Growing on seaweed.
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Angela - I'm sorry to be so late in arriving!
This is quite unusual pattern for an Ascidian
It looks a little like a batch of Sea Star eggs; but I agree, logic suggest Ascidian.
There is not much evidence on the web for my proposal but Dave Fenwick's site (ALWAYS good to check) has this http://www.aphotomarine.com/starfish_asterina_gibbosa_cushion_star.html.
My yet to be IDd post here http://www.ispotnature.org/node/335412 shows similar (just!) shapes or colours.
Now you have four years more experience, do you have any more thoughts?
Lucy, you seem certain - did you read my comment above?
I have looked hard again and it still has the look of small spherical egg masses and not a degraded ascidian. I am NO expert - honest.
Can you say what swung it for you?
I did read your comment - but I really don't think egg mass. The round "eggs" are more close to "petals".
If I was pushed to change the identification - I would be inclined to put forward B. leachi instead - as the patterns are more in lines than "stars".
Lat/Lng: 50.5, -2.5
OS grid ref: SY6676