Calanoida ( Order )
    Clausocalanoidea ( Superfamily )
Mesaiokeratidae Matthews, 1961 ( Clausocalanoidea )
Ref.: Matthews, 1961 (p.37,38); Bradford, 1969 b (p.502, Table 3); Andronov, 1974 a (p.1005, 1009); Fosshagen, 1978 (p.182: Definition); Razouls, 1982 (p.373); 1993 (p.310); Bowman & Abele, 1982 (p.9); Huys & Boxshall, 1991 (p.419); Bradford-Grieve, 1994 (p.133, Def.); Bradford-Grieve & al., 1999 (p.881, 902, 904, 935); Ohtsuka & Huys, 2001 (p.461); Boxshall & Halsey, 2004 (p.16; 49; 142: Def., Rem.: p.144); Vives & Shmeleva, 2007 (p.674)
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.: 1 G.: Mesaiokeras.
For Boxshall & Halsey (2004, p.144) the Mesaiokeratidae is closely related to the Stephidae. This family was placed in the superfamily Clausocalanoidea (as Pseudocalanoidea) by Andronov (1974) and Park (1966). The validity of this family has been assumed but never tested by a comprehensive phylogenetic analysis.
Family Mesaiokeratidae - Plate 1issued from : E.L. Markhaseva & F.D. Ferrari in Invert. Zool., 2005, 2 (2). [p.162, Table 4]
Setation of oral parts in females Mesaiokeratidae (Clausocalanoidea) and ancestral condition of setation.

Family Mesaiokeratidae - Plate 2Issued from : G.A. Boxshall & S.H. Halsey in An Introduction to Copepod Diversity. The Ray Society, 2004, No 166, Part. I. [p.142].
Armature formula of swimming legs P1 to P4.
Setation sometimes reduced; outer spine on 1st and 2nd exopodal segments of P1 lacking in most species.

Nota: Female typically asymmetrical with right leg missing and left leg rudimentary, 2-segmented, with 2nd segment forming a bifid process: rarely both legs present.
- Male P5 asymmetical; right leg reduced to 1 to 3-segmented process; left leg elongate; comprising coxa, elongate basis and 3-segmented exopod; 2nd exopodal seglment sometimes armed with outer spine, ornamented with row of long spinules in Mesaiokeras mikhailini Andronov; distal segment with up to 2 inner elements.
- Eggs released into water.

Family Mesaiokeratidae - Plate 3Issued from : G.A. Boxshall & S.H. Halsey in An Introduction to Copepod Diversity. The Ray Society, 2004, No 166, Part. I. [p.143, Fig.30].
Mesaiokeratidae. A, Mesaiokeras heptneri habitus female; B, habitus male (lateral); C, P1; D, female P5; E, male P5; F, Mesaiokeras kaufmanni male Md. [Andronov, 1973a: A-E; Fosshagen, 1978: F].
(1) Mesaiokeras Matthews, 1961
Ref.: Matthews, 1961 (p.33, 38); Fosshagen, 1978 (p.183: Def.); Razouls, 1982 (p.373); Huys & Boxshall, 1991 (p.49, 324); Razouls, 1993 (p.310); Bradford-Grieve, 1994 (p.133, Def.); Mauchline, 1998 (p.87, 88: F; p.90: M); Bradford-Grieve & al., 1999 (p.935: spp. Key); Bradford-Grieve, 2004 (p.287); Boxshall & Halsey, 2004 (p.144, spp. Key); Vives & Shmeleva, 2007 (p.675, spp. Key)
Rem.: Hyperbenthic forms.
On the occasion of the description of this new genus, Matthews discusses the validity of two sections Amphascandria and Isokerandria.
The family Mesaiokeratidae shows characters that are similar to those of the Scolecitrichidae and the Diaixidae.
Type: Mesaiokeras nanseni Matthews,1961. Total: 9 spp.

Diagnosis from Bradford-Grieve (1994, p.95) :
- As for the family definition.
- Small rostral filaments present.
- Setation on the caudal rami may be asymmetrical.
- Last endopodal segment of A2 elongate, with no distinct lobe.
- Endopod of P1 with a straight outer margin and exopod usually with a reduced number of spines on the outer margin.
- Left P5 male very long, uniramous and 5-segmented; right leg short, not extending beyond exopodal segment 1 of the left leg.

Key to species after Boxshall & Halsey (2004, p.144) (not included : M. hurei Krsinic, 2003, and M. spitsbergensis Schulz & Kwasniewski, 2004) :
1 – Female A1 24-segmented (apical segment double XXVII-XXVIII) ; 1st exopodal segment of P1 with outer spine ……… M. heptneri.
1’ – Female A1 23-segmented (apical segment triple XXVI-XXVIII) ; 1st exopodal segment of P1 without outer spine …….. 2.
2 – 2nd exopodal segment of P1 with outer spine ; penultimate segment of male left P5 with row of large spinules along margin …… M. mikhailini.
2’ – 2nd exopodal segment of P1 without outer spine ; penultimate segment of male P5 without row of large spinules along margin …… 3.
3 – Male left P5 with truncated distal margin of terminal segment armed with at least 2 strong spines plus a rounded setal element subapically ……. 4.
3’ – Male left P5 with terminal segment tapering to acute point ; segment armed with 1 or 2 setal elements …… 5.
4 – Left caudal ramus of female with long naked seta positioned proximally near inner margin and directed laterally ; 4th and 5th pedigerous somites fused ; distal segment of male left P5 armed with 1 strong spine and 1 rounded element …… M. nanseni.
4’ – Left caudal ramus of female without such a modified seta ; 4th and 5th pedigerous somites separate ; distal segment of male left P5 armed with 2 strong spines ……M. semiplenus.
5 – Terminal segment of male left P5 armed with 2 strong spines at mid-length ; penultimate segment lacking distal spinous process …… M. kaufmanni.
5’ – Terminal segment of male left P5 with 1 subapical spine and 1 proximal spine ; penultimate segment lacking distal spinous process …… M. marocanus.
5’’ – Terminal segment of male left P5 with 2 closely adpressed spines subapically ; penultimate segment produced into distal spinous process ….. M. tantillus.
Remarks on dimensions and sex ratio:
The mean female size is 0.520 mm (n = 11, SD = 0.231), and the mean male size is 0.490 mm (n = 2, SD = 0.217). The size ratio male: female is 0.942.
If the arctic species (M. spitsbergensis Schulz & Kwasniewski, 2004) is not included, we obtain: mean size female: 0.420 mm (n = 9; SD = 0.0630), and mean male size 0.409 mm (n = 12; SD = 0.0735) and a size ratio male:female 0.973.

 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., 2005-2022. - Biodiversity of Marine Planktonic Copepods (morphology, geographical distribution and biological data). Sorbonne University, CNRS. Available at http://copepodes.obs-banyuls.fr/en [Accessed May 26, 2022]

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