Megacalanidae Sewell 1947

Family MEGACALANIDAE Sewell, 1947

Differential diagnosis. Large copepods> 8 mm long. Female urosome of 4 free somites, of male 5 free somites. ‘Cephalic dorsal hump’ present in male. Female antennule with segments I and II separate, at least segments II–III, III–IV, X–XI fused. Hair sensilla on dorsal surface of all or some of ancestral segments I–V. Right antennule of male geniculate between ancestral segments XX and XXI, segments I and II usually separate, at least segments II–III, III–IV, IX–XI and XXII–XXIII fused, segments around geniculation (XIX–XXI) with gripping elements. Clavate (club-shaped) seta present on ancestral segment XI on both male antennules. Aesthetascs larger than in female, and longest on proximal segments (longer than the width of segments). Antenna exopod 10-segmented with ancestral segments I and II separate, segments II–IV and IX–X fused. Male mouthpart setation not reduced (except in Elenacalanus nom. nov.). Mandible endopod segment 1 with large inner lobe. Maxillule basal exite usually with 1 seta (except in some Elenacalanus nom. nov.). Maxilla coxal epipodite with 1 seta. Maxilliped ancestral endopod segment 5 without enlarged outer setae in male. Legs 1–5 with 3-segmented rami in both sexes. Exopod segment 3 of legs 2–4 with 3 outer edge spines and terminal spine with serrated outer border. Legs 1–5 basis with outer distal seta or spine. Leg 5 of male and female similar to other legs; inner distal border of male left exopod segment 2 with specialised seta.

Description. Large calanoid copepods up to 17 mm total length, pedigerous somites 1 to 5 all separate. Pair of rostral filaments of varying form; anterior head in dorsal view of varying shapes. ‘Cephalic dorsal hump’ present in male. Urosome of female comprising 4 free somites, genital double-somite with small anteroventral genital operculum covering genital atrium into which opens pair of seminal receptacles and gonopores (Barthélémy 1999); male with 5 free urosomites. 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. Mouthparts usually identical in both sexes, setation not reduced except in Elenacalanus nom. nov.

Antennule of female usually extending well beyond caudal rami. Female with ancestral segments II–IV, X–XI fused, segments XXVII and XXVIII separate. Most setation elements modified setae (ms) or aesthetascs (a), very few of naked simple type (ss). Setation of segments as follows: I—1 ms, 1a, 2ss (varying in size), II to XXI—2ms, 1a; XXII—1 ms, 1a; XIII—1ms, 1/0a; XXIV to XXV—2ms 1a; XXVI—2 ms; XXVII—2 ms; XXVIII—3ms, 1a, 1ss. Male antennule geniculate on right. Left antennule with ancestral segments II–IV, IX–XI fused, XXVII and XXVIII separate, aesthetascs doubled on at least segments III, V, VII, IX, XI–XIV. Right antennule with ancestral segments II–IV fused, IX–XI, XIV–XV fused or separate, XXI/XXII–XXIII fused, XXVII and XXVIII separate, aesthetascs doubled on at least segments III, V, VII, IX, XI–XIII/XIV.

Antenna with separate coxa and basis, coxa with 1 inner plumose seta and basis with 2 inner setae, each ornamented with short setules. Exopod and endopod approximately equal in length. Endopod 2-segmented although line of fusion between segments 2 and 3 visible on posterior surface; segment 1 with 2 inner setae and short longitudinal row of outer setules, terminal segment with 9+7 setae. Exopod ancestral segment I separate from segment II, segments II–IV and IX–X fused; compound terminal segment IX–X with 1 + 3 terminal setae, segments V–VIII each with long plumose seta, ancestral segments I–IV each with seta of variable development.

Mandible coxal gnathobase with five complex teeth with opaline tips. Largest tooth ventrally situated, separated from adjacent tooth by wide gap, form and orientation of which varies with genus; 4 following teeth progressively decreasing in size, followed by 3 simpler teeth, dorsal-most tooth longest; and finally, 1 long, lashlike element situated dorsally, bordered with wide setules. Basis with 4 inner setae; endopod 2-segmented, segment 1 with large inner lobe and 4 distal inner setae (1 or 2 setae may be very small or absent), segment 2 with up to 11 terminal setae, 2 of them short on one surface or absent; exopod 5-segmented with 1, 1, 1, 1, 2 setae.

Maxillule praecoxal arthrite with up to 15 setae (up to 2 of the 4 ancestral posterior surface setae may be absent in Bathycalanus and Elenacalanus nom. nov.); coxal endite with 0, 1, 2, or 5 terminal setae, coxal epipodite with 9 setae (2 proximal setae shorter, vestigial or absent); basal endites 1 and 2 with 2–4 and 2–4 setae respectively, exite usually represented by 1 seta except for some Elenacalanus nom. nov.; basis and endopod separate or fused; endopod with segments 1 and 2 fused—fusion line visible on anterior surface, segment 3 separate, setation of endopod segment varying with genus and species, with 1–3; 0, 1, 2, 4; 5–6 + 1 small anterior surface seta, respectively; exopod usually with 11 setae.

Maxilla praecoxal endites 1 and 2 with 6+1 small, spine-like seta and 3 setae, respectively; coxal endites 1 and 2 each with 3 setae, coxal epipodite with 1 seta (vestigial in Bradycalanus); basal endite longest, with 4 setae; endopod segment 1 with short lobe bearing 3 small + 1 large setae, endopod segments 2–4 apparently with 2 (1 often vestigial), 2, 3 setae. Inner surfaces of praecoxal to basal endites each with 1 seta shorter and more densely lined with long spinules; longer setae of at least basal endite and endopod of form varying with genus.

Maxilliped directed ventrally so that setae on syncoxa and basis directed towards midline; syncoxa with 1, 2, 4, 4 setae; basis with 3 setae, patch of setules on anteroproximal surface. Endopod segment 1 usually separate, bearing 2 setae; endopod segments 2–6 plesiomorphically with 4, 4, 3, 3+1 (outer seta directed proximally), 4 setae (some setae reduced in size or absent in different genera), respectively.

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

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

Leg 1 with inner seta on coxa, outer distal seta of basis vestigial, basis anterodistal surface usually naked but with hook-like process in Megacalanus. Distolateral corner of endopod segment 1 rounded, spine usually present on outer distal corners of exopod segments 1 and 2 except in Bathycalanus and Elenacalanus nom. nov. Legs 2–4 with seta on inner margin of coxa; basis with outer distal spine; pore openings located on anterior surfaces at base of outer edge spine of exopod segments 1–3. Leg 5 of female with outer distal corner of endopod segment 1 rounded; pore openings on anterior surface at base of outer border spines. Leg 5 of male slightly asymmetrical because of specialised seta on inner distal corner of left side exopod segment 2 (on exopod segment 2 on both sides in Bathycalanus unicornis), setulose inner border of left exopod segment 3, and asymmetrically placed spine on inner borders of exopod segment 3 on both sides, interpreted here as being the terminal spine of other legs.

Type genus. Megacalanus Wolfenden, 1904

Remarks. Sewell (1947) implies the type genus for the Megacalanidae is Megacalanus Wolfenden, 1904 because the family name is formed from this genus (ICZN 1999, Article 63).