How to recognize an Acartia female:

Using a binocular :

Top of the head quite flat or slightly triangular.

Prosome with a typical slender shape and 4 visible thoracic segments.

A1 longer than the prosome but shorter than the boby, with many quite long and spaced setae.

Urosome 3-segmented.

Furca rather short, approximativelly as long as one urosome segment.

Furcal setae rather robust with a typical fan shape

Tiny P5.

Using a microscope :

No rostrum.

P5 uniramous, 3-segmented, with the last segment modified into a spine.

How to differenciate A. negligens from other Acartia

Using a binocular :

Corners of the last thoracic segment with one small spine (rarely more).

Using a microscope :

First two urosome segments with a dorsal row of tiny spinules on the posterior border.

Third segment of P5 forming a short, straight, spine with serrated outer edges.

Second segment of P5 longer than wide and bearing a very long setae
(X5 longer than the spine).

 

Ecological Notes :