Small to medium-sized larvae, up to 5.5 mm long. Coloration bluish with dark brown or black head.
Antenna with 5 segments; segment 3 shorter than 4 (and often 5) in length, segment 5 with apical constriction. Ring organ in basal 1/4 of segment 1. Blade as long or slightly shorter or longer than flagellum. Lauterborn organ weak. Style reaching middle of segment 4.
Labrum with SI seta strong, simple, bristle- to scale-like, other S setae simple. Dorsal part of labrum strongly sclerotized. Chaetae and spinulae simple or very weakly serrated. Labral lamellae absent. Pecten epipharyngis consisting of 3 simple, subequal scales. First pair of chaetulae laterales of similar shape giving the impression of 5 scales in pecten epipharyngis; remaining chaetulae laterales and chaetulae basales absent or vestigial. Ungula squarish; basal sclerite narrow and very elongate, well sclerotised. Premandible heavily sclerotized with simple apex and indication of low and broad inner tooth; brush absent.
Mandible with apical tooth shorter than combined width of second and third inner teeth (of 3-4 inner teeth total). Seta subdentalis apparently short. Seta interna probably with long unbranched basal part bearing 5 smooth branches. Mola with 3 long spines, occasionally serrate distally, or spines reduced to points.
Mentum with 1 double median tooth and 5 pairs of lateral teeth, outermost teeth very small; lateral areas strongly sclerotized. Ventromental plate relatively well-developed, but narrow, with posterior sharp bend; anterolateral margin evenly convex; beard absent.
Maxilla with palpiger with pointed leaf-like lamellae. Lacinial chaetae plumose. Pecten galearis absent.
Body with anterior parapods with longer claws simple, shorter claws serrate. Posterior parapods well developed with some small and several large black claws. Procercus short, sclerotized, with 7 anal setae. Shape and size of anal tubules unknown.
Diagnosis after Halvorsen and Saether (1987). The combination of simple SI seta, shape of the mentum, strongly sclerotised mentum, labrum and premandible, short procercus and elongate basal sclerite will separate Tokunagaia from relatives, perhaps excepting some Cardiocladius. There appear to be no differences between Tokunagaia and Cardiocladius in the antenna, the epipharyngeal area, maxilla and procercus and differentiation is uncertain.
The genus is based on the Eukiefferiella rectangularis group expanded by addition of T. tonollii (Rossaro), T. parexcellens Tuiskunen & Lindeberg and T. obriaini Hayes & Murray. Liu and Wang (2006) added 5 new species as adult males from China and 1 species is known from Darjeeling, India.
Tokunagaia larvae are found in gelatinous cases adhering to surfaces of immersed stones in fast-flowing creeks, in algal mats and in the hygropetric zone.