Gray see slug

Had an unexpected day free Sunday so headed to Seaford Head for a spot of bird-watching when I suddenly realised the tide was pretty good. Bird-watching soon turned into rock-pooling and about the third rock I turned over had a lifer on it! It was a Grey Sea Slug Aeolidia papillosa. It really reminds me of those weird floral vintage swimming caps that used to give me the creeps. It's my fourth sea-slug, all of which I have seen in the last two years and all from between Seaford and Beachy Head. 

I like this last shot. It looks like it's just devoured a tiny Human and the only bits left are two tiny fingers giving the peace sign as they too are slowly absorbed. So long, tiny Human!

Later on, I found this purple triangular crab under a rock. Pretty sure this is Pisa armata, not a species I have seen before.

This thing had me scouring the Handbook though. I thought it was some bizarre mollusc. Then I thought it was a pistachio macaron for a while. Now I believe it's actually a fossilised mollusc. Thanks to Robin Shrubsole for pointing me in this direction.