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Rebecca (your first day!) this is not a fish but a sea star, a much preferred name than starfish.
Re-designate it as an invertebrate - Edit/Group.
Nice record - keep it up.
Happy or not, I do not think there are many candidates for this greenish cushion star other than A. gibbosa.
Nice spot but agree with dejayM - it needs to go in the invertebrate section, which can be done by editing your record. In future (and this is worth pointing out to others out on the rocky shore), keep an eye out for a related species called Asterina phylactica. This was first described from British waters in 1979 and is found predominantly on coasts of the south and west of Britain so Devon is a very likely candidate location to find them. It is difficult to distinguish from A. gibbosa when smaller than 4.5 mm in size. Apparently prefers rock pools with a lot of seaweed. All information here is from A Student's Guide to the Seashore by the very appropriately named Fish and Fish. For a photo check out http://www.aphotomarine.com/starfish_asterina_phylactica_green_starlet.html
To find out about what's going on in the south west check out http://www.ispot.org.uk/forum/8409
Lat/Lng: 50.3177, -4.0846
OS grid ref: SX516484