Megacalanus ohmani, 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 : 44-51

Megacalanus ohmani n. sp.

( Figs 9 , 17–23 )

Type locality. 5.350o S, 133.583o E.

Material examined. Indopac VIII, Stn 6, IKMT, 0–2 0 96 mwo, 33♀ (9.5–11.0 mm), 8♂, holotype female 11.0 mm, paratypes.

Type specimens. Deposited in the collection of the Scripps Institution of Oceanography , California: Holotype female: PIC- 140409 -0004-HT; Paratype male: PIC- 140409 -0005-PT; Paratype lot of 33 females and 7 males PIC- 140409 -0006-PT.

Morphological description. Following description based on holotype and paratype from Indopac VIII, Stn 6. As for genus with following specific level features.

Female ( Figs 17–20 ). Total length 11.0 mm (mean 10.4 mm, range 9.5–11.0 mm, n=10). Head bluntly rounded, posterior corners of pedigerous somite 5 with short bluntly rounded posterior borders extending one quarter of distance along genital double-somite in dorsal view and only as far as genital bulge of genital double-somite in lateral view, posterior borders appearing rounded in dorsal view.

Antennule ( Fig. 18 ) segment length (µm) as follows. Measurements taken along posterior border of each segment but two (posterior (shortest) and anterior) measurements taken of ancestral segment I. I (441, 691); II–IV (765); V (376); VI (371); VII (418); VIII (418); IX (483); X–XI (928); XII (654); XIII (666); XIV (787); XV (938); XVI (951); XVII (968); XVIII (1020); XIX (1020); XX (1012); XXI (1005); XXII (708); XXIII (651); XXIV (691); XXV (658); XXVI (431); XXVII (503); XXVIII (67). Dorsal surface of segments I–V each with very small hair sensillum of which those on segments I–IV each accompanied by macula cribrosa ( Fig. 20 G); ventral surface of ancestral segments XIV to XVII with distal row of 19/20, 31/31, 30/36, 30/30 teeth, respectively ( Fig. 18 B); ancestral segments XV and XVI smooth, without distoposterior row of blunt teeth.

Leg 1 ( Fig. 17 E, F) outer spines on exopod segments 1 and 2 extending only slightly beyond base of following, more distal spine and exopod segment 2 has macula cribrosa located at distal one third from base of distolateral spine on lateral border.

Male ( Figs 21–23 ). Total length 10.7 mm. Anterior margin of head bluntly rounded in dorsal view.

Right antennule ancestral segment XI with distal clavate (club-shaped) seta (indicated by arrow) ( Fig. 23 A); ancestral segment XIX with 1 fused gripping element extending beyond base of aesthetasc, 1ms, 1a ( Fig. 23 C, D); and segment XXI with 2 long overlapping gripping elements (proximal element with tip extending to base of aesthetasc, distal element extending well beyond distal border of segment), 1a ( Fig. 23 C, E). Left antennule ( Fig. 22 ) with clavate seta broken off ancestral segment XI in figured specimen leaving setal vestige, but present in other specimens.

Leg 5 inner distal border of basis without setules, left leg 5 exopod segment 2 specialised seta with basal part longer than wide and lash longer than basal part ( Fig. 21 E–G). Right exopod segment 3 inner border with series of hyaline ridges along distal two thirds, proximal one third lined with fine setules ( Fig. 21 E).

Distribution. Megacalanus ohmani is a bathypelagic species known so far only from east of Irian Jaya ( Fig. 9 , Table 1 ).

Species comparisons. Female of M. ohmani n. sp. and M. ericae n. sp. are similar in that ancestral segments XV and XVI of the antennule have a smooth distoposterior border ( Table 7). Females (and males) of these two species may be distinguished by the shape of the posterior borders of pedigerous somite 5: in M. ohmani n. sp. these borders are short and round and in M. ericae n. sp. longer and triangular. Male M. ohmani n. sp. is easily distinguished from males of all other species by the pair of overlapping fused gripping elements on right antennule ancestral segment XXI and the inner border of the right leg 5 exopod segment 3 which is setulose only on the proximal one third and has a series of hyaline ridges along the remainder of the inner border — no other species has these features.

The specialised seta on the male left leg 5 exopod segment 2 is similar to that of M. princeps and M. frosti n. sp. but differs from the Pacific species M. ericae n. sp. which has a much stumpier specialised seta with a lash that is shorter than the basal part. Leg 1 of M. ohmani n. sp. has two unique features: the outer spines on exopod segments 1 and 2 extend only slightly beyond base of following, more distal spine (in the other species these outer spines are longer); and exopod segment 2 has the macula cribrosa located at distal one third from base of distolateral spine on lateral border (whereas in the other species the macula cribrosa is situated close to the base of the outer border spine and on the anterior surface).

Etymology. This species has been named for Professor Mark Ohman who made available the extensive collection of Megacalanidae from the Scripps Institution of Oceanography ( USA) collection of pelagic invertebrates, on which this work is partly based.

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 17. Megacalanus ohmani n. sp. female: A, dorsal view; B, lateral view; C, lateral view of anterior head; D, dorsal view of caudal ramus; E, anterior view of leg 1 (mc = macula cribrosa); F, detail of large spine on anterior surface of leg 1 basis. Scale bars represent 1.0 mm on figures A, B; 0.1 mm on remaining figures. Illustrated specimen from Indopac VIII, Stn 6.

FIGURE 18. Megacalanus ohmani n. sp. female antennule ancestral segments: A, ventral surface of segments I – XI; B, segments XII – XVII; C, segments XVIII – XXIII; D, segments XXIV – XXVIII; E, detail of aesthetasc (a), modified setae (ms) and macula cribrosa (mc) distally on segments XXV; F, detail of terminal part of antennule (a = aesthetasc, mc = macula cribrosa). Scale bars represent 1.0 mm on figures A – D; 0.1 mm on remaining figures. Illustrated specimen from Indopac VIII, Stn 6.

FIGURE 19. Megacalanus ohmani n. sp. female: A, antenna; B, mandible; C, maxillule (mc = macula cribrosa); D, inner view of maxilla; E, detail of inner view of terminal part of maxilla (B = basal endite, Ri 1 – 4 = endopod segments 1 – 4); F, outer view of endopod of maxilla. Scale bars represent 1.0 mm on figure D; 0.1 mm on remaining figures. Illustrated specimen from Indopac VIII, Stn 6.

FIGURE 20. Megacalanus ohmani n. sp. female: A, maxilliped; B, detail of bifurcate spinule from basis of maxilliped; C, leg 2; D, leg 3; E, leg 4; F, leg 5; G, dorsal view of antennule ancestral segments I – V showing hair sensilla (hs) and maculae cribrosae (ms). Scale bars represent 1.0 mm on figures A, C – G; 0.1 mm on remaining figures. Illustrated specimen from Indopac VIII, Stn, 6.

FIGURE 21. Megacalanus ohmani n. sp. male: A, dorsal view; B, lateral view of anterior head; C, lateral view of posterior corners of pedigerous somite 5; D, dorsal view of caudal ramus; E, posterior view of leg 5; F, distal inner corner of left exopod segment 2; G, distal inner corner of left exopod segment 2 of another specimen. Scale bars represent 1.0 mm on figures A, C, E; 0.1 mm on remaining figures. Illustrated specimen from Indopac VIII, Stn 6.

FIGURE 22. Megacalanus ohmani n. sp. male left antennule ancestral segments: A, segments I – VIII; B, segments IX – XV, note clavate seta on segment XI apparently knocked off but present in other specimens; C, segments XVI – XX; D, segments XXI – XXVIII. Scale bars represent 1.0 mm on figures A – D; 0.1 mm on figure E. Illustrated specimen from Indopac VIII, Stn 6.

FIGURE 23. Megacalanus ohmani n. sp. male right antennule ancestral segments: A, segment I – XII, arrow indicates ‘ clavate seta’; B, segments XIII – XVIII; C, segments XIX – XXVIII; D, detail of segments XIX – XX; E, detail of segments XXI – XXIII; F, detail of terminal part. Scale bars represent 1.0 mm figures A – C; 0.1 mm on remaining figures. Illustrated specimen from Indopac VIII, Stn 6.