Megacalanus princeps Wolfenden, 1904

Megacalanus princeps Wolfenden, 1904

( Figs 7–9 View FIGURE 7 View FIGURE 8 View FIGURE 9 )

Megacalanus princeps Wolfenden, 1904 , p. 112–113, pl. ix, fig. 1. Macrocalanus longicornis Sars, 1905 , p. 26.

Megacalanus longicornis: Farran, 1908 , p. 21.

Megacalanus princeps: Wolfenden, 1911 , p. 196–198, fig. 1.

Megacalanus princeps: With, 1915 , p. 41–44, pl. 1, figs 3a–i, textfig. 8a–d. Megacalanus longicornis: Sars, 1924 –25, p. 11–14, pls I–II.

Megacalanus princeps var. inermis Sewell, 1947 , p. 25–27, text-fig. 2. Non Calanus princeps Brady, 1883 , pp 36–37, pl. x, figs 3–7. Non Megacalanus princeps Wolfenden, 1905b , p. 1–3, pl. I, figs 1–6.

Type locality. 51–60o N, 6–13o W.

Material examined. Oceanus Cr 473, MOC1: Stn 1, 100– 866 m, 1♀ (8.8 mm), Co119.3.1; Stn 8, 801– 1001 m, 1♂ (9.7 mm), Co119.4.1; Stn 21, 600– 798 m, 2♀ (9.1, 9.5 mm), Co.119.5.1. ANTXIV/1, Stn 2, MOC10: 2000–3000 m, 1♀ (10 mm), 1♂ (10.4 mm); 1000–2000 m, 1 NVI (2.56 mm), 1 CII (4.0 mm), 2 CV (7.8, 8.0 mm). MOC1: 797–1003 m, 1♀ (10.5 mm), Co119.2.1; 399–500 m, 2♀ (9.7, 9.1 mm), 1CV, 1CIV. RHB0603, Stn 3, MOC10, 1000–3000 m 1♂. Indopac VII, Stn 5, IKMT, 0–2121 mwo, 5♀ (9.1–9.8 mm), 2♂ (9.5, 9.7 mm). Indopac VIII, Stn 6, IKMT, 0–2096 mwo, 1♂ (9.6 mm). Southtow IV, Stn 36, IKMT, 0–2000 m, 2♂ (9.8, 10.2 mm). Records from Natural History Museum , London: Sr 224, off Ireland, 700 fathoms, 3♀ (see Farran 1908 — as Megacalanus longicornis ) , BMNH 1908.7.6.1-3. John Murray Expedition: Stn 96, 1CV, BMNH 1949.12.31.24; Stn 98, 1♂ , BMNH 1949.12.31.25; Stn 131D, 1♂ , BMNH 1949.12.31.27, 1♀, 3CV, 2CIV, BMNH 1949.12.31.28; Stn 172, 2665 mwo, 1CV, BMNH 1949.12.31.29; Stn 172, 1500 mwo, 1♂, 1CV , BMNH 1949.12.31.30. Discovery Stn 7406#24, BT8, 600– 700 m, 3♀ (8.6, 8.5, 8.5 mm), BMNH 1994.5752-5755. Additional records from Smithsonian Institution, USNM numbers: 69930, 262489, 262491, 298332, 299632, 1027487–89, 1027491, 1027492, 1027514–21, 1027685, 1027688–92, 1027731–37, 1027740, 1027741, 1207752

Genetic material. Co119.2.1, Co119.3.1, Co119.4.1, Co119.5.1. GenBank numbers in Table 6.

Morphological description. Following description based on specimens from Oceanus 473, 100– 866 m and ANTXXIV/1 Stn 2. As for genus with following specific characters.

Female ( Fig. 7 View FIGURE 7 ). Total length 8.5–10.5 mm (mean 9.3 mm, n=12). Anterior margin of head in dorsal view slightly produced into short rounded projection dorsal to base of rostrum ( Fig. 7 View FIGURE 7 A–C). Posterior borders of pedigerous somite 5 ( Fig. 7 View FIGURE 7 B) extend into triangular lappets reaching one third of way along genital double-somite, in dorsal view ( Fig. 7 View FIGURE 7 A) lappets appear pointed.

Antennule length of segments (µm) as follows (note last few segments missing). Measurements taken along posterior border of each segment but two (posterior (shortest) and anterior) measurements are taken of ancestral segment I: I (676, 228); II–IV (654); V (329); VI (314); VII (366); VIII (379); IX (389); X–XI (814); XII (545); XIII (567); XIV (681); XV (787); XVI (765); XVII (782); XVIII (834); XIX (871); XX (881); XXI (884); XXII (646). Dorsal surface of ancestral segments I–V each with very small hair sensillum of which those on segments II and III accompanied by macula cribrosa ( Fig. 7 View FIGURE 7 H); ancestral segments XIV–XVII with 13/13, 20/21, 25/26, 24/26 distoventral teeth, respectively, distoposterior borders of segments XV and XVI with 26/32 and 29/33 minute blunt teeth, respectively; in some specimens, observed in dish without coverslip, these blunt teeth appear to be crosssection of posterior border ridges that run at right angles to main axis of segment.

Leg 1 ( Fig. 7 View FIGURE 7 J) outer spines on exopod segments 1 and 2 extend half way between bases of following 2 more distal spines; 1 macula cribrosa at base of outer spine on exopod segment 2.

Male ( Fig. 8 View FIGURE 8 ). Total length 9.5–10.4 mm (mean 9.8 mm, n=7). Anterior margin of head in dorsal view slightly produced into short rounded projection dorsal to base of rostrum which extends into two long, ventroposteriorlydirected, tapering points that appear to be direct extensions of cuticle ( Fig. 8 View FIGURE 8 A–C).

Antennule right ancestral segment XIX without fused gripping element ( Fig. 8 View FIGURE 8 H), instead, 2ms, 1a; XXI—1 short fused element (arising just proximal to midlength but fused to its segment only at its base), 1ms, 1a ( Fig. 8 View FIGURE 8 I); fused segments XV–XVI ( Fig. 8 View FIGURE 8 G) on right antennule each with longitudinal row of 16, 32 ventral teeth, respectively.

Leg 5 inner distal border of basis with setules, specialised seta on inner distal border of exopod segment 2 on left with basal part longer than wide and lash longer than basal part ( Fig. 8 View FIGURE 8 E, F). Right exopod segment 3 inner border completely lined with fine spinules to just short of inner articulated spine.

Remarks. The possibility there may be another species of Megacalanus was considered because Sewell (1947) described Megacalanus princeps var. inermis : p. 25–27, text-fig. 2. This female, from the John Murray Expedition Stn 98 in the Arabian Sea, did not have an anterodistal hook-like process on the basis of leg 1 and was also characterised by the presence of macula cribrosa on the anterior surface at the base of the outer distal spine on exopod segments 1 and 2. In M. princeps , M. frosti n. sp., M. ericae n. sp. and M. ohmani n. sp. there is a macula cribrosa only on exopod segment 2, but in the first three species the macula cribrosa is situated close to the base of the distolateral spine on the anterior surface, whereas, in M. ohmani n. sp. the macula cribrosa is situated more proximally on the lateral border. Re-examination of leg 1 of Sewell’s specimen (GAB) revealed that there is no macula cribrosa on exopod segment 1, contrary to the original observation ( Sewell 1947). The general shape of the head and the nature of antennular ancestral segments XIV–XVII are consistent with M. princeps . We also note Gueredrat’s (1969) observation that a number of Megacalanus from the tropical western Pacific do not have an anterodistal hook-like process on the basis of leg 1 or have various versions of reduction or atrophy. Therefore, we consider var. inermis to be a deformed M. princeps .

Distribution. Megacalanus princeps is a bathypelagic species that also extends into the mesopelagic zone (see also Mauchline & Gordon 1991). It is known mainly from the Atlantic and Indian Oceans based on presently examined specimens. It extends into the Pacific Ocean since it has been recorded from the Celebes Sea and a few specimens have been found in the eastern Pacific as well as one in the tropical Pacific at 150o W ( Fig. 9 View FIGURE 9 , Table 1 View TABLE 1 ). The vertical distribution of M. princeps extends from 500–2800 m.

Species comparisons. Both Wolfenden (1911) and Sewell (1947) mention the presence of a row of very small teeth on the distal part of the posterior border of the female antennular segments XV and XVI in M. princeps . The female of M. princeps shares this character with the female of M. frosti n. sp., nevertheless, M. princeps can be distinguished from M. frosti n. sp. by the absence of a crest on the head, a characteristic found only in M. frosti n. sp. ( Table 7). Megacalanus princeps males may be distinguished from males of all other species by the absence of a gripping element on ancestral segment XIX of the right antennule.