There were 3 or 4 of these in a rock pool I was surveying off of east Devon, not sure what it is but left a purple trail. It's approx 20cm long
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Almost certainly Aplysia punctata, the commonest UK species by far, up to 20cms but I've mostly seen them more like 8-12cms. Aplysia species are known for defensive purple dye emissions and punctata comes in various colour forms including chocolate brown. A. depilans and fasciata are very rare visitors from more southerly water and can be much bigger. Maybe can't be ruled out from this pic as their colour forms vary too and can be all dark, but are unlikely.
Nick Upton, naturalist and photographer.
Lat/Lng: 50.705, -3.23
OS grid ref: SY132902