Megacalanus frosti, Bradford-Grieve, Janet M., Blanco-Bercial, Leocadio & Boxshall, Geoffrey A., 2017

Bradford-Grieve, Janet M., Blanco-Bercial, Leocadio & Boxshall, Geoffrey A., 2017, Revision of Family Megacalanidae (Copepoda: Calanoida), Zootaxa 4229 (1), pp. 1-183 : 37-40

Megacalanus frosti n. sp.

( Figs 9 , 12–14 )

Megacalanus longicornis: Miller, 2002 , pp 129–142, figs 2–10.

Megacalanus princeps: Sewell, 1947 , p. 25, Stn 131, female 8.53 mm.

Type locality. 2.867o N, 80.850o W.

Material examined. Francis Drake III, Stn 4, 8 May 1975, IKMT, 0–3000 m, 11♀ (9.3–10.7 mm), 6♂ (9.5–10.4 mm), holotype female (10.5 mm), paratypes. Southtow IV, Stn 36, IKMT, 0–2000 m, 7♀ (10.1–11.0 mm), 7♂ (10.5–11.2 mm), 1CV (8.9 mm). MV 66-II, Stn 5, IKMT, 0–3889 mwo, 6♀ (10.6–11.0 mm), 2♂ (10.9, 10.6 mm). MV 73-I, Stn 53, IKMT, 0–1900 m, 12♀ (9.9–11.0 mm), 5♂ (10.6–11.2 mm), 1CV (8.3 mm). CCE PO810 Cycle 1 Tow 1, Net 1, 500– 700 m, 1♀, Co439.2.1, Co439.2.2. NH1208, MOC1: Stn 18, 399– 600 m 1♀, Co439.5.1, NIWA105276; Stn 34, 400– 600 m, 1♀, Co439.10.1. Record from Natural History Museum, London: John Murray Expedition, Stn 131, 0–500 m, 1♀ (8.53 mm), BMNH1949.12.13.26. Additional records from Smithsonian Institution, USNM numbers: 67211, 67213, 67218–24, 299531, 310291, 310301 –03, 1027730, 1027738.

Type specimens. Deposited in the collection of the Scripps Institution of Oceanography , California: Holotype female: PIC- 140409 -0001-HT; Paratype male: PIC- 140409 -0002-PT; Paratype lot of 9 females and 4 males PIC- 140409 -0003-PT.

Genetic material. Co439.2.1, Co439.2.2, Co439.5.1, Co439.10.1. GenBank numbers in Table 6.

Morphological description. Following description based on holotype and paratype specimens from Francis Drake III, Stn 4. As for genus with following specific level features.

Female. Total length 10.5 mm (mean 10.2 mm, range 8.53–11.0 mm, n = 9) ( Fig. 12 A, B). Anterior head with low crest produced into short triangular projection extending beyond rostrum ( Fig. 12 A–C). Posterior borders of pedigerous somite 5 extending into triangular lappets reaching one third of way along genital double-somite, in dorsal view, lappets appearing pointed ( Fig. 12 A).

Length 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 (260, 688); II–IV (765); V (394); VI (379); VII (433); VIII (433); IX (465); X–XI (913); XII (619); XIII (631); XIV (738); XV (847); XVI (829); XVII (866); XVIII (904); XIX (891); XX (931); XXI (965); XXII (728); XXIII (654); XXIV (644); XXV (644); XXVI (371); XXVII (433); XXVIII (63). Dorsal surface of segments I–V each with very small hair sensillum of which those on segments I–III each accompanied by macula cribrosa ( Fig. 12 E); ancestral segments XIV to XVII with 15, 21, 32, 25 ventral teeth, respectively ( Fig. 12 F, G); and segments XV and XVI with variable numbers of blunt teeth along distoposterior border.

Leg 1 outer spines on exopod segments 1 and 2 extending half way between bases of following 2 more distal spines; 1 macula cribrosa at base of outer spine on exopod segment 2.

Male ( Fig. 13 A, B). Total length 10.4 mm (mean 10.6 mm, range 9.5–11.2 mm, n=8). Head with low crest ( Fig. 13 A–C). Posterior borders of pedigerous somite 5 extending as short triangular lappets, in lateral view, as far as posterior border of urosomite 1 ( Fig. 13 B).

Antennule ( Fig. 14 A–D) on right with ancestral segment XIX with 1 fused griping element extending beyond base of aesthetasc, 1ms, 1a; XXI with 1 short gripping element at about midlength fused only at its base, 1ms, 1a.

Leg 5 as in generic description, inner distal border of basis with setules, left exopod segment 2 specialised seta with basal part usually longer than wide, lash longer than basal part; right exopod segment 3 with inner border completely lined with fine setules to just short of inner border articulated spine ( Fig. 13 E, F).

Morphological variation. Crest usually prominent, but sometimes small. Ancestral segments XIV to XVII of female antennules with 15–18, 20–26, 24–28, 24–32 ventral teeth, respectively (n=6 antennules). Male leg 5 left specialised seta with lash of variable length, sometimes shorter than basal part. Male ancestral segments XIV–XV ventral teeth with 17–29 and 35–39, respectively (n=4).

Distribution. Megacalanus frosti n. sp. is a bathypelagic species that extends into the mesopelagic zone from depths <500 m to> 2000 m. It is known mainly from the eastern Pacific Ocean from off California to off Chile just south of 45o S and extends westwards at least to about 135o W at mid latitudes in the North Pacific ( Fig. 9 , Table 1 ) (see also Miller 2002). A single stray female was found in the Arabian Sea during the John Murray Expedition (Stn 131).

Species comparisons. Females of M. frosti n. sp. and M. princeps are similar in that antennule ancestral segments XV and XVI have a distoposterior row of blunt teeth, a feature not found in M. ericae n. sp. or M.

ohmani n. sp. ( Table 7). Both sexes of M. frosti n. sp., nevertheless, are easily distinguished by their crested heads whereas both sexes of M. princeps lack crests. Also, males of M. frosti n. sp. can be distinguished from the males of M. princeps by the presence of a fused gripping element on ancestral segment XIX on the right antennule (this element is not present in M. princeps ).

The single crested female from the John Murray Expedition, examined by GAB, corresponds well to the description of M. frosti n. sp. It has ventral teeth on antennule segments XIV to XVII inclusive, and has blunt teeth on the posterior margin of XV and XVI, as figured for M. frosti n. sp. The posterior borders of pedigerous somite 5 are as figured. The crest is prominent. The body length for this adult female is only 8.53 mm total length which is small although specimen was from the “southern area of the Arabian Sea” (Stn 131).

Etymology. At the suggestion of Professor Charlie Miller of Oregon State University, who first recognised this form of Megacalanus , this species is named in honour of Professor Bruce Frost.

Figures

FIGURE 9. Distribution of Megacalanus species: filled triangle = Megacalanus princeps; open triangle = M. frosti n. sp.; filled square = M. ericae n. sp.; open square = M. ohmani n. sp..

FIGURE 12. Megacalanus frosti n. sp. female: A, dorsal view; B, lateral view; C, lateral view of head; D, dorsal view of caudal ramus; E, antennule ancestral segments I – V (hs = hair sensillum, mc = macula cribrosa), dorsal view; F, antennule ancestral segments XIV – XV; G, antennule ancestral segments XVI – XVII. Scale bars represent: 1.0 mm on figures A – E, 0.1 mm on remaining figures. Illustrated specimen from Francis Drake III, Stn 4.

FIGURE 13. Megacalanus frosti n. sp. male: A, dorsal view; B, lateral view; C, lateral view of anterior head; D, dorsal view of caudal ramus; E, posterior view of leg 5; F, inner distal margin of exopod segment 2 of left leg. Scale bars represent 1.0 mm on figures A – C; 0.1 mm on remaining figures. Illustated specimen from Francis Drake III, Stn 4.

FIGURE 14. Megacalanus frosti n. sp. male, details of selected right antennule ancestral segments: A, segments XIX – XXIII; B, segments XIV – XV; C, segments XIX – XX; D, segments XXI – XXIII. Scale bars represent 1.0 mm on figure A; 0.1 mm on remaining figures. Illustrated specimen from Francis Drake III, Stn 4.