These are my local rock pools at the Trefusis headland near Flushing. On the right is Penryn River (which is not a river at all) with Falmouth on the other bank. Further to the right around Falmouth is the open sea. To the left the Fal Estuary (or Carrick Roads) going all the way to Truro, the capital of Cornwall.
The seaweed composition had again changed since I last visited. This time there was one red weed dominating the pools, I believe it is the Pacific invader Devil’s tongue weed Grateloupia turuturu (which appeared in my aquarium at some point as well):
The most common sea slug (nudibranch) here is the Sea lemon Archidorus pseudoargus:
An Bollenessor 
You have a very attractive and interesting blog. I think your Archidoris pseudoargus 24 cOct 2013 is in fact Lamellaria perspicua, a relative of the cowries. I can’t be sure as it is not in water when Lamellaria curls its mantle into a respiratory siphon at one end. Have a look at http://www.conchsoc.org/spAccount/lamellaria-perspicua for detailed description and images (all live ones in water) for next time you see one. There’s a large selection of sea slugs, including Archidoris, on the same site. Best wishes Ian
Dear Ian,
thanks for your comment! I had actually consulted this great website and was not 100% sure about my ID. I went for the Sea lemon as it is common and known to retract its gills. I might have misidentified another seaslug; if you could comment on that picture that would be great: https://anbollenessor.wordpress.com/2013/03/28/some-serious-aquascaping/)
cheers, Michiel