Megacalanus Wolfenden, 1904

Bradford-Grieve, Janet M., Blanco-Bercial, Leocadio & Boxshall, Geoffrey A., 2017, Revision of Family Megacalanidae (Copepoda: Calanoida), Zootaxa 4229 (1), pp. 1-183 : 26-30

Genus Megacalanus Wolfenden, 1904

Macrocalanus Sars, 1905 , p. 26.

? Pseudolovenula Marukawa, 1921 , p13, pl. 1 figs 10–13, pl. 2 figs 1–4.

Differential diagnosis. As for Megacalanidae plus following character states: Anterior head without spine-like processes, rostral filaments tapering to point. Female antennule ancestral segment XXIII without aesthetasc, and segments XIV–XVII have ventral surface tooth row. Female and male ancestral segments I–V each with very small hair sensillum on dorsal surface. Right male antennule segments XIV–XV and XXII–XXIII fused, at least segments XX and XXI with fused gripping elements. Antennal exopod ancestral segments I–IV bearing relatively well-developed setae, on segments 1-III each seta longer than its segment, on segment IV seta extends beyond distal border of exopod. Mandibular gnathobase with ventral tooth set at right angles to main plane of gnathobase therefore appears tapering and similar to other teeth; endopod segment 1 with 4 setae, endopod segment 2 with 9 large and 2 short proximal setae. Maxillule praecoxal arthrite with 4 posterior surface setae, coxal endite with 5 setae, basal endites 1 and 2 with 4 setae each, endopod segment 2 with 4 setae. Maxilla modestly developed; longest setae extend as far as anterior labrum; most setae bear auxilliary setules. Maxilliped modestly developed, longest setae extending to rostrum, endopod segments 3–5 with proximal seta shortest. Leg 1 basis usually with anterior inner surface bearing large hook-like process onto which straight inner seta inserted, exopod segments 1 and 2 each bear distolateral articulated spine, segment 3 with 2 outer border spines. Specialised seta on male left leg 5 with base greatly enlarged into outer border bulge then tapering into short terminal section bordered by long setules. Circular or oval distoanterior ‘macula cribrosa’ present: between rostral points; at base of all antennular aesthetascs, associated with some dorsal surface hair sensilla on ancestral segments I–V depending on species, and distoposteriorly on ancestral segment XXVIII; on mandibular gnathobase, proximally near insertion of basis; on maxillule outer proximal surface of exopod; on maxilliped inner surface of basis and endopod segment 5; on anterior surface of basis of legs 1–5 adjacent to insertion of exopod segment 1; and leg 1 exopod segment 2.

Type species. Megacalanus princeps Wolfenden, 1904

Type locality. 51–60o, 6–13o W.

Description. Female: Anterior margin of head rounded or crested without anterior spine-like processes. Rostrum ventroposteriorly-directed, extending into two long cuticular extensions, tapering to point, 1 macula cribrosa situated at base, between rostral points. Urosome of four free somites. Genital double-somite symmetrical in dorsal view, approximately as long as wide with widest width at about anterior one third; with small anteroventral genital operculum and pair of seminal receptacles. Posterior borders of pedigerous somite 5 bluntly triangular or rounded in lateral view. Caudal rami with seta I absent, setae II and III lateral, setae IV–VI terminal (seta V longest), seta VII inserted at inner distal corner on small projection.

Antennule extending up to 8 segments beyond caudal rami, ancestral segments II–IV, X–XI fused; XXVII and XXVIII separate. Most setation elements modified setae (ms), aesthetascs (a), or few naked simple setae (ss); proximal setae on ancestral segment I small but developed. Posterior setae on ancestral segments XXIV to XXVI pseudoannulate as well as plumose, and posteriormost seta of terminal pair on segment XXVIII, pseudoannulate. Setation of segments as follows: I—1 ms, 1a, 2 ss; II to XXI—2ms, 1a; XXII—1ms, 1a; XXIII—1ms; XXIV to XXV—1+1ms, 1a; XXVI—1+1ms; XXVII—1+1ms; XXVIII—3ms, 1ss, 1a. Distoanterior maculae cribrosae located at base of all aesthetascs and also distoposteriorly on ancestral segment XXVIII. Dorsal surface of ancestral segments I–V each with hair sensillum which may or may not be accompanied by macula cribrosa, depending on species (see Figs 7 H, 20G). Ventral surface of ancestral segments XIV to XVII with longitudinal distoventral row of small teeth, distoposterior border of segments XV and XVI may or may not be lined by blunt teeth.

Antenna with separate coxa and basis; coxa with 1 long inner plumose seta and inner tuft of setules, basis with 2 inner setae each ornamented with rows of short setules. Exopod slightly longer than endopod; endopod 2- segmented although line of fusion between segments 2 and 3 visible on posterior surface; segment 1 with 2 inner, almost naked setae and short longitudinal row of outer setules, terminal segment with 9+7 setae and patch of long setules; exopod in anterior view with segments I and II separate, segments II–IV fused, although ancestral segments I–IV appear to be fused in posterior view; terminal segment IX–X with 1 + 3 terminal setae, segments V–VIII each with long plumose seta, ancestral segments II–IV with 3 short but well developed setae bearing short setules and segment I with 1 short but well developed seta.

Mandible coxal gnathobase with five complex teeth oriented at right angles to main plane of gnathobase, with opaline tips, largest tooth ventrally situated, tapering when viewed at right angles to broad plane of gnathobase, separated from adjacent tooth by wide gap; 4 following teeth progressively decreasing in size; then, 3 simpler, asymmetrically bicuspid teeth without opaline tips follow, dorsal-most tooth longest; and finally, 1 long, lash-like element situated dorsally, bordered by wide setules; macula cribrosa situated proximally on gnathobase near insertion of basis; rows of setules lying on both distal anterior and posterior surfaces at bases of dorsal-most 6 teeth. Basis with 4 inner setae; endopod 2-segmented, segment 1 with large inner lobe and 4 distal inner setae distal of which well-developed, segment 2 with 9 terminal setae plus 2 short setae both on the same surface and transverse row of setules; exopod 5-segmented with 1, 1, 1, 1, 2 setae.

Maxillule praecoxal arthrite with full complement of 15 setae (4 of them on posterior surface); coxal endite with 5 terminal setae, epipodite with 7 long and 2 short setae; basal endites 1 and 2 with 4 and 4 setae, respectively; exite with 1 seta; basis and endopod separate; endopod with ancestral segments 1 and 2 fused — fusion line visible on anterior surface, with 3, 4, 6 + 1 small posterior surface seta, segment 3 with row of anterior surface setules; exopod with 11 setae, 1 macula cribrosa on outer proximal surface.

Maxilla modestly developed, longest setae extend as far as anterior labrum, praecoxa with macula cribrosa on outer surface. Praecoxal endites 1 and 2 with 6+1 small and 3 setae, respectively; coxal endites 1 and 2 with 3 setae each, coxal epipodite with 1 long plumose seta; basal endite longest with 4 setae, one of them more heavily developed than others; endopod segment 1 with short lobe bearing 1 large plus 3 short, but well developed setae, endopod segments 2–4 with 3, 2, 2 setae, respectively, setae bordered by two rows of moderately spaced long spinules and distal part of concave border differs in appearance from convex border, possibly ridged. Inner surfaces of praecoxal endite 2, coxal endites 1 and 2 and basal endite each with 1 seta that is shorter and more densely lined with long spinules; endopod segments 2 and 3 with inner setae shorter than outer setae, segment 2 inner proximal seta well-developed, inner distal seta convex border bearing spinules. Terminal setae gently curving along whole length. All setae (apart from heavily built seta on basal endite), as well as having very small closely spaced spinules, bordered by long, widely spaced auxiliary spinules; distal seta on coxal endite 2 similar although closely spaced spinules much longer than on remaining setae.

Maxilliped directed ventrally so that setae on syncoxa and basis directed into animal’s midline; syncoxa with 1, 2, 4, 4 setae, longest seta of endite 4 extending short of distalmost border of basis and lined by very short small spinules; basis with 3 setae, patch of bifurcate setules on anteroproximal surface; endopod segment 1 separate bearing 2 setae; endopod segments 2–6 with 4, 4, 3, 3+1 (outer seta directed proximally), 4 setae; endopod segments 3–5 with proximal seta shortest. One macula cribrosa on inner surface of basis and on endopod segment 5.

Legs 1–5 biramous, each ramus 3-segmented with following setal formula (Roman numerals indicate spines, Arabic numerals setae, outer border setation listed to left in each group separated by ‘;’):

Leg 1 (Coxa 0-1. Basis 1-1. Exopod I-1; I-1; II,1,4. Endopod 0-1; 0-2; 1,2,3);

Leg 2 (Coxa 0-1. Basis I-0. Exopod I-1; I-1; III,1,5. Endopod 0-1; 0-2; 2,2,4);

Leg 3 (Coxa 0-1. Basis I-0. Exopod I-1; I-1; III,1,5. Endopod 0-1; 0-2; 2,2,4);

Leg 4 (Coxa 0-1. Basis 1-0. Exopod I-1; I-1; III,1,5. Endopod 0-1; 0-2; 2,2,3);

Leg 5♀ (Coxa 0-0. Basis 1-0. Exopod I-0; I-1; II,1,4. Endopod 0-1; 0-1; 2,2,2).

Leg 1 coxa and basis with long setules on anterior surface, basis usually with large anterodistal hook-like process from which straight short seta arises, macula cribrosa situated on anterior surface adjacent to articulation socket of exopod segment 1. Distolateral corner of endopod segment 1 rounded. Exopod segment 2 with macula cribrosa on lateral surface at base of outer distal spine and macula cribrosa also on anterior surface of endopod segment 2. Legs 2–4 with maculae cribrosae on: basis adjacent to region of articulation of exopod segment 1, at base of outer spines on exopod segments 1 and 2, at base of 2 proximal outer edge spines on exopod segment 3, and on endopod segment 2; pore openings (presumably of bioluminescence glands, see Herring 1988) located on anterior surfaces at base of outer edge spines of exopod segments 1–3; endopod segment 1 outer distal corner extends into sharp point. Leg 5 extending just beyond third free urosomite, with outer distal corner of endopod segment 1 rounded; pore openings on anterior surface at base of outer border spines; maculae cribrosae on anterior surface of basis near articulation with exopod segment 1, on endopod segment 2, and just proximal to pore on exopod segments 1–3.

Male: Anterior margin of head rounded or crested, without anterior spine-like processes. Rostrum extending into two long, ventroposteriorly- or ventrally-directed, tapering points that appear to be direct extensions of cuticle. Urosome of five free somites, genital somite short. Posterior borders of pedigerous somite 5 bluntly triangular or rounded in lateral view. Caudal rami seta I absent, setae II and III lateral, setae IV–VI terminal (seta V longest), seta VII inserted at inner distal corner onto small projection; tufts of setules decorate inner border, row of setules located at base of seta VII, and tufts on lateral border between setae II and III.

Antennules asymmetrically developed, geniculate on right. Aesthetascs larger than in female with stiffened posterior border. Left antennule with ancestral segments II–IV, IX–XI fused, segments XXVII and XXVIII separate, with aethetascs doubled on segments III, V, VII, IX, XI–XIV. Most setal elements modified setae (ms) or aesthetascs (a); plus few naked simple setae (ss). Setation of segments as follows: I—1 ms, 1a, 2ss; II– IV—6 ms, 4a; V—2 ms, 2a; VI—2 ms, 1a; VII—2 ms, 2a; VIII—2 ms, 1a; IX– XI—5 ms, 5a, 1 clavate (club-shaped) seta (on segment XI, damaged in specimen figured); XII—2 ms, 2a; XIII—2ms, 2a; XIV—2ms; 2a; XV to XXI—2ms, 1a; XXII to XXIII—1ms, 1a; XXIV—1+1ms, 1a; XXV—1+1ms, 1a; XXVI—1+1ms; XXVII—1+1ms; XXVIII—3ms, 1a, 1ss. Segments XIV and XV with row of distoventral teeth. Maculae cribrosae at base of every aesthetasc including on segment XXVIII. Right antennule geniculate between segments XX and XXI, with ancestral segments II–IV, IX–XI, XIV–XV, XXI–XXII fused on ventral surface with arthrodial membrane present on dorsal surface; XXII–XXIII completely fused, XXVII and XXVIII separate. Setation of segments as follows: I—1 ms, 1a, 2ss; II– IV—6 ms, 4a; V—2 ms, 2a; VI—2 ms, 1a; VII—2 ms, 2a; VIII—2 ms, 1a; IX– XI—5 ms, 5a, 1 clavate seta (on segment XI based on M. ohmani and M. princeps ); XII to XIII—2ms; 2a; XIV–XV—4ms, 3a; XVI to XVIII—2ms, 1a; XIX—2ms, 1a or 1ms, 1 fused gripping element, 1a; XX—1ms, 1 fused gripping element, 1a; XXI—1ms, 1 fused gripping element, 1a or 2 gripping elements, 1a; XXII–XXIII—1ms, 1ss; XXIV to XXV—1+1ms, 1a; XXVI—1+1ms; XXVII—1+1ms; XXVIII—3ms, 1a, 1ss. Marker seta interpreted as indicating fusion of segments XXII–XXIII minute, proximally-inserted at first 1/6th of this double segment. Maculae cribrosae located at base of all aesthetascs, and distally on segments XXIV and XXVI. Fused segments XIV and XV on right antennule each with longitudinal row of teeth on ventral surface.

Antenna, mandible, maxillule, maxilla, maxilliped and legs 1–4 as in female. Leg 5 with following setal formula (Roman numerals indicate spines, Arabic numerals setae, outer border setation listed to left in each group separated by ‘;’):

Leg 5♂ left (Coxa 0-0. Basis 1-0. Exopod I-0; I-1; II,1,0. Endopod 0-0; 0-1; 2,2,2);

Leg 5♂ right (Coxa 0-0. Basis 1-0. Exopod I-0; I-0; II,1,0. Endopod 0-0; 0-1; 2,2,2).

Leg 5 almost symmetrical apart from specialised seta on inner distal border of left exopod segment 2 and asymmetrically inserted single spine on inner border of exopod segment 3 (homologous with terminal spine of female). Male left leg 5 exopod segment 2, specialised seta composed of two sections: wide basal part and lash arising from proximodistal border of basal part. Basal part of variable proportions and bearing pit on anterodistal corner. Lash bearing longitudinal rows of long setules extending onto basal part and tuft of setules arising from pit. Outer distal corner of endopod segment 1 rounded. Pore openings on anterior surface at base of outer spines. On posterior surface, 1 macula cribrosa on endopod segment 3 and just proximal to each outer spine on exopod segments 1 to 3. Basis inner distal border naked or with patch of elongate setules.

Type species. Megacalanus princeps Wolfenden, 1904

Remarks. Type species. The identity and nomenclature of Atlantic Ocean Megacalanus had not been stabilised when this work began. A great deal of confusion was caused by the incomplete description of Calanus princeps by Brady (1883) , making its true identity the subject of ongoing speculation. Also the generic diversity of the Megacalanidae was not recognised until A. Scott’s (1909) description of Bradycalanus , a genus which is morphologically closer to Megacalanus than to Bathycalanus .

Damkaer (2000) concluded that the correct name for Atlantic Megacalanus is M. longicornis ( Sars, 1905, as Macrocalanus ) although the first recognisable description of this species was actually Megacalanus princeps Wolfenden, 1904 ; the presence of the anterodistal hook-like process on the basis of leg 1 clearly indicates Wolfenden’s species is a Megacalanus . The most important question requiring an answer to untangle this confusion is: should Megacalanus princeps Wolfenden, 1904 be considered to be a homonym of Calanus princeps Brady, 1883 ? Our answer is ‘no’ based on the following reasoning.

Wolfenden (1904) reported on page 113 that: “the only described copepod at all resembling it [ Megacalanus princeps Wolfenden, 1904 ] is C. princeps of Brady … in which the feet are very similar, but there are not such setae on the anterior foot jaws as Brady figures, the maxilla is totally different as regards its bristles, and the segmentation of the anterior antennae and abdomen is also different. It is therefore certainly not Brady’s species.”

In using the same species name as Brady (1883) , Wolfenden implied his Megacalanus princeps was not in the same genus as Brady’s species. Nevertheless, he did not resolve the issue of generic affinities of C. princeps Brady, 1883 except to say that, in his opinion, Brady’s species was not a Calanus because of the extra outer border spine on exopod segment 3 of legs 2–4. Thus, it was Sars’ (1905) opinion, a year later, which erroneously designated Megacalanus princeps Wolfenden, 1904 as a junior homonym of Megacalanus princeps ( Brady, 1883) .

By 1906, Wolfenden (P. 4) had examined Brady’s type specimen of Calanus princeps and again recognised it was not a Megacalanus and concluded it resembled Heterocalanus Wolfenden, 1906 although the totality of his comments are contradictory. We agree with Wolfenden’s (1906) concept of a separate genus to take C. princeps Brady, 1883 and develop this idea further in a later section along with a discussion of the need for a replacement name.

One of us (GAB) recently examined two of Brady’s type specimens of Calanus princeps Brady, 1883 (BMNH 1884.4. c.c.2/5), each dissected on two slides, and found that most limbs were in good condition. The maxillule setal formula is: praecoxal arthrite, coxal endite and basal endite 1 with 11, 0, 2 setae, respectively; basal endite 2 with 2 setae, and endopod segments with 1, 1, 4+1 setae. The first endopod segment carried 1 seta on one side of the body but 2 setae on the other side. Leg 1 exopod segments 1 and 2 both lack an outer border spine and exopod segment 3 has 2 outer border spines.

We propose that there is no value in attempting to untangle all the opinions expressed concerning synonymies between the dates of 1905–1925. We recommend the use of Wolfenden’s (1904) name Megacalanus princeps since his is the first recognisable description of this species and it is clearly not in the same genus as C. princeps Brady, 1883 .

Species. Four species of Megacalanus are now included in this genus: the type species and three new species described herein: Megacalanus frosti n. sp. is the crested form recorded by Miller (2002) and M. ohmani n. sp. and M. ericae n. sp. These species may be distinguished by a combination of female and male characteristics relating to: the general shape of the posterior borders of pedigerous somite 5 in the female, the shape of the anterior head and rostrum in lateral view, the presence or absence of small blunt teeth on the distoposterior border of antennular segments XV and XVI in the female, the shape of the specialised seta on the male left leg 5 inner distal corner of exopod segment 2, and the armature of the right male antennular segments XIX to XXI.

Vervoort (1949) considered a copepodite stage IV, described as Pseudolovenula magna by Marukawa (1921) and placed in the Centropagidae , to be a Megacalanus . Marukawa’s (1921) figure (plate 1 fig. 13) of leg 1 does not have a large hook-like process on its basis which is present in at least copepodite stage II (see Fig. 11 J). Therefore, it is not possible to be certain it is a Megacalanus and the species name magna is available for use.

The mouthparts and legs of Megacalanus species are generally indistinguishable. Thus, these limbs are fully illustrated for M. ohmani alone. Distinguishing features only, are illustrated for the other three species.

Distribution. The type species was found in the Atlantic and Indian Oceans, the southwest Pacific close to Chile, and strays have been found in the Celebes and Banda Seas and the tropical Central Pacific (see Fig. 9 , Table 1 ). Megacalanus frosti n. sp. is the crested form recorded by Miller (2002) from the eastern Pacific, another new species is found east of Irian Jaya ( M. ohmani n. sp.), and M. ericae n. sp. is widespread in the Pacific and Indian Oceans.

Figures

FIGURE 7. Megacalanus princeps Wolfenden, 1904 Female: A, dorsal view; B, lateral view; C, lateral view of anterior head; D, inner seta of maxilla praecoxal endite 2; E, seta of maxilla praecoxal endite 1 (and most of other setae on maxilla); F, antennule ancestral segments XIV – XV; G, antennule ancesstral segments XVI – XVII; H, antennule ancestral segments I – IV, dorsal view; I, hair sensilla (hs) and macula cribrosa (mc) on ancestral segment III; J, leg 1. Scale bars represent: 1.0 mm on figures A, B, H, J; 0.1 mm on remaining figures. Illustrated specimen from Oceanus Cr 473, 100 – 866 m (A – C).

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 11. Megacalanus princeps Wolfenden, 1904 copepodite II: A, dorsal view; B, lateral view; C, antennule; D, rostral filaments; E, antenna endopod segment 2; F, antenna exopod; G, mandible endopod; H, maxillule; I, maxilliped; J, leg 1 — arrow indicates hook – like process; K, leg 2; L, leg 3. Scale bars represent: 1.0 mm on figures A – C; 0.1 mm on remaining figures. Illustrated specimens from ANTXXIV / 1, Stn 2, 1000 – 2000 m.