Calanoida ( Order )
    Calanoidea ( Superfamily )
Megacalanidae Sewell, 1947 ( Calanoidea )
Ref.: Sewell, 1947 (p.20, G. & spp. Key ); Farran & Vervoort, 1951 a (n°33, p.3); Johnson, 1958 a (p.257); Vervoort, 1963 b (p.85, Rev.); Andronov, 1974 a (p.1005); Razouls, 1982 (p.84); Bowman & Abele, 1982 (p.10); Brodsky & al., 1983 (p.142, 146, 190); Mauchline, 1988 (p.718: cuticular pores); Nishida, 1989 (p.175); Bradford-Grieve, 1994 (p.17, Def.); Michel, 1994 (p.175, Genera and spp Key); Chihara & Murano, 1997 (p.834); Barthélémy, 1999 a (p.30); Bradford-Grieve & al., 1999 (p.877, 902, 904, 905, Genera Key); Ohtsuka & Huys, 2001 (p.445, 461); Boxshall & Halsey, 2004 (p.13: 49; 139: Def.; p.140: Genera Key); Vives & Shmeleva, 2007 (p.909, Genera Key); Bradford-Grieve & Ahyong, 2010 (p.287: Rem., phylogeny); Blanco-Bercial & al., 2011 (p.103, Table 1, Fig.2, 3, 4, molecular biology, phylogeny); Bradford-Grieve & al., 2017 (p.1-183: familie's revision; Diagnosis: p.25-26, molecular biology, phylogeny); Laakmann & al., 2019 (p.330, fig. 2, 3, phylogenetic relationships)
Bradford-Grieve J.M., (2002 onwards). Key to calanoid copepod families. Version 1 : 2 oct 2002. http://www.crustacea.net/crustace/calanoida/index.htm
Rem.: Type genus: Megacalanus Wolfenden, 1904. 4 G.: Bathycalanus, Bradycalanus, Megacalanus, Elenacalanus.

After Vervoort (1946, p.47-49) much confusion exists with regard to the genera Megacalanus (Wolfenden, 1904, p.112); Macrocalanus (Sars, 1905, p.7) and Bradycalanus (A. Scott, 1909, p.14). Farran (1939) gives a critical review of the question concerning the nomenclature of these genera.
After Boxshall & Halsey (2004, p.140) the Megacalanidae was treated as the most plesiomorphic family within the superfamily Calanoidea (as Megacalanoidea) by Fleminger (1983).
For Bradford-Grieve & Ahyong (2010, p.287) it is accepted in the superfamily Calanoidea that the Megacalanidae (represented by Megacalanus princeps Wolfenden, 1904) is the sister group to the Calanidae.
Megacalanidae have P5 similar to the other swimming legs with each branch 3-segmented and myelinated nerve axons. Nevertheless, there are major morphological and apparent behavorial differences between the two families:
Exopod segment 3 has 3 outer edge spines on P2-P4 in M. princeps compared with 2 spines in the Calanidae.
In males of the M. prnceps the A1 is geniculate on the right and the outer setae on free endopod segments 4 and 5 on Mxp are unmodified. In the Calanidae male, A1 are identicalon the right and left and the male Mxp has the outer setae on free endopod segments 4 and 5 directed proximally and extremely enlarged. This difference may be related to differencesin mating behaviour. Tsuda and Miller (1998) observed mating behaviour in Calanus and inferred that the Mxp is used to grasp the female prosome while the male position itself dorsally above the female; it is suggested that the enlarged setae assist in this process and perhaps replace the function of the geniculate A1 in some Megacalanidae in grasping the female (see Ohtsuka & Huys, 2001).
Key to genera (based on female characters except where stated) after Boxshall & Halsey (2004, p.140) :
1 - Rostrum with cylindrical round-ended filaments; distal setae of Mx2 elongate, densely spinulose, with curved tips; P1 lacking outer spine on 1st and 2nd exopodal segments; male right A1 geniculate ............. Bathycalanus.
1' - Rostrum with tapering, pointed filaments; distal setae of Mx2 normal, sparsely spinulose, not modified; P1 with outer spine on 1st and 2nd exopodal segments; male right A1 non-geniculate ........... 2.
2 - Large hook-like process present on anterior surface of basis of P1; A1 exceeding body length by distal 7 to 9 segments ........... Megacalanus.
2' - No hook-like process on basis of P1; A1 exceeding body length by distal 4 to 6 segments ......... Bradycalanus.

Differencial diagnosis from Bradford-Grieve, Blanco-Bercial & Boxshall (2017, p.25):
- Large copepods > 8 mm.
- Female urosome of 4 free somites; of male 5 somites;
- Cephalic dorsal hump present in male.
- Female A1 with segments I and II separate, at least segments II and III, III-IV, X-XI fused. hair sensilla on dorsal surface of all or some ancestral segments I-V. Right A1 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, segmets around geniculation (XIX-XXI) with gripping elements. Clavate (club-shaped) seta present on ancestral segment XI on both male A1. Aesthetascs larger than in female, and longest on proximal segments (longer than the width of segments).
- A2 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).
- Md endopod segment 1 with large inner lobe.
- Mx1 basal exite usually with 1 seta (except in some Elenacalanus.
- Mx2 coxal epipodite with 1 seta.
- Mxp ancestral endopod segment 5 without enlarged outer setae in male;
- P1-P5 with 3-segmented rami in both sexes;
- Exopod segment 3 of P2-P4 with 3 outer edge spines and terminal spine with serrated outer border.
- P1-P5 basis with outer distal seta or spine.
- P5 of male and female similar to other legs; inner distal border of male left exopod segment 2 with specialised seta.

Key to Megacalanidae genera, females and males (from Bradford-Grieve & al., 2017, p.198) :
1 - Exopod of P1 in both sexes with 1, 1, 2 outer marginal spines; Mx2 terminal setae gently curving, extending short of mouth ..... 2.
1' - Exopod of P1 in both sexes with different arrangement of outer marginal spines; Mx2 terminal setae, curled, extending to or beyond rostrum .......... 3.
2 - P1 basis usually with inner anterodistal hook-like processes; Mx1 endopod segment 2 with 4 setae; Md gnathobase with ventral tooth set at right angles to main plane of gnathobase thus appearing narrow; male right A1 ancestral segments XXII-XXIII fused ..........Megacalanus.
2' - P1 basis without inner anterodistal hook-like process; Mx1 endopod segment 2 with at most 2 setae; Md gnathobase with ventral tooth set obliquely to maun plane of gnathobase thus appearing btoad; male right A1 ancestral segments XXI-XXIII fused ....... Bradycalanus;
3 - Exopod of P1 in both sexes with 0, 0, 2 outer marginal spines; anterior margin of head of both sexes without pair of anteriorly-directed spine-like processes but female may be crested; Mx2 terminal setae curled distally into semicircle ....... Elenacalanus.
3' - Exopod of P1 in both sexes with 0, 0, 1 outer marginal spines; anterior head of both sexes with pair of anteriorly-directed spine-like processes or single spine-like process; Mx2 terminal setae curled completely on themselves distally .....Bathycalanus.
Family Megacalanidae - Plate 1Issued from : G.A. Boxshall & S.H. Halsey in An Introduction to Copepod Diversity. The Ray Society, 2004, No 166, Part. I. [p.139].
Armature formula of swimming legs P1 to P4.
Female P5, biramous with 2-segmented protopod and 3-segmented rami.
Male P5 slightly asymmetrical; both biramous with 2-segmented protopod and 3-segmented rami; inner seta of 2nd exopodal segment (* in formula) modified, present on lelft leg only.
Eggs released into water.

Family Megacalanidae - Plate 2Issued from : G.A. Boxshall & S.H. Halsey in An Introduction to Copepod Diversity. The Ray Society, 2004, No 166, Part. I. [p.141, Fig.29]. Megacalanidae.
A, Bradycalanus sarsi (as Megacalanus princeps) habitus female; B, Bradycalanus richardi habitus male; C, rostral filaments; D, Megacalanus princeps (as Megacalanus longicornis, rostral filaments; E, P1; F, P5.
[After Sars, 1924].

Family Megacalanidae - Plate 3Issued from : J.M. Bradford-Grieve, L. Blanco-Bercial & G.A. Boxshall in Zootaxa, 2017, 4229 (1). [p.26].
Setal formula of swimming legs P1 to P5 female and male.

Family Megacalanidae - Plate 4Issued from : J.M. Bradford-Grieve, L. Blanco-Bercial & G.A. Boxshall in Zootaxa, 2017, 4229 (1). [p.160, Fig. 112].
Strict consensus (A), 50% majority-rule (B) trees, of 84 trees, length 102, consistency index (CI) = 0.75, retention index (RI) = 0.90. After one round of successive rewighting (50% majority rule of 12 trees), the topology of tree 1 is same as unweighted 50% majority rule tree (B). The trees are rooted to the outgroup calanus helgolandicus.
After Bradford-Grieve & al. (2014), the phylogenetic hypothesis concerning calanoid coppepod families has the Calanidae as sister to the Megacalanidae.. The exemplar chosen is Calanus helgolandicus because this is one of the taxa used in the outgroup for the genetic analysis to realyse comparison with the cladistic analysis.
Clade number above the line and bootstrap support see author's text.

Among characters with a high consistency index (RC) are several that are recognised by the authors as defining the genera. For example, the terminal setae of Mx2: curled completely on themselves) defines Bathycalanus; outer spines of P1 exopod segments 1-3: present and P1 basis anterodistal hook-like process: present define Megacalanus; proximal inner seta of endopod segment 2 of Mx2: vestigial with oval base defines Bradycalanus; outer distal spines of P1 exopod segments 1 and 2: absent denine Bathycalanus; and a number of characters define the remamed genus Elenacalanus.
In tree B, 1: Megacalanus (Clade 1) is sister to the remaining members of the family. It is united by two unambiguous, unique characters: A1 ancestral segments XIV-XVII with ventral surface with row of teeth and P1 basis with hook-like process. Clade 2 is united by four unique, unambiguous character changes that are uniform in the tree: On Mx1, basal endite 1 with 2 setae and endopod segment 3 lacks seta 7. On Mx2, the endopod setae do not have sparse auxiliary spinules and endopod segment 1 smallest setae are vestigial. Several other characters change unambiguously on a branch, change above the node in the tree, but are uniform outside the branch: an aesthetasc is present on female A1 ancestral segment XXIII. A2 ancestral segment IV seta is short. On Md endopod segment 1 and 2 the surface setae are vestigial. On Mx1, endopod segment 2 has 2 setae. On Mxp endopod segment 3, setae 2 and 3 are reduced in size and on endopod segment 5 iinner seta 2 is reduced in size.
Clade 2 divides into two sister clades - 3 and 4.:
Clade 3 (Bradycalanus) is united by two unique unambiguous characters: Mx2 coxal epipodite seta vestigial and on endopod segment 2 the distal inner seta convex border is naked.
Clade 4 is united by three unambiguous, unique changes uniform: there is a macula cribrosa adjacent to the hair sensillum on ancestral segment V of the female A1; the setae of praecoxal endites 1 and 2 and coxal endite 1 of Mx2 are without sparse auxiliary spinules and there is no outer distal spine on exopod segments 1 and 2 of P1. Two characters that change unambiguously on its branch but are homoplasious in the tree are: on Mx1, absence of the 2 distalmost setae on posteriorsurface and presence of 2 setae on basal endite 2.
Within Clade 4 there are two sister clades (5 and 6) with variable bootstrap support:
Clade 5 (Bathycalanus) is united by two unambiguous, unique changes uniform: the hair sensillum on ancestral segment IV is absent and P1 exopod segment 3 is without a proximal outer spine. One character changes but not outside: anterior head with 2 spines and one character is homoplasious* outside: anterior head is decorated in dorsal view.
Clade 6 (the renamed genus Elenacalanus nom. nov.) is united by eleven unambiguous unique changes uniform: on Md endopod segment 1, the distal and proximal setae are absent; endopod segment 2 both surface setae are absent; on Mx1 endopod segment 3, seta 6 is absent; on Mxp, setae 2, 3 and 4 of endopod segment 3 are absent; seta 3 of endopod segment 4 is absent and inner seta 2 of endopod segment 5 and the outer border seta are both absent. Three other characters are homoplasious* outside and one character that is homoplasious also unite this clade.
Generally the present morphological data set is insufficiebnt to resolve most relationships at the species level. Of course, the necessity of the molecular analysis (See pp. 22-25).
*: Structural resemblance due to parallelism or convergent evolution rather than to common ancestry
(1) Bathycalanus Sars, 1905
Syn.: Heterocalanus Wolfenden, 1906 (p.26); 1911 (p.201)
Ref.: Sars, 1905 b (p.8); 1925 (p.16); Wolfenden, 1911 (p.198); Sars, 1920 c (p.1); 1925 (p.16); Sewell, 1929 (p.30); Rose, 1929 (p.11, Rem.); 1933 a (p.65); Farran, 1939 (p.359); Vervoort, 1946 (p.63); Sewell, 1947 (p.32); Brodsky, 1950 (1967) (p.97); Farran & Vervoort, 1951 a (n°33, p.3); Tanaka, 1956 (p.264); Vervoort, 1963 b (p.85); Razouls, 1982 (p.86); Brodsky & al., 1983 (p.193, spp. Key); Mauchline, 1988 (p.718); Razouls, 1993 (p.308); Bradford-Grieve, 1994 (p.18, Def.); Michel, 1994 (p.188, spp. Key); Chihara & Murano, 1997 (p.834); Mauchline, 1998 (p.71); Boxshall & Halsey, 2004 (p.140); Bradford-Grieve & al., 2017 (p.74, Redefinition., phylogeny, species key F & M: p.169)
Rem.: type: Bathycalanus richardi Sars,1905. 10 spp. + 1 uncertained.

Diagnosis after H.B. Michel (1994, p.188) :
1 - Exopod of P1 with 0, 0, 2 outer marginal setae on the three, occasionally two, segments: 2nd inner lobe of Mx1 lacks setae; setae on terminal part of Mx2 ribbon-like and densely covered with spinules.

Diagnosis from Bradford-Grieve (1994, p.18) :
- Rostrum composed of two stout sausage-shaped, not tapering processes, located on a conspicuously thickened base of the anterior head.
- Mx1 inner lobes 1, 2 and 3 with 11, 0, and 2 setae respectively; basipod with 2-3 setae; endopod segments 1, 2, and 3 with 1-2, 1 and 4-6 setae respectively; exopod with 11 setae; outer lobes 2 and 1 with 1 and 9 setae respectively.
- Mx2 with setae very long, curved at the tips, each provided with a dense row of hairs on the inner margin forming a ribbon-like lemella.
Mxp with the 4 terminal segments with reduced setation compared to the other two genera.
- Male P5 very similar to that of Megacalanus
Remarks on dimensions and sex ratio:
The mean female size is 10.792 mm. (n = 17; SD = 1.584), and the mean male size is 9.344 mm. (n = 9, SD = 1.843). The size ratio (male: female) is approximately 0.887.
(2) Bradycalanus A. Scott, 1909
Ref.: A. Scott, 1909 (p.14); Farran, 1939 (p.359); Vervoort, 1946 (p.47, 48, 49); Sewell, 1947 (p.27); Farran & Vervoort, 1951 a (n°33, p.3); Tanaka, 1956 (p.264); Vervoort, 1963 b (p.85); Owre & Foyo, 1964 (p.343, Rem.); 1967 (p.34); Razouls, 1982 (p.88); Brodsky & al., 1983 (p.198); Mauchline, 1988 (p.718); Razouls, 1993 (p.308); Bradford-Grieve, 1994 (p.20, Def.); Michel, 1994 (p.188, spp. Key); Mauchline, 1998 (p.71); Bradford-Grieve & al., 1999 (p.905, 906: spp. Key); Boxshall & Halsey, 2004 (p.140); Vives & Shmeleva, 2007 (p.911); Bradford-Grieve & al., 2017 (p.51, Diagnosis, Redefinition., phylogeny, species key: p.169)
Rem.: Type: Bradycalanus typicus A. Scott,1909. Type locality: 0°17.6' S, 129°14.5' E. Total: 4 spp. (of which 1 doubtful).
For Owre & Foyo (1964) this genus is synonymous with Megacalanus, a position that will be given up by the authors in 1967.

Diagnosis after H.B. Michel (1994, p.187) :
1 - Exopod of P1 with 1, 1, 2 outer marginal setae on the three segments; 2nd inner lobe of Mx1 with 2-5 setae.
2 - A1 not so long that in Megacalanus; endopod 2 of Mx1 with 2 setae, 1 extremely small; terminal setae of Mx2 scythe-like, with rows of short, stiff hairs; rostrum extends into acuminate prongs of varying length and breadth.

Diagnosis from Bradford-Grieve (1994, p.20) :
- The genus resembles Megacalanus but:
- A1 apparently not modified for grasping in the male.
- Setae on the terminal parts of Mx2 scythe-like and densely ciliated.
- Mx1 with inner lobes 1, 2, and 3 with 13, 4, and 2 setae respectively; basipod with 4 setae; endopod segments 1, 2 and 3 with 2, 1, and 5 setae respectively; exopod with 11 setae; outer lobes 2 and 1 with 1 and 9 setae respectively.
- P1 basipod segment 2 without a hooked spine.
- Male left P5 exopod segment 2 with 1 simple spine without hairs on the inner border.

Differential diagnosis after Bradford-Grieve & al. (2017, p.51):
- Anterior head rounded or crested.
- Rostral filaments slender and tapering to point.
- Female A1 ancestral segment XXIII with aesthetasc. Dorsal surface of ancestral segments I-V each with small hair sensillum without maculae cribrosae.
- Right male A1 ancestral segments XXI-XXIII fused, segments XIX, XX and XXI with gripping elements.
- A2 Exopod ancestral segments I-III each bearing relatively well-developed seta each longer than its segment, or shorter than its segment (Bradycalanus abyssicolus), seta on segment IV extending short of distal border of exopod. Bathycalanus); endopod segment 1 with 4 setae, endopod segment 2 with 9 large setae and 2 vestigial setae
- mx1 praecoxal arthrite with 4 posterior surface setae, coxal endite with 4 + 1 small or 2 setae (Bradycalanus abyssicolus), basal endites 1 and 2 with 2, 4 setae, respectively; endopod segments 1-3 usually with 2, 1 + 1 vestigial, 5 + 1 small posterior surface setae, respectively.
- Mx2 relatively enlarged compared with other megacalanidae; longest setae extend as far as mid labrum and only setae of praecoxal endites 1 and 2 and coxal endite 1 with auxilliary setules; proximal inner seta of endopod segment 2 vestigial, convex border of distal inner seta of endopod segment 2 naked.
- Mxp not enlarged compared with other megacalanids, longest setae extend to rostrum, endopod segments 3-5 with variable numbers of setae poorly developed;
- P1 basis without hooked process, exopod segments 1 and 2 each bearing distolateral articulated spine, segment 3 with 2 outer border spines. Maculae cribrosae absent.
- Male P5 specialised seta on left leg tapering evenly, bordered by very long setules.
Remarks on dimensions and sex ratio:
The mean female body size is 13.063 m (n = 8, SD = 2.610) and the mean male body size is 10.233 mm (n = 3; SD = 3.427). The length ratio male : female is approximately 0.8 . Sex ratio 2 but probably = 1owing to the fact that the abyssal situation and the paucity individuals catched.
(3) Elenacalanus Bradford-Grieve, Blanco-Bercial & Boxshall, 2017
Ref.: Bradford & al. (2017, p.129, Def.)
Rem.: Type species: Heterocalanus medius Wolfenden, 1906. Total: 5 spp.
Mainly bathypelagic.
For Bradford-Grieve & al, (2017, p.132) Heterocalanus medius Wolfenden, 1906 is clearly a junior synonym of Calanus princeps Brady, 1883 since H. medius has a head crested and P1 exopodal segment 3 with 2 outer spines. The authors propose the new genus name Elenacalanus as replacement name for Heterocalanus in the sense of Wolfenden (1906) and designate H. medius Wolfenden (1906) as the type species, because the genus Heterocalanus T. Scott, 1894 ( in reality Pseudodiaptomus Herrick, 1889) was preoccupied.

Diagnosis after Bradford-Grieve, Blanco-Bercial & Boxshall (2017, p/129) :
As for Megacalanidae plus following character states:
- Anterior head usually rounded and without anterior spine-like processes but may be crested (E. princeps).
- Rostral filaments bluntly tapering.
- Genital double-somite bulbous, widest in dorsal view at about midlength, usually as wide as long.
- Posterolateral corners of pedigerous 5 usually bluntly triangular or rounded in E. sverdrupi.
- Female A1 ancestral segment XXIII with aesthetasc.
- Female and male ancestral segments I-V with hair sensillum on dorsal surface and adjacent macula cribrosa except sometimes on segment IV.
Right male A1 ancestral segments XIV-XV and XXI-XXIII fused,ancestral segments XIX, XX and XXI with fused gripping elements.
- A2 exopod ancestral segments I-III without seta, on ancestral segment IV seta absent (E. princeps) or, more usually, extending short of the distal end of exopod.
- Md gnathobase with ventral tooth set at right angles to main plane of gnathobase therefore appearing tapering, similar to other teeth; endopod segment 1 with 2 setae, endopod segment 2 with 9 setae.
- Mx1 with 2 or 3 posterior surface setae on praecoxal arthrite, coxal endite without setae, basal endites 1 and 2 with 2, 1-3 setae, respectively, endopod segments 1 and 2 with 1 seta each at most, endopod segment 3 with 4+1 small setae (often 4 in male), basal exite with or without seta, coxal epipodite usually with 7+2 proximal reduced setae (sometimes absent).
- Mx2: endopod with curled setae into semicircle distally but not completely curled on themselves, with row of fine, long dense setules along distal half of concave surface; long setae extend anteriorly as far as rostrum, none of setae with auxiliary spinules; 1 inner seta of endopodal segment 2 vestigial.
- Mxp endopod segments 2-6 with 4, 1, 1, 1 (outer seta absent), 2 long + 0-2 small setae (sometimes outer setae absent).
- P1 basis without anterodistal hook-like process ; exopod segments 1 and 2 without outer border spines; expopd segment 3 with 2 outer border spines.
- Setae on some male mouthparts reduced in size.

Remaks from Bradford-Grieve & al. (2017, p.131): The morphology-based cladistic analysis and the molecular-based analysis clearly identify two separate monophyletic clades within what was previously called Bathycalanus. The name Bathycalanus is retained for the group of species, including the type species Bathycalanus richardi, with a maximal number of setae on the endopod of the Md, Mx1 and Mxp and 1 outer border spine on P1 exopodal segment 3.
Remarks on dimensions and sex ratio:
The mean body size female: 14.372 mm (n = 9; SD = 2.1256) and male: 11.433 mm (n = 3; SD = 2.1939). The length ratio male/female = 0.853 (n = 3; SD = 0.1225. Sex ratio female/male = 1.5
Heterocalanus Wolfenden, 1906
Ref.: Wolfenden, 1906 (p.26); 1911 (p.211); Farran, 1908 b (p.21); Vervoort, 1946 (p.63); 1963 b (p.85, Rem.); Razouls, 1982 (p.88)
Rem.: Cf. Bathycalanus
Macrocalanus Sars, 1905
Ref.: Sars, 1905 b (p.7); With, 1915 (p.36, clé G); Farran, 1939 (p.359, Rem.)
Rem.: Cf. Megacalanus
(4) Megacalanus Wolfenden, 1904
Syn.: Macrocalanus Sars, 1905 b (p.7); With, 1915 (p.36)
? Pseudolovenula Marukawa, 1921 (p.13, figs.)
Ref.: Wolfenden, 1904 (p.112); 1905 a (p.1); van Breemen, 1908 a (p.13); A. Scott, 1909 (p.10); Wolfenden, 1911 (p.195); With, 1915 (p.40); Sars, 1925 (p.10, Rem.); Sewell, 1929 (p.27, Rem.); Wilson, 1932 a (p.26); Rose, 1933 a (p.63); Farran, 1939 (p.355, Rem.); Vervoort, 1946 (p.47, Rem.); Sewell, 1947 (p.20, Rem.); Farran & Vervoort, 1951 a (n°33, p.3); Tanaka, 1956 (p.262); Vervoort, 1963 b (p.85); Guérédrat, 1969 (p.67, Rem.); Razouls, 1982 (p.84); Brodsky & al., 1983 (p.191); Mauchline, 1988 (p.717); Razouls, 1993 (p.308); Bradford-Grieve, 1994 (p.17, Def.); Michel, 1994 (p.187, spp. Key); Chihara & Murano, 1997 (p.834); Mauchline, 1998 (p.71); Boxshall & Halsey, 2004 (p.140); Vives & Shmeleva, 2007 (p.911); Bradford-Grieve & al., 2017 (p.26, Redefinition., Diagnosis, phylogeny, species key F & M: p.168)
Rem.: Type species : Megacalanus princeps Wolfenden, 1904. For Vervoort (1946, p.48) Wolfenden's Megacalanus princeps represents the type of the genus Megacalanus and has priority over Macrocalanus longicornis Sars, 1905 (p.7). Total: 4 spp. + 1 variety (doubtful)..

Bradford-Grieve & al. (2017, p.29) note a great deal of confusion caused by the incomplete description of Calanus princeps by Brady (1883), . 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. Wolfenden (1906) recognised that the Brady's type Calanus princeps was not a Megacalanus (resemble Heterocalanus Wolfenden, 1906, as Heterocalanus medius. The authors cited agree with Wolfenden's (1906) concept of a separate genus to take C. princeps Brady (1883) in a new genus Elenacalanus.

Diagnosis after H.B. Michel (1994, p.187) :
1 - Exopod of P1 with 1, 1, 2 outer marginal setae on the three segments; 2nd inner lobe of Mx1 with 2-5 setae.
2 - A1 very long, nearly twice the body length, extending beyond the caudal rami by about 8 segments; endopod 2 of Mx1 with 4 setae; terminal setae of Mx2 armed with moderately spaced spinules; bifurcated rostrum extends into slender, tapering filaments.

Diagnosis after Bradford-Grieve (1994, p.17) :
- As for the family definition.
- Head and pedigerous segment 1 and pedigerous segments 4 and 5 separate.
- Rostrum composed of 2 sharp, slightly tapering spines.
- A1 twice as long as the metasome.
- Mx1 inner lobe 1 (praecoxal arthrite) with 14 marginal spines and setae including 4 on the posterior surface and 1 on the anterior surface; inner lobes 2 and 3 (coxal and basal endites) with 4 and 4 setae respectively; basis with 4 setae; endopod segments 1, 2 and 3 with 3, 4 and 7 setae respectively; exopod with 11 setae; outer lobe 2 with 1 seta; outer lobe 1 with 9 setae.
- Mx2 with setae of moderate length, sparsely ciliated.
- P1 with exopod segments 1, 2, and 3 each with 1, 1, and 2 outer edge spines and a reflexed hook on each basipod 2 (= basis) (absent in some varieties).
- Males have slight differences in rostrum and appendages up to P4.
- A1 male asymmetrical, similar to female on the left, but with larger aesthetascs, with segments 7-9 imperfectly separated; 24-segmented on the right, with segments 7-9, 12-13, and 17-18 fused, segments 17+18, 19, and 20 edge expanded slightly into a flange; segment 9 bearing a clavate seta.
- Male P5 almost symmetrical, distinguished from those of the female only by the reduction in inner edge setae on exopod segment 3 and in the hairy projection terminated in a fine whip on the inner edge of the left exopod segment 2.

Differencial diagnosis from Bradford-Grieve & al. (2017, p.26):
- Anterior head without spine-like processes, rostral filaments tapering to point.
- Female A1 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 A1 ancsetral segments XIV-XV and XII-XXIII fused, at least segments XX and XXI with fused gripping elements.
- A2 exopod ancsetral segments I-IV bearing relatively well-developed setae on segments I-III each seta longer than its segment, on segment IV seta extends beyond distal border of exopod;.
- Md gnathobase with ventral tooth set at right angles to main plane of gnathobase therefore appears tapering andsimilar to other teeth; endopod segment 1 with 4 setae, endopod segment 2 with 9 large and 2 short proximal setae.
- Mx1 praecoxal arthrite with 4 posterior surface setae, coxal endite with 5 setae, basal endites 1 and 2 with 4 setaze each, endopod segment 2 with 4 setae.
- Mx2, modestely developed; longest setae extend as far as anterior labrum; most setae bear auxilliary setules.
Mxp mosestly developed, longest setae extending to rostrum, endopod segments 3-5 with proximal seta shortest.
- P1 basis usually with anterior inner surface bearing large hook-like process o,to which straight inner seta inserted, exopod srgments 1 and 2 each bear distolateral articulated spine , segment 3 with 2 outer border spines.
- Specialised seta on male left P5 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 ancsetral segment XXVIII; on Md gnathobase, proximally near insertion of basis; on Mx1 outer proximal surface of exopod; on Mxp inner surface of basis and endopod segment 5; on anterior surface of basis of legs 1-5 adjacent to insertion of exopod segment1; and P1 exopod segment 2.
Remarks on dimensions and sex ratio:
Mean body size female: 10.104 (n = 8; SD = 1.877) and male: 10.114 (n = 7, SD = 0.979. Length ratio male : female = 1.011 (n = 4, SD = 0.071). Sex ratio = 1.

 Any use of this site for a publication will be mentioned with the following reference :

Razouls C., Desreumaux N., Kouwenberg J. and de Bovée F., 2005-2021. - Biodiversity of Marine Planktonic Copepods (morphology, geographical distribution and biological data). Sorbonne University, CNRS. Available at http://copepodes.obs-banyuls.fr/en [Accessed July 26, 2021]

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