Appear to me as ascidians (tunicates), most likely Didemnum sp. A quick search on tunicates in northern Norway leads me to suggest Didemnum albidum, but I'm not 100% confident on the species ID from the photo. See the attached paper by J.A. Marks (1994) on Didemnum tunicates in northern Norway. Might also be a sponge.

It can be Leucosolenia (Spongia) - calcareous tube-sponge- maybe Leucosolenia variabilis (Haeckel, 1870)

The bryozoan Electra pilosa?

I don't think Electra pilosa would be feasible here. Bryozoans are encrusting species that would cover the outside surface of the rockweed in a thin layer. Given the thickness of the white animals in comparison to the rockweed blades in the photo, I'm doubtful that the white projections are bryozoan-encrusted rockweed blades.

I think Leucosolenia sponge or Didemnum tunicate are most likely.

Thank you all for your inputs. I agree that it might be a ascidian or a sponge, but I do not think it is an ectoprocta (bryozoa). The animals seemed to have soft bodies, which rules out the ectoprocta. Do any of you have a suggestion to a paper showing the morphology of the Leucosolenia sponge?

Here are some close-up pictures of the animals:

Close-up picture nr 2:

Some classic publications on Leucosolenia variabilis:

http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=4333320&fileId=S0025315400024668

http://jcs.biologists.org/content/s3-102/60/543.short

http://jcs.biologists.org/content/s3-100/52/557.short

http://jcs.biologists.org/content/s3-102/60/531.short

Thank you!

It certainly is a calcareous sponge, probably Leucosolenia variabilis (Haeckel, 1870). You need a microscope to see if the sponge spicules are monaxons (oxeas) and tetraxons with 3 and 4 axis (triactines and tetractines). Bryozoa and Ascidiacea Didemnidae are colonies of zooids and appear as small holes in the surface. In the picture there are no holes, only one oscule at the end of each branch.