Dana, 1853
Females
Total length: 0.36 mm-1.90 mm. Pr short oval to long fusiform in dorsal view, of 5 somites. Anterior end of head quadrate, rounded or produced into pointed R. Ur slender, of 5 somites; Gns longest. CR frequently with 1 dorsal, 4 apical and 1 outer marginal setae; 2 of apical setae may be absent in Paroithona. A1 slender, with proximal part indistinctly segmented making it difficult to determine precise number of segments. A2 uniramous, bent outward at medial geniculation; proximal part 1-segmented, with 1-3 inner and 0-2 outer marginal setae; distal part 1- or 2-segmented. Proximal segment of distal part short, with 2 medial and 3 distal inner marginal setae; distal segment long, usually with 7 terminal setae. Md ( Oithona attenuata 3 ) consisting of gnathobase with 1-2 transverse rows of setules on anterior surface near distal margin. Basis transversally elongated, with 1 inner seta; fused to Enp1 which has 2-3 elements varying in shape and size; Enp2 with 2-5 setae; Exp 4-segmented, with 5 setae. Mx1 ( Oithona hamata 2 ) Li1 with maximum of 11 inner setae; Li2 seta present or absent; Li3 with 2-3 setae distally; Li4 with 0-2 setae; Le1 seta present or absent; Enp 1-segmented, with 0-4 setae; Exp 1-segmented, with 3-4 setae. Mx2 with praecoxa and Coxa fused, plane of fusion marked by incomplete transverse line, best observed on inner surface; Li1 with 3 setae; Li2 with 1 spinulose seta; Li3-Li4 each with 3 setae; Basis produced into inner lobe (Li5) bearing 2-3 setae or with 1 claw fused to lobe, and 2 setae; Enp1 with 1 inner marginal proximal seta and 3 inner marginal distal setae; Enp2 with 2 strong, curved setae; Enp3 minute, with 1 strong curved seta and 2-3 more or less reduced setae. Mxp 4-segmented; praecoxa and Coxa incompletely separated; seta formula: 1-3-2 (1-2-2 in Limnoithona); Basis with 1 medial and 1 distal inner marginal setae; 1 midmarginal row and/or 1 distal posterior surface row of minute setae present or absent; Enp1 with 1 or 3 setae; Enp2 with 3-4 setae. P1-4 usually with 3-segmented Enp and Exp except for Paroithona where P1 Exp2 and 3 and P1-4 Enp2 and 3 are partly or completely fused. In most species P4 Enp2-3 with 1-2 modified setae which are thick and slightly curved towards tip, with distal flange.
P5 represented by 2 lateral processes on Ur1; its dorsal process small, with 1 terminal seta and ventral process usually a free segment with 1-4 terminal setae. P6 reduced into lateral process with 1-2 setae on anterior part of genital segment.
Males
Total length: 0.37 mm-1.24 mm; usually smaller than female. Pr longer than Ur, of 5 somites. Forehead rounded in lateral view, truncate in dorsal view. Lateral posterior margin of Ce usually produced into flap with well-developed rows of pores. Ur of 6 somites excluding CR. A1 with 2 geniculations. A2, Md, Mx1, Mx2 and Mxp similar to those of female in basic structure however A2, Md, and P6 sexually dimorphic in some species. P1-4 Exp generally with I,I,II-III outer marginal spines. P5 as in female.
Oithonids of marine and estuarine waters are restricted to the sub-family Oithoninae Kiefer, 1928, which contains Oithona Baird, 1843 and Paroithona Farran, 1908. Limnoithoninae Keifer, 1928 was created for the freshwater genus Limnoithona Burckhardt, 1912. Fine details of the morphology of Oithonidae as well as keys have been produced by Nishida (1985) and Wellershaus (1970). Rocha (1985, 1986) helped to elucidate the taxonomic details of common species in estuaries along the Brazilian coast both on systematic and biological grounds. In the South Atlantic Ocean, considering inlet and embayment environments, and neritic and oceanic provinces together, 22 species have been reported (see table Co. Tab. 28 and the tables on the page Co.12 Tables. Morphometric and distributional data for the species treated. Morphometric and distributional data for the species treated).
Readers interested in knowing more about the distribution and taxonomy of Oithonidae on a broad basis should refer to Giesbrecht (1892), Rosendorn (1917), Wellershaus (1970), Ferrari and Bowman (1980), and Nishida (1985).