I think it most be a relativ of Noctiluca. I show you two specimens (with the diameter 180 µm and 250 µm). The movements in the cell were very fast. Could it be the marine species Kofoidinium (velleloides?)?
I agree that it cannot be Noctiluca, the outline is too irregular. Noctiluca has a smooth envelope or if shrunken shows smooth folds or creases.
So, I agree that it must be a species of Kofoidinium.
Kofoidinium pavillardii, Kofoidinium splendens and Kofoidinium velelloides are recorded from the Mediterranean. But the appearance of each species is so variable that I think even professionals would be reluctant to name your specimen....perhaps a species new to science!!
i am sending you a paper on Kofoidinium by pm, which I think will show you how poorly this genus is known and understood.
I cannot wait for the next member of your amazing menagerie.
I agree with Brian, there are not enough details to get to species. There is more change of preserving them when fixing samples in the field, the appearance of live spec is so variable upon stress. From this image it is going toward the Spathulodinum stage of Kofoidinium (we regard them nowadays as one species), but for K. velleloides it lacks all typical characteristics. It is not at all rare in the North Sea, but often neglected.