented by observations made on material held at the Natural History Museum, London. As for genus with following specific level features.

Female (Fig. 87 A–C). Total length 12.4 mm (mean = 12.5 mm, range = 11.5–13.4 mm, n = 15). Anterior margin of head in dorsal and lateral views with low crest. In lateral view, posterior corners of pedigerous somite 5 short and rounded. Genital double-somite symmetrical in dorsal view, bulbous, wider than long.

Antennules (Figs 87 C, 88) extending beyond caudal rami by 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 (207, 569); II–IV (429); V (244); VI (261); VII (283); VIII (308); IX (330); X–XI (638); XII (603); XIII (621); XIV (761); XV (975); XVI (1015); XVII (1089); XVIII (1143); XIX (1143); XX (1214); XXI (1196); XXII (906); XXIII (840); XXIV (850); XXV (764); XXVI (315); XXVII (734); XXVIII (39). Segments I, II, III, and V with dorsal surface hair sensillum and adjacent macula cribrosa, segment IV has only hair sensillum.

Antenna (Fig. 87 D) ancestral exopod segment IV without seta.

Maxillule (Fig. 89 C) basal endites 1 and 2 with 2 and 2 setae, respectively; endopod segments with 1–2, 0–1, 4+1 small anterior surface seta; basal exite without seta, epipodite with 7 long and 2 vestigial setae.

Maxilliped (Fig. 89 F) syncoxal endites with 1, 2 (1 vestigial), 4, 4 (one vestigial) setae, longest seta on endite 4 extending as far as distal border of basis; endopod segments 3–6 with 1, 1, 1 (no outer border seta but 1 macula cribrosa), 3 terminal setae (2 large and 1 small, no seta on outer border), respectively.

Male (Fig. A–D). (From Rose, 1929, pp. 9–14, Pl. 1, 25 figs (as Ba. rigidus) and male USNM 122568, augmented by observations made on material held at the Natural History Museum, London). Total length: mean = 9.6 mm, range = 8.9–11.0 mm, n = 50. Anterior margin of head in dorsal and lateral views rounded, without crest. Rostrum ventrally-directed, bluntly tapering.

Antennule (Figs 91 D, 92) ancestral segments 1 and II fused. On right side (Fig. 92 D–G), segments XVI to XIX with narrow proximal anterior border ridge; fused gripping element on segment XIX extending distally well beyond insertion of aesthetasc; segment XX curved on shorter radius than in E. eltaninae; proximal fused gripping element of fused segments XXI–XXIII overlapping second fused gripping element by about 40% of its length and gripping element 2 extending distally beyond base of small seta of incorporated segment XXII. Left antennule (Fig. 92 A–C) segments XXV–XXVIII broken off.

Antenna (Fig. 93 A) ancestral exopod segments I–IV without setae.

Mandible basis with 3 setae, endopod segment 2 with 9 setae (Fig. 91 E, F).

Maxillule (Fig. 93 B, C) praecoxal arthrite with about 10 reduced setae, apparently none on posterior surface, basal endites 1 and 2 with 2, 0 setae, respectively; endopod segments with 1/0, 0, 4 setae, exopod with 11 setae; basal exite without seta, epipodite with 7 long setae, proximal 2 setae absent.

Maxilla (Fig. 93 D) as in female but setae weakly developed; 2 setae of basal endite and setae of endopod segments 1–4 very long, distally weak and narrow therefore not holding their distal curl, lined distally with very dense, fine spinules.

Maxilliped (Fig. 93 E) syncoxal endites with 1, 2 (1 vestigial), 4, 3 setae, respectively, longest seta on endite 4 extending to distal seta of basis; endopod segments 3–6 with 1, 1, 1 (no outer border seta), 2 large setae and 1 short seta, respectively.

Leg 1 (Fig. 91 G) exopod segment 2 proximal outer spine not extending to base of terminal outer spine.

Leg 5 (Fig. 91 H–N) slightly asymmetrical. Right exopod segment 3 with tuft of proximal setules on inner border whereas left exopod segment 3 inner border densely setulose. Exopod segment 2 on left variable with characteristic specialised inner distal seta made of 2 parts: basal part and setulose lash; basal part usually rectangular, distally setulose (in some specimens base slightly drawn out) and with setules at base of lash; setulose lash arising from proximal border of basal part. Left exopod segment 3 carrying 1 terminal, 1 inner (at about distal ¼) and 1 outer border spine (at distal 1/3rd); right exopod segment 3 with 1 terminal spine, 1 inner and 1 outer border spine positioned opposite each other. Macula cribrosa present on each of basis and exopod segments 1–3.

Remarks. There has been much previous confusion about the identity of this species because Brady’s (1883) description and drawings were sufficient only to place this taxon in the Megacalanidae. That is, the maxillule was not accurately described, only a seta from the maxilla was illustrated and leg 1 was not described. One of us (GAB) recently re-examined two of Brady’s type specimens, each dissected on two slides. Most of the limbs are in good condition in the female from Stn 50 (BMNH 1884.4 c.c.2) although not perfectly orientated. The maxillule formula is: coxal endite without setae, basal endites 1 and 2 with 2, 1 setae respectively, endopod with 1, 1, 4+1. Leg 1 exopod segments 1 and 2 lack outer border spines, while segment 3 has 2 outer spines. These character states confirm that Calanus princeps Brady, 1883 belongs to a genus separate from Megacalanus, Bradycalanus and Bathycalanus. Brady does not mention the crest on the head of the female but With (1915) and Wolfenden (1906) do. Brady (1883) does not describe the structure of leg 1, but we confirm that exopod segment 3 of leg 1 of the type specimens possesses 2 outer border spines in addition to a long terminal element and 4 inner setae, as illustrated by Rose (1929) and Wolfenden (1906). With (1915) states that leg 1 exopod segment 3 has only 1 outer border spine, but two spines are present in the types. The slide of the mouthparts of the syntype from Challenger Stn 50 has 7 + 2 short setae on the coxal epipodite of the maxillule on one side but 7 + 3 short setae on the other side. This is interpreted here as an aberrant state, given that the total of 9 setae is the maximum known anywhere in the Copepoda (Huys & Boxshall 1991).

Farran (1939) recognised that the specimens called Calanus princeps by Brady (1883) were the same as the specimen described as Bathycalanus rigidus and illustrated by Sars (1924/25). Bathycalanus rigidus consequently becomes a junior subjective synonym of Elenacalanus princeps (Brady, 1883).

The present female specimens agree with the description Bathycalanus rigidus of Sars (1924, 1925), although some details differ from the present description. His illustration of the maxilla has 5 setae on praecoxal endite 1 whereas presently examined specimens have 6 + 1 small seta. In the text, Sars (1925) says that the antenna, mandible and maxillule show no differences from the preceding species (Ba. richardi) which is not true. Sars’ (1924) illustrations of these limbs of Ba. richardi omit several of the smaller setae that are present in currently examined specimens. For example, mandible endopod segments 1 and 2 are illustrated as having 3 and 8 setae, respectively; actually 4 and 11 setae, respectively, in currently examined specimens. Lack of this detail obscured differences from E. princeps where these mandible endopod setae are 2 and 9 respectively, a consistent characteristic of all Elenacalanus.

Rose (1929) contains the only description of a male to date. Among the specimens examined at the Smithsonian Institution there was one male (USNM299526) that agrees with most details given by Rose (1929) although leg 5 was not intact (the two left terminal exopod segments are missing) so the hook on the specialised seta could not be confirmed. Examination of males held by the Natural History Museum, London does not corroborate the presence of a hook on the basal part. Rather, the basal part is of variable shapes although one specimen examined (BMNH1993.809-813, Fig. 91 N) had a slight distal notch. Males were assigned to females mainly based on the fact that E. princeps females were the most common Elenacalanus species in the R.R.V. Discovery material as were the males (total length: range 9–10 mm; mean 9.5 mm) which are slightly smaller on average than in E. eltaninae (total length: range 10–10.5 mm; mean 10.2 mm).

Sexual dimorphism of the limbs appears to be more extensive in E. princeps than in most other Megacalanidae. Compared with the female, the male, in addition to the antennules and leg 5, has 1 fewer setae on the mandible basis, fewer setae on the praecoxal arthrite, basis and endopod segments 1 and 2 of the maxillule; and the distal parts of the long setae of the maxilla are more weakly developed.

Morphological variation. The setation of the endopod of the female maxillule was variable on each side of the same individual examined here. The shape of the male inner distal specialised seta of left leg 5 exopod segment 2 is variable; the base may be very rectangular (Fig. 91 K–M), drawn out distally (Fig. 91 I, J) or with a slight notch (Fig. 91 N).

Distribution. Elenacalanus princeps seems to be a bathypelagic species that extends into the mesopelagic zone in the North Atlantic Ocean (800–2000 m (With 1915), and Natural History Museum, London records), in the southeastern Pacific off Chile (present results), the southern Indian Ocean 350–2394 m (Grice & Hulsemann 1967) and Atlantic sector of the Antarctic 366–3074 m (Michel 1994). Specimens held by the Smithsonian Institution are from the Atlantic and the Antarctic sector of the Pacific, and Indian Oceans (Fig. 94, Table 1). Michel (1994) summarises its depth of occurrence as 366–3074 m.

Species comparisons. Females of Elenacalanus princeps are closely related to E. eltaninae, E. sverdrupi, E. inflatus and an undescribed species from the northwest Pacific. Elenacalanus princeps may be distinguished from the other four species (Table 11) by the crested head of the female (head rounded in the other four species), proximal seta of maxillule coxal epipodite absent (present in all other species), and the maxilliped endopod segment 6 with 3 setae (4 in most other species).

Males are known for E. princeps, E. eltaninae and E. tageae n. sp.. The male leg 5 of E. princeps has the right exopod segment 3 inner border with a proximal tuft of setules (in the other known species this border is naked). The specialised seta on the left exopod segment 2 of leg 5 in E. princeps is rather variable and the form of this seta on E. eltaninae fits within the range of variability found in M. princeps (compare Fig. 91 I–N and Fig. 100 H). Males of E. princeps and E. eltaninae are also without a seta on exopod segment 4 of the antenna. The male geniculate antennule of E. princeps has ancestral segments IX–XI fused but segment XII separate (in E. eltaninae segments IX–XII are fused), the radius of curvature of segment XX of E. princeps is shorter than in E. eltaninae and the anteroproximal border of segments XVII–XIX are thickened into flange-like extensions (these are absent in E. eltaninae).