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. plumifera from other Oithona / Dioithona

Using a binocular :

Rostrum is visible in dorsal view.

Plumose setae arise from swimming legs of some specimens but are not always present (broken ?).

Genital segment is twice as long as wide.

Furca is slightly shorter than the anal segment.

O. Plumifera is one of the biggest Oithonidae with O. setigera which is very similar.

Using a microscope :

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

A characteristic tuft of hairs is present on the ventral side of the Genital segment.

 

Ecological Notes :

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