KEY TO GENERA
1. Trichomes always without heterocytes.......................................................................................................................RAPHIDIOPSIS
- Trichomes capable of producing heterocytes......................................................................................................................................2
2. Heterocytes terminal.............................................................................................RAPHIDIOPSIS (in part, ex. Cylindrospermopsis)
- Heterocytes intercalary........................................................................................................................................................................3
3. Heterocytes develop in pairs which, following trichome breakage, are in the terminal position.......................... ANABAENOPSIS
- Heterocytes solitary.............................................................................................................................................................................4
4. Apical cells generally differentiated, elongated, tapered, or hyaline..................................................................................................5
- Apical cells generally undifferentiated................................................................................................................................................6
5. Apical cells elongated, attenuated and acuminate................................................................................................... CUSPIDOTHRIX
- Apical cells conical, or tapered and elongated, often hyaline..............................................................................CHRYSOSPORUM
6. Vegetative cells always distinctly shorter than broad....................................................................................................NODULARIA
- Vegetative cells ± isodiametric or longer than wide............................................................................................................................7
7. Akinetes spherical, always developing either side of the heterocytes........................................................ SPHAEROSPERMOPSIS
- Akinetes adjacent to or remote from heterocytes................................................................................................................................8
8. Akinetes long cylindrical, often with a collar-like extension at either end which extends over the adjacent vegetative cells.............
.............................................................................................................................................APHANIZOMENON (in part A. gracile)
- Akinetes oval to cylindrical.............................................................................................................................DOLICHOSPERMUM

KEY TO SPECIES

Cuspidothrix issatschenkoi (Usachev) Rajaniemi, Komárek, Willame, Hrouzek, Kaštovskà, Hoffmann & Sivonen
(2005: 388) Fig. 8 A–E.
Basionym: Anabaena issatschenkoi Usachev (1938: 109)
Filaments planktonic; trichomes solitary or slightly flexuous, cylindrical, continually narrowed towards both ends, not
or slightly constricted at the cross walls. Vegetative cells long cylindrical, with aerotopes, 4–8 (–15) μm long × 2.5–4.0
μm broad; apical cells narrowed, hair-like, often hyaline. Heterocytes solitary, sub-cylindrical 6–11 μm long × 2.5–4.0
μm broad. Akinetes cylindrical with rounded ends, solitary or in series, remote from the heterocytes, 6–20 μm long ×
2.5–4.5 μm broad.