Cyclopoida ( Order )
Oithonidae Dana, 1853 ( Cyclopoida )
Syn.: Cyclopidae : Giesbrecht, 1892 (p.36) (part.) ; van Breemen, 1908 a (part., p.166)
Ref.: Sars, 1900 (p.119);1913 (1918) (p.4); Kiefer, 1929 g (p.2); Rose, 1933 a (p.279); Gonzalez & Bowman, 1965 (p.268); Wellershaus, 1970 (p.474); Shuvalov, 1980 (p.13, 85, Genera Key); Ferrari & Bowman, 1980 (p.3); Björnberg & al., 1981 (p.663); Razouls, 1982 (p.630); Bowman & Abele, 1982 (p.11); Zheng Zhong & al., 1984 (1989) (p.261); Reid, 1985 a (p.25); Nishida, 1985 a (p.11); Dussart & Defaye, 1985 (p.9); Sazhina, 1985 (p.85); Ho, 1986 (p.177); Nishida, 1986 (p.385: cuticular pores); Huys & Boxshall, 1991 (p.465); Razouls, 1993 (p.311); Chihara & Murano, 1997 (p.935); Bradford-Grieve & al., 1999 (p.886, 964, clé spp.); Boxshall & Halsey, 2004 (p.16, 610: Def.; p.611: Genera key); Vives & Shmeleva, 2010 (p.43, Rem., genera key); Hirai & al., 2020 (p.1, Fig.4: metabarcoding, Fig.8: OTUs distribution patterns, Fig.9: phylogenetic analysis)
Rem.: 2 Sub-fam : Limnoithonidae, Oithoninae.

Key to genera after Boxshall & Halsey (2004, p.611) :
1 - P5 2-segmented, with 3 or 4 spines/setae on distal (exopodal) segment. ......Limnoithona.
2 - P5 with at most 2 setae on the free exopodal segment......3.
3 - Free endopodal segment of mandibular palp with 5 setae; free exopodal segment of P5 with 2 apical setae; male lacking cephalosome flap organ ......Dioithona
3' - Free endopodal segment of mandibular palp with 2 to 4 setae; free exopodal segment of P5 usually with only 1 seta, if 2 setae present then outer spines on middle exopodal segment of P3 and P4 lacking; male with cephalosome flap organ.......Oithona.

Key after Vives & Shmeleva (2010, p.45) :
1- Free segment (Exopod) of P5 with 2 apical setae; males without flattened organ ('flap organ) on the cephalosome ............ Dioithona
2 - Free segment (exopod) of P5 mainly with 1 apical seta (if 2 setae, lacking external spines on the 2rd segment of P3 and P4 (see O. frigida and O. pseudofrigida); males with flattened organ on the cephalosome ......................................... Oithona
Family Oithonidae - Plate 1Issued from : G.A. Boxshall & S.H. Halsey in An Introduction to Copepod Diversity, Part II, The Ray Society, 2004, Part II, No 166. [p.610].
Armature formula of swimming legs P1 to P4.
Nota: Swimming legs typically with 3-segmented rami, occasionally with 2-segmented endopods and with 2-segmented exopod in P1 in some species. P1 to P4 joined by intercoxal sclerites. Inner seta on basis of P1 present. Inner coxal seta present in all legs. Setation of P1 to P4 often secondarily reduced ; often by loss of outer margin spines from exopods of the more posterior leg pairs; setation strongly reduced in 2-segmented endopods.
- P5 in both sexes with protopod incorporated into somite and 1-segmented exopod; outer protopodal seta situated on papilla on surface of somite dorsal to free exopodal segment; exopod bearing 1, 2, 3 or 4 setae.
- P6 represented by 2 setae on genital operculum of female; by 2 setae on genital operculum of male.
- Egg sacs paired, multiseriate.

Family Oithonidae - Plate 2Issued from : G.A. Boxshall & S.H. Halsey in An Introduction to Copepod Diversity, Part II, The Ray Society, 2004, Part II, No 166. [p.612, Fig.198].
Oithonidae: A, female with egg sacs; B, Oithona nana A2; C, Mxp; D, Md palp; E, Mx1.
From Huys & Boxshall, 1991.
Limnoithoninae Kiefer, 1928
Ref.: Kiefer, 1929 g (p.11); Nishida, 1985 a (p.11); Dussart & Defaye, 1985 (p.10)
Rem.: 1 G.: Limnoithona
Oithoninae Kiefer, 1913
Ref.: Kiefer, 1929 g (p.2); Nishida, 1985 a (p.11, clé spp.); Dussart & Defaye, 1985 (p.9); Al-yamani & al., 2011 (p.94)
Rem.: 2 G.: Oithona, Dioithona
(1) Dioithona Kiefer, 1935 ( Oithoninae )
Syn.: Dioithona Kiefer, 1935 f (p.322, espèce type: Dioithona alia )
Ref.: Lindberg, 1950 (clé spp.F); Vervoort, 1964 a (p.25, Rem.); Gonzalez & Bowman, 1965 (p.268); Wellershaus, 1970 (p.472); Shuvalov, 1980 (p.86, 169); Razouls, 1982 (p.645); Nishida, 1985 a (p.48, Rem.); Dussart & Defaye, 1985 (p.9); Vives & Shmeleva, 2010 (p.45, Rem.)
Rem.: Vervoort (1964) believes that the creation of this genus rests on a character that is too restrictive and suggests to consider it only as a subgenus of Oithona. This opinion is followed by Wellershaus (1970) who reviews several characters in some Oithona species. Shuvalov (1980, p.86, 169) considers the genus as justified. Meanwhile, after examination of the males and on the basis of a phylogenetic analysis (Abiahy, 2000), Boxshall & Halsey (2004, p.611) restore the validity of the genus Dioithona; for the authors the diagnosis is : Free endopodal segment of mandibular palp with 5 setae; free exopodal segment of P5 with 2 apical setae; male lecking cephalosome flap organ.
5 spp. (including 1 doubtful).
Remarks on dimensions and sex ratio:
The mean female size is 0.691 mm. (n = 8, SD = 0.172), and the mean male size is 0.613 mm (n = 6, SD = 0.179). The size ratio male: female is 0.887.
(2) Limnoithona Burckhardt, 1913 ( Limnoithoninae )
Ref.: Burckhardt, 1913 (p.421); Rosendorn, 1917 a (p.6); Kiefer, 1929 g (p.11); Razouls, 1982 (p.645); Shuvalov, 1980 (p.86, 187); Dussart & Defaye, 1985 (p.10)
Rem.: Diagnosis after Boxshall & Halsey (2004, p.6121): P5 2-segmented, with 3 or 4 spines/setae on distal (exopodal) segment.
Remarks on dimensions and sex ratio:
The mean female size is 0,503 mm and the mean male size is 0,440 mm. The size ratio (M/F) is 0,876 or 87,6 %.
(3) Oithona Baird, 1843 ( Oithoninae )
Syn.: Paroithona Farran, 1908 b (p.89); 1913 a (p.192, clé spp.); Rosendorn, 1917 (p.5); Sars, 1918 (p.207); Kiefer, 1929 g (p.3, 10); Rose, 1933 a (p.284); Shuvalov, 1980 (p.86, 190); Razouls, 1982 (p.644); Nishida, 1985 a (p.11,15, 16, 137, clé spp., Rem.); 1986 (p.388); Dussart & Defaye, 1985 (p.10); Razouls, 1993 (p.311); Chihara & Murano, 1997 (p.939)
Ref.: Brady, 1878 (p.90); Giesbrecht, 1892 (p.77, 537); Sars, 1900 (p.119); Wheeler, 1901 (p.186); Esterly, 1905 (p.207); van Breemen, 1908 a (p.166, spp. Key); A. Scott, 1909 (p.193); Sars, 1913 (1918) (p.4); Burckhardt, 1913 (p.420, 439); Farran, 1913 a (p.182, 191, spp. Key); Rosendorn, 1917 a (p.4, 6, 50: spp. Key, biogeography); Pesta, 1920 (p.548, spp. Key); Kiefer, 1929 g (p.3, spp. Key); Wilson, 1932 a (p.311); Rose, 1933 a (p.279); Mori, 1937 (1964) (p.108, spp. Key); Dakin & Colefax, 1940 (p.115, spp. Key); Oliveira, 1946 (p.478); Davis, 1949 (p.73); Lindberg, 1950 e (p.273, spp.F Key); Carvalho, 1952 a (p.165); Lindberg, 1955 e (p.464, key spp.); Björnberg, 1963 (p.73); Gonzalez & Bowman, 1965 (p.268, spp. Key); Owre & Foyo, 1967 (p.107, spp. Key); Wellershaus, 1970 (p.477, 478, Rem. spp.); Sheldon & al., 1972 (p.327, fig. 13: size particle vs production's rate); Nishida & al., 1977 (p.125, key spp.); Ferrari & Bowman, 1980 (p.5, Rem.); Shuvalov, 1980 (p.86, spp. Key); Björnberg & al., 1981 (p.663, spp. Key); Razouls, 1982 (p.632); Gardner & Szabo, 1982 (p.107); Zheng Zhong & al., 1984 (1989) (p.261, spp. Key); Dussart & Defaye, 1985 (p.9); Reid, 1985 a (p.26, spp. Key); Nishida, 1985 a (p.14, spp. Key); 1986 (p.385: cuticular pores); Huys & Boxshall, 1991 (p.465); Yoo & Lim, 1993 (p.91, Rem.: p.97, Table 1, spp. Key); Chihara & Murano, 1997 (p.935, spp. Key); Bradford-Grieve & al., 1999 (p.965: spp. Key) ; Vives & Shmeleva, 2010 (p.45, 47 Rem., spp. Key)
Rem.: Gurney (1927, p.159) points that Sars (1918, p.5) has proposed a new genus Oithonina to receive O. nana. The oithonas without rostrum, of which there are 7 species, agree in having the same spine-formula for the swimming legs (except O. simplex Farran), whereas there is considerable diversity among the rostrate species; but the rostrate group includes species with the formula of O. nana (O. robusta, O. brevicornis), and the absence in the female of the rostrum does not seem sufficient by itself to characterise a genus.
Kiefer (1935) distinguishes two groups of species within the genus Oithona. The first group, Oithona (sensu stricto), is characterised by the very small unique segment of the P5 (narrow and long, or reduced to a protuberance), having only two setae on its extremity. Also, one seta projects at each side on the external angle of the last thoracic segment.
The second group comprises the species in which the segment of the P5 is better developed (oviform) and of which the extremity bears a long plumose seta and a second, smaller seta, directed to the body axis and which has the aspect of a subtle spine (Dioithona). The author underlines the importance of the structure of the P5 in the Cyclopoids of fresh water and thinks that it is, mistakenly, neglected in the Oithonidae family.
The ecophenotypic variations could be important in the Oithonidae family making the identification of the species difficult and the synonymies confusing (Nishida & al., 1977).
These small carnivores, often very abundant, and generally underestimated because of the used mesh size in sampling, deserve a special attentiion because of their role in the pelagic food web. (cf.: Gallienne & Robins, 2001, p.1421).
For Boxshall & Halsey (2004, p.611) following Abiahy (2000), Paroithona is a synonym of Oithona. The diagnosis is : Free endopodal segment of mandibular palp with 2 to 4 setae; free exopodal segment of P5 usually with only 1 seta, if 2 setae present then outer spines on middle exopodal segment of P3 and P4 lacking; male with cephalosome flap organ.

Type: Oithona plumifera Baird,1843. Total: 44 spp. + 6 unidentified.
Remarks on dimensions and sex ratio:
The mean female size is 0,789 mm (n= 39; S= 0,307; Cv= 0,389) and the mean male size is 0,608 mm (n= 23; S= 0,171; Cv= 0,281). The size ratio (M/F) is 0,833 ou 83,3 % (n= 23; S= 0,139; Cv= 0,167)
Oithonina Sars, 1913
Ref.: Sars, 1913 (1918) (p.5); Wilson, 1932 a (p.316); 1942 a (p.197); Razouls,1982 (p.644); 1993 (p.311)
Rem.: Cf. Oithona
Paroithona Farran, 1908
Ref.: Boxshall & Halsey, 2004 (p.611: Rem.)
Rem.: Cf. Oithona

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