Sars, 1902
Females
Total length 2.0-8.0 mm. Body usually slender, Pr oval, Ce and Pd1 separated, Pd4 and 5 fused, posterolateral corners rounded. Ce rounded anteriorly; rostral filaments slender. Ur elongate, of 4 somites; Gns with ventral process sometimes protruding greatly and sometimes occupying whole length of somite; CR usually asymmetrical (largest on left) with varying degrees of fusion between them and anal somite, with 7 setae, one of terminal setae often slightly or extraordinarily elongate on the left side. A1 symmetrical, 25-segmented. A2 Basis with 2 outer edge setae; Exp and Enp separated from Basis; Enp 2-segmented, Enp1 with 2 distally placed setae, Enp2 with 6-8 inner setae medially and 6-7 setae terminally; Exp 7-9-segmented, proximal 2 segments sometimes without setae. Md gnathobases usually asymmetrically developed on each side with 3-5 (or more) long, sharp teeth, in Heterorhabdus, Hemirhabdus, and Neorhabdus one tooth set far apart from remaining teeth; Enp 2-segmented, Enp1 with 2-4 setae, Enp2 with 6-9 setae terminally; Exp1-4 with 1 seta each, Exp5 carrying 2 setae. Mx1 tends to have Li2-4 and Enp reduced; Li1 with 5-13 spines and setae (most reduced setation in Hemirhabdus, least reduced in Disseta); Li2 small or absent with 0-2 setae; Li3 small or absent with 0-2 setae, Li4 with 1-4 setae; Enp small, 1-segmented, with 4-11setae; Exp large, with 5-11 setae (least reduced setation in Disseta and Mesorhabdus); Le1 with 3-8 setae. Mx2 usually modified as a grasping, piercing limb (least modified in Disseta and most modified in Hemirhabdus); Li1 and 2 with 3-5 and 0-3 reduced setae respectively; Li3 and 4 with 2-3, 2-3 usually small setae respectively; Li5 with 1-3 setae, 1 or 2 of which may be greatly enlarged and spine-like; Enp1 usually with an enlarged spine-like seta; remainder of Enp with greatly reduced setae. Mxp Coxa with 3-7 setae proximal one of which is greatly enlarged and spine-like in Heterorhabdus; Basis slender with 3 small setae medially; Enp 6-segmented, segment 1 almost fully incorporated into Basis with 2 setae, segments 2-5 with 3, 3, 2-3, 2-3 setae respectively, segment 6 with 3-4 setae outermost 1-2 of which are reduced. P1-4 distinctly 3-segmented, Exp of P2-4 flattened and leaf-like.
Spine and seta formula: Spine and seta Heterorhabdidae.
P5 natatory, symmetrical, very similar to other swimming legs but smaller; inner edge seta on Exp2 modified into long slender spine articulated with its segment; Exp2 posterior distal border expanded into toothed flange in Heterostylites.
Males
Body similar to female, but Ur of 5 somites. Left A1 geniculate, 21-23-segmented. Mouthparts and legs 1-4 similar to female. P5 with a 3-segmented Enp and Exp on both sides, right Basis usually with inner extensions of variable shape and usually fringed by hairs or spinules, sometimes with inner extension on left; right Exp2 with inner expansion of variable shape but usually in form of thick spine; left Exp3 often but not always terminated by an elongate spine.
This family was revised by Heptner (1972) and the genera redefined based on the structure of the mouthparts which are related to feeding type. Later Grice (1973) added a further genus, although it has been only provisionally placed in this family pending discovery of the male.
As currently defined the family includes the following genera, of which Alrhabdus, Hemirhabdus, and Microdisseta have not been taken in the South Atlantic:
Genus Alrhabdus Grice, 1973
Genus Disseta Giesbrecht, 1892
Genus Hemirhabdus Wolfenden, 1911
Genus Heterorhabdus Giesbrecht, 1892
Genus Heterostylites Sars, 1920
Genus Mesorhabdus Sars, 1905
Genus Microdisseta Heptner, 1972
Genus Neorhabdus Heptner, 1972