Appearance
The species can grow to a length of 50 mm. It feeds primarily on species of Eudendrium, a very common hydroid genus in the Mediterranean Sea.
Flabellina affinis is often confused with Paraflabellina ischitana, but can be distinguished as follows:
Flabellina affinis: the outer part of the digestive gland in each ceras is not visible as the area beneath the subapical white ring is opaque violet in colour.
Paraflabellina ischitana: the ceratal surface has no purple-violet colouration and is translucent.

