Achnanthidium rivulare is difficult to distinguish from Achnanthidium crassum, except under optimal illumination. Achnanthidium crassum has a slightly larger length-to-width ratio and radiate striae at the apices on the raphe valve. Achnanthidium deflexum has linear-elliptic valves and often shortened marginal striae in the middle portion of the valve. Rossithidium pusillum has parallel striae throughout most of the valve and no terminal raphe fissures. Achnanthidium convergens has strongly convergent striae near apices of the raphe valve, is larger in size, and has expanded external central raphe ends.

LM scalebar = 10 µm
Achnanthidium rivulare LM4 Achnanthidium rivulare LM5 Achnanthidium rivulare LM3 Achnanthidium rivulare LM2 Achnanthidium rivulare LM1
Length Range
5.4-23.8 µm
Width Range
2.6-4.4 µm
Striae in 10 µm
19-25 in the middle of raphe valve, 19-28 in the middle of the rapheless valve

Achnanthidium crassum

(Hust.) Potapova and Ponader 2004

Achnanthidium crassum LM5 Achnanthidium crassum LM4 Achnanthidium crassum LM1 Achnanthidium crassum LM2 Achnanthidium crassum LM3 Achnanthidium crassum LM6
Length Range
7.3-19.6 µm
Width Range
3.1-4.5 µm
Striae in 10 µm

Achnanthidium deflexum

(Reimer) Kingston 2003

Achnanthidium deflexum LM4 Achnanthidium deflexum LM1 Achnanthidium deflexum LM3 Achnanthidium deflexum LM5 Achnanthidium deflexum LM2
Length Range
7.4-28.3 µm
Width Range
3.1-5.3 µm
Striae in 10 µm
15-28 in the middle of raphe valve

Rossithidium pusillum

(Grunow) Round and Bukht. 1996

Rossithidium pusillum LM6 Rossithidium pusillum LM8 Rossithidium pusillum LM10 Rossithidium pusillum LM2 Rossithidium pusillum LM4 Rossithidium pusillum LM12
Length Range
10–16 µm
Width Range
3.1-4.1 µm
Striae in 10 µm

Also compare to

Achnanthidium convergens

(H.Kobayasi) H.Kobayasi