I theses are different individuals - but perhaps the same species?
No interactions present.
Thanks for ID Peter.
Alan Brampton (Benson)
I am surprised that iSpot gives Great Red Slug as an alternative to Great Black Slug for Arion ater. Have I missed something?
Anyway, I do not believe you can give a precise species in this group without behavioural details and dissection. The individuals shown are probably not adult. They would be better described as Arion (Arion) ater agg.
Have a look at http://idtools.org/id/mollusc/factsheet.php?name=Arion%20ater%20group:%2...
Lat/Lng: 51.625, -1.105
OS grid ref: SU620921