How to recognize an Oithona female:

Anterior and posterior section of prosome gradually narrowing.

A1 as long as or longer than prosome but shorter than the whole body.

Urosome usually as long as prosome, with 5 segments.

 

How to differenciate O. nana from other Oithona / Dioithona

Using a binocular :

Top of the head flattened in dorsal view.

Rostrum rounded in lateral view.

A1 as long as prosome.

Genital segment twice as long as wide.

Anal segment as long as wide.

Furca as long as anal segment and twice as long as wide.

O. nana is among the small Oithonidae

Using a microscope :

Rostrum shape in lateral view is a useful distinctive feature (see figures).

 

Ecological Notes :

Very common all the year long.
Probably carnivorous.
As other Oithona, this species can numerically dominate the samples.