It looks like a Berthella to me as well, also possibly Berthella platei. Based on the paper attached, it seems like the specific locality information is important to determining which.

Article The genus Berthella Blainville, 1825 (Notaspidea: Pleurobran...

Thanks for the help Jessica!

I requested the full text to Michael Schroedl for further checking, although your opinion is a good guide already.

Dear Nahuel

I recommend you to use the service of Diagnostic Assistance of the CDC. This is very useful. I have used it before with good results.

Please check this link: http://www.cdc.gov/dpdx/contact.html

There, is also the information required to send the images for diagnosis.

Finally, please if you consider this answer appropriate, please upvote it using the green up arrow click. Thanks.

 Dear Alfonso,

Thanks for your answer but there seems to be something wrong with that link: when open it it says " Laboratory Identification of Parasitic Diseases of Public Health Concern" (?)

Hello Nahuel, the shell looks large, so I agree it is likely Berthella. Check out sea slug forum, Berthella platei.  The shell is a thin transparent brown, and it does say it has a distribution to the Falklands.

Best,

Nancy

Thanks Nancy!